Saturday, August 31, 2013

AP Chemistry Question of the Day --15


Here comes the answer to question 14 I posted yesterday. The correct answer is B
To Solve an equilibrium question, ICE table is always very helpful. ICE stands for Initial, Change and Equilibrium. 
Assume x atm of PCl5 converted to product at equilibrium, then
According to Dalton's Law of partial pressure, the total pressure is the sum of the individual pressures. At equilibrium, the total pressure is 1.40atm,


1.0-x+x+x=1.4

From the above equation, we can figure out that x=0.4. The equilibrium pressures of the three gases are 0.6atm. 0.4atm and 0.4atm. Kp can then be calculated:
So, the correct answer is B, Kp<1.


Following is Question 15 of the series. It is in continuation with question 12, 13 and 14. 

13. PCl5(g) decomposes into PCl3(g) and Cl2(g) according to the equation above. A pure sample of PCl5(g) is placed in a rigid, evacuated 1.00 L container. The initial pressure of the PCl5(g) is 1.00 atm. The temperature is held constant until the PCl5(g) reaches equilibrium with its decomposition products. The figures below show the initial and equilibrium conditions of the system. If the decomposition reaction were to go to completion, the total pressure in the container will be:


(A) 1.4 atm
(B) 2.0 atm
(C) 2.8 atm
(D) 3.0 atm


I will post the answer and explanation on Monday, Sept. 2nd. 

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Friday, August 30, 2013

AP Chemistry Question of the Day --14


Here comes the answer to question 13 I posted yesterday. The correct answer is C
The rates of the reaction, both forward and backward are affected by the concentration of the reactant (forward reaction) and product (backward reaction). Since initially, there was only PCl5 in the system, the forward reaction rate was at the maximum. The backward reaction rate was zero. As reaction goes, the concentration of PCl5 decreases and that of PCl3 and Cl2 increase, causing the forward reaction to slow down and the backward reaction to speed up. When the forward and backward reaction rate are the same, the equilibrium is achieved. The chemical equilibrium is a dynamic equilibrium and the rate will never be 0. 



Following is Question 14 of the series. It is in continuation with question 12 and 13. 

13. PCl5(g) decomposes into PCl3(g) and Cl2(g) according to the equation above. A pure sample of PCl5(g) is placed in a rigid, evacuated 1.00 L container. The initial pressure of the PCl5(g) is 1.00 atm. The temperature is held constant until the PCl5(g) reaches equilibrium with its decomposition products. The figures below show the initial and equilibrium conditions of the system. Which of the following statements about Kp, the equilibrium constant for the reaction, is correct?



(A) Kp > 1
(B) Kp < 1
(C) Kp = 1
(D) It cannot be determined whether Kp > 1, Kp < 1, or Kp = 1 without additional information


I will post the answer and explanation tomorrow.

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Thursday, August 29, 2013

AP Chemistry Preparation Class


The next AP Chemistry exam is scheduled on May 5, 2014.  This year, there is a new curriculum framework in effect and some major changes are introduced. Nuclear Chemistry and Organic Chemistry are eliminated and Photoelectric Spectroscopy and Mass Spectroscopy are included. The exam will focus more on data interpretation and graph demonstration. There are some other changes as well. It is highly recommended to start the preparation early to guarantee a satisfying score (preferably a 5).

I will start two small group AP chemistry classes with a max. of 4 students in each group in early October as follows:

Class one: Saturday afternoons from 2pm to 5pm, starting on Oct. 5, 2013.
Class two: Sunday  afternoons from 2pm to 5pm, starting on Oct. 6, 2013.

Spaces are limited, please call me on my cell 604-808-6827 or email me at vancouverchemistrytutor@gmail.com to register.

Thank you very much!

Visit my website at www.vancouverchemistrytutor.com

AP化学小班辅导


明年的AP化学考试定在5月5号,这次考试将启用新的考试大纲,与以往的考试大纲有比较大的差别,题型和考试侧重点也有比较大的变化。例如核化学和有机化学的部分不再列入考试范围,另外新增加了光电光谱和质谱的内容。考试将更注重数据,图表的分析。由于AP化学考试涵盖很多内容,早做准备对考取满意成绩(5分)尤为重要。

我将于10月初开始举办两个AP化学的补习班,每班最多四名学生,具体日期如下:

星期六下午班, 2点到5点, 10月5号开班。
星期天下午班, 2点到5点, 10月6号开班。

名额有限,请依据您的时间安排尽快报名。请致电张老师 604-808-6827 或电邮vancouverchemistrytutor@gmail.com

AP Chemistry Question of the Day --13


Here comes the answer to question 12 I posted yesterday. The correct answer is C

According to ideal gas law,
PV=nRT
Since the temperature and volume are held constant, pressure (P) is directly proportional to total mole number. As one molecule of PCl5 will produce 2 molecules of product.  At equilibrium, there are more moles of gas in the container, causing the pressure to increase. So, C is the right answer.

Following is Question 13 of the series. It is in continuation with question 12. 

13. PCl5(g) decomposes into PCl3(g) and Cl2(g) according to the equation above. A pure sample of PCl5(g) is placed in a rigid, evacuated 1.00 L container. The initial pressure of the PCl5(g) is 1.00 atm. The temperature is held constant until the PCl5(g) reaches equilibrium with its decomposition products. The figures below show the initial and equilibrium conditions of the system. As the reaction progresses toward equilibrium, the rate of the forward reaction


(A) increases until it becomes the same as the reverse reaction rate at equilibrium
(B) stays constant before and after equilibrium is reached
(C) decreases to become a constant nonzero rate at equilibrium
(D) decreases to become zero at equilibrium



I will post the answer and explanation tomorrow.

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Wednesday, August 28, 2013

AP Chemistry Question of the Day --12


Here comes the answer to question 11 I posted yesterday. The correct answer is C

This is a question about intermolecular forces as I discussed earlier in one of my blogs regarding the prediction of physical properties.

When the pressure of the gas is increased and the gas is about to condense, the gas can no longer deemed as ideal gas. At this point, the intermolecular forces between the gas molecules are no longer negligible. The stronger the intermolecular forces are, the less pressure is needed for the gas to condense.

All  gases A, B, and  C are non polar covalent bonded molecules. The only intermolecular force is London dispersion force. London dispersion force is caused by the instantaneous dipoles as molecules move around. The more complex the electronic structure of the molecule, the stronger the London dispersion force. Normally in a series of compound with similar structures, the higher the molar mass, the stronger the London dispersion force. As a result, answer C is the right answer.


Following is Question 12 of the series. 

12. PCl5(g) decomposes into PCl3(g) and Cl2(g) according to the equation above. A pure sample of PCl5(g) is placed in a rigid, evacuated 1.00 L container. The initial pressure of the PCl5(g) is 1.00 atm. The temperature is held constant until the PCl5(g) reaches equilibrium with its decomposition products. The figures below show the initial and equilibrium conditions of the system. Which of the following is the most likely cause for the increase in pressure observed in the container as the reaction reaches equilibrium?

(A) A decrease in the strength of intermolecular attractions among molecules in the flask
(B) An increase in the strength of intermolecular attractions among molecules in the flask
(C) An increase in the number of molecules, which increases the frequency of collisions with the walls of the container
(D) An increase in the speed of the molecules that then collide with the walls of the container with greater force


I will post the answer and explanation tomorrow.

Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Tuesday, August 27, 2013

AP Chemistry Question of the Day --11


Here comes the answer to question 10 I posted yesterday. The correct answer is B
According to ideal gas law: 
PV=nRT
Mole number n can also be written as 
where m is the mass of the gas and MW is the molar mass of the gas respectively.
If we rewrite the equation, we can see that
As a result, density is directly proportional to the product of pressure and molar mass when temperature is kept unchanged. So, gas be has the highest density. B is the right answer. 

Following is Question 11 of the series. It is in continuation with Question 9 and 10. 

10. Three gases are kept in three identical rigid containers under the conditions given in the table below. If the pressure of each gas is increased at constant temperature until condensation occurs, which gas will condense at the lowest pressure?
A. gas A
B. gas B
C. gas C
D. All three gases will condense at the same pressure

I will post the answer and explanation tomorrow.


Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Monday, August 26, 2013

AP Chemistry Question of the Day --10


Here comes the answer to question 9 I posted yesterday. The correct answer is D
Average kinetic energy of any substance is only dependent on the temperature. Temperature  is the measure of average kinetic energy of atoms and molecules.


Following is Question 10 of the series. It is in continuation with Question 9. 

10. Three gases are kept in three identical rigid containers under the conditions given in the table below, which one has the greatest density?
A. gas A
B. gas B
C. gas C
D. Their average kinetic energy are the same

I will post the answer and explanation tomorrow.


Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Sunday, August 25, 2013

AP Chemistry Question of the Day --9


Here comes the answer to question 8 I posted yesterday. The correct answer is B

As we discussed earlier, HA is a weak acid and the chemical reaction is:
Since HA is a monoprotic acid, HA reacts with NaOH in a 1:1 ratio. At equivalent point, the mole number of NaOH should equal to the mole number of HA in the original sample. Mole number can be calculated by the product of their molar concentration and volume. Thus:


If the indicator changes colour slightly past the equivalent point, this means the volume of the NaOH solution recorded will be a little over 40ml. As a result, the calculated concentration of HA will be slightly more than 0.800M.


Following is Question 9 of the series:

9. Three gases are kept in three identical rigid containers under the conditions given in the table below, which one has the greatest average kinetic energy?
A. gas A
B. gas B
C. gas C
D. Their average kinetic energy are the same

I will post the answer and explanation tomorrow.


Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Saturday, August 24, 2013

AP Chemistry Question of the Day --8


Here comes the answer to question 7 I posted yesterday. The correct answer is C

As we discussed earlier, HA is a weak acid and the net ionic reaction is:
If HA and A-  ion coexist, the reaction has not reached equivalent point yet. answer D can be eliminated. From the graph, we can see that at equivalent point, 40ml of NaOH solution is consumed. This means S is the half point, when half of the HA is neutralized. point R is before the half point. At point R, HA concentration is higher than A- ion. At point T, there are more A- on than HA. So answer C is the correct answer.


Following is Question 8 of the series, it is the continuation of question number 5, 6 and 7. 

8. A 50.0 mL sample of an acid, HA, of unknown molarity is titrated, and the pH of the resulting solution is measured with a pH meter and graphed as a function of the volume of 0.100 M NaOH added. A student carries out the same titration but uses an indicator instead of a pH meter. If the indicator changes colour slightly past the equivalence point, what will the student obtain for the calculated concentration of the acid?



A. Slightly less than 0.0800M
B. Slightly more than 0.0800M
C. Slightly less than 0.125M
D. Slightly more than 0.125M

I will post the answer and explanation tomorrow.


Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Friday, August 23, 2013

AP Chemistry Question of the Day --7


Here comes the answer to question 6 I posted yesterday. The correct answer is B

As we discussed yesterday, HA is a weak acid and the net ionic reaction is:

Point U is after the equivalent point. At this point, all the HA has been neutralized and the mixture should also contain excessive amount of NaOH. As a result, the dominant species other than the solvent water should be NaA and NaOH. SInce both NaA and NaOH are ionic compounds, they dissociate completely in water. In addition, Na ion and Anion A, and Na ion and hydroxide should exist in 1:1 ratios.  Thus, B is the right answer. 

One thing I need to point out is that even at point U, there should still be a small amount of HA in the solution. This is due to the hydrolysis of the anion A- as shown in the following equation.


Following is Question 7 of the series, it is the continuation of question number 5 and 6. 

7. A 50.0 mL sample of an acid, HA, of unknown molarity is titrated, and the pH of the resulting solution is measured with a pH meter and graphed as a function of the volume of 0.100 M NaOH added. At which point on the titration curve is [A] closest to twice that of [HA]?



A. R
B. S
C. T
D. U



I will post the answer and explanation tomorrow.


Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Thursday, August 22, 2013

AP Chemistry Question of the Day --6


Here comes the answer to question 5 I posted yesterday. The correct answer is A

This is a tricky question! This is the direction the new AP chemistry curriculum is leading to. You  have to be able to read the graph and understand what is going on. 
First of all, you have to decide whether the acid HA is a strong acid or a weak acid, since it is not stated in the question. How are we going to figure that out? Look at the pH of the equivalent point. Because NaOH is a strong base, the pH of the equivalent point would be 7 if HA is a strong acid and the pH of the equivalent point would be higher than 7 if HA is a weak acid. When you examine the graph, you will see that the equivalent point pH is at around 8. Therefore, HA is a weak acid. 

Now we look at point R on the titration curve, it is at the beginning of the curve and has not reached the half point yet. As a result, it is still mainly undissociated HA molecules dominant in the mixture. 

So, the right answer is A.



Following is Question 6 of the series, it is the continuation of question number 5. 

6. A 50.0 mL sample of an acid, HA, of unknown molarity is titrated, and the pH of the resulting solution is measured with a pH meter and graphed as a function of the volume of 0.100 M NaOH added. Which of the following is the best particulate representation of the species (other than H2O) that are present in significant concentrations in the solution at point U in the titration?







I will post the answer and explanation tomorrow.


Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Wednesday, August 21, 2013

AP Chemistry Classes


The next AP Chemistry exam is scheduled on May 5, 2014.  This year, there is a new curriculum framework in effect and some major changes are introduced. Nuclear Chemistry and Organic Chemistry are eliminated and Photoelectric Spectroscopy and Mass Spectroscopy are included. The exam will focus more on data interpretation and graph demonstration. There are some other changes as well. It is highly recommended to start the preparation early to guarantee a satisfying score (preferably a 5).

I will start two small group AP chemistry classes with a max. of 4 students in each group in early October as follows:

Class one: Saturday afternoons from 2pm to 5pm, starting on Oct. 5, 2013.
Class two: Sunday  afternoons from 2pm to 5pm, starting on Oct. 6, 2013.

Spaces are limited, please call me on my cell 604-808-6827 or email me at vancouverchemistrytutor@gmail.com to register.

Thank you very much!

Visit my website at www.vancouverchemistrytutor.com

AP化学小班辅导


明年的AP化学考试定在5月5号,这次考试将启用新的考试大纲,与以往的考试大纲有比较大的差别,题型和考试侧重点也有比较大的变化。例如核化学和有机化学的部分不再列入考试范围,另外新增加了光电光谱和质谱的内容。由于AP化学考试涵盖很多内容,早做准备对考取满意成绩(5分)尤为重要。

我将于10月初开始举办两个AP化学的补习班,每班最多四名学生,具体日期如下:

星期六下午班, 2点到5点, 10月5号开班。
星期天下午班, 2点到5点, 10月6号开班。

名额有限,请依据您的时间安排尽快报名。请致电张老师 604-808-6827 或电邮vancouverchemistrytutor@gmail.com

AP Chemistry Question of the Day --5


Here comes the answer to question 4 I posted yesterday. The correct answer is C

To answer this question right, first we need to know that both hydrogen and oxygen gas exist as diatomic molecules. Apart from hydrogen and oxygen, nitrogen, fluorine, chlorine, bromine and iodine also exist as diatomic molecules. Another detail we should notice is that oxygen molecules are bigger than hydrogen molecules in size. This is because oxygen is in the second period and hydrogen is in the first period. If you are good at molecular geometry, you can see that the water molecules are drawn as bent molecules. This is due to the sp3 hybridization of the central oxygen atom. In addition, we need to write the balanced equation correctly as follows.

From this equation,we can see that hydrogen and oxygen and the water produced obey a 2:1:2 ratio.
Only choice C is correct in both ways, so C is the right answer.


Following is Question 5 of the series:

5. A 50.0 mL sample of an acid, HA, of unknown molarity is titrated, and the pH of the resulting solution is measured with a pH meter and graphed as a function of the volume of 0.100 M NaOH added.At point R in the titration, which of the following species has the highest concentration?





.

I will post the answer and explanation tomorrow.


Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Tuesday, August 20, 2013

AP Chemistry Question of the Day --4

Here comes the answer to question 3 I posted yesterday. The correct answer is B

First Law of Thermodynamics states that the total quantity of energy in the universe is assumed to remain constant. Energy can not be destroyed or generated, it can only be converted from one form to the other. Chemical systems undergo three main processes that change their energy: heating/cooling, phase transitions, and chemical reactions. In Question 3, the container is insulated, so it is considered a closed system. Therefore, there is no energy exchange with the surroundings. The energy the metal lost should be equal to the energy the water gained. The temperature changes of the metal and water are different because they have different heat capacities. Specific heat (c) is defined as the amount of heat required to raise the temperature of 1g of a substance by 1 degree in temperature.
Here q is the heat amount, m is the mass of the substance and Delta T is the temperature change. Since the heat the water gained is the same amount as the metal lost and both the metal and the water has a mass of 100g. We can conclude that since the metal temperature changed more than the water temperature, the metal has a smaller specific heat than water.

This is also the key behind Calorimetry. Always remember  Energy can not be destroyed or generatedit can only be converted from one form to the other.


Following is Question 4 of the series:

4.Which of the following particulate diagrams best shows the formation of water vapor from hydrogen gas and oxygen gas in a rigid container at 125C?




I will post the answer and explanation tomorrow.


Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Monday, August 19, 2013

AP Chemistry Question of the Day --3

Here comes the answer to question 2 I posted yesterday. The correct answer is D


Conditions affecting reaction rate include:
  • Nature of the reactants. The stronger the bonds in the reactants, the more difficult to break them and the slower the reaction will be. 
  • Temperature: the higher the temperature, the faster the reaction.
  • Catalyst: Increase the rate of the reaction by taking an alternate reaction path and lowering the activation energy.
  • Concentration of the reactants: The higher the concentration, the faster the reaction goes. This is not always true though. It is true only if the substance is part of the rate law.
  • Surface area: in a heterogeneous reaction, the bigger the surface area, the faster the reaction goes.
So, D is the correct answer.

Following is Question 3 of the series:

3. A 100 g sample of a metal was heated to 100C and then quickly transferred to an insulated container holding 100 g of water at 22C. The temperature of the water rose to reach a final temperature of 35C. Which of the following can be concluded?

A.  The metal temperature changed more than the water temperature did; therefore the metal lost more thermal energy than the water gained.
B.  The metal temperature changed more than the water temperature did, but the metal lost the same amount of thermal energy as the water gained.
C.  The metal temperature changed more than the water temperature did; therefore the heat capacity of the metal must be greater than the heat capacity of the water.
D.  The final temperature is less than the average starting temperature of the metal and the water; therefore the total energy of the metal and water decreased.


I will post the answer and explanation tomorrow.


Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Sunday, August 18, 2013

AP Chemistry Question of the Day --2

Here comes the answer to question 1  I posted yesterday. The correct answer is C.
Atom is the smallest unit that retain the properties of the individual element. Each atom consists of a nuclear and electron(s) surrounding the nuclear. The nuclear is very dense and takes up only a tiny portion of the total volume of the atom. The vast space outside the nuclear is empty and electrons move around in it. In the nuclear, there are protons and neutrons. The number of protons are called the atomic numbers. Each element in the periodic table has a characteristic atomic number, which is the same as the protons in the nuclear. Each proton carries a charge of +1.  Neutrons are neutral and carry no charge. The number of electrons orbiting the nuclear is the same as the atomic number when the atom is neutral. Atoms of the same element can have different number of neutrons. They are called isotopes. For each isotope, the mass of the atom is roughly the total number of protons and neutrons in the nuclear since electrons have negligible mass compared to proton and neutron. The atomic mass we observe from the periodic table is the weighted mass of all the naturally occuring isotopes of that element. It is calculated as follows which is the sum of the product of the mass of each isotope and its abundance:
In Question 1, we know the weighted average is 35.5 and we know the mass of the two major isotopes are 35 and 37. Assume 35Cl is x%, then
which gives x to be 66%.

Following is AP Chemistry Question of the Day --2:

2. Relatively slow rates of chemical reaction are associated with which of the following:

A. The presence of a catalyst
B. High Temperature
C. High concentration of reactants
D. Strong bonds in reactant molecules


I will post the answer and explanation tomorrow.


Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Saturday, August 17, 2013

AP Chemistry Question of the Day--1

As September is around the corner, I decided to start  a series called " AP Chemistry Question of the Day" on my blog.  Each day I am going to post a question on my blog and give out the answer and the key points related to the question the next day. Follow me and understand the keys to AP chemistry bits by bits and your success in next May's AP Chemistry exam is guaranteed.

Here is my first  " AP Chemistry Question of the Day".

1. The atomic mass of chlorine is 35.5. Given that 35Cl and 37Cl are the two most naturally abundant isotopes of chlorine. The abundance of 35Cl is close to:

A. 50%
B. 33%
C. 66%
D. 40%

 I will post the answer and explanation tomorrow.


Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!

Wednesday, August 14, 2013

vapour pressure in AP Chemistry

Questions involving vapour pressure appear each year in the AP chemistry exam. To understand the concept of vapour pressure, the following should be kept in mind:

Vapour pressure is the pressure exerted by the gas above a liquid in a closed container when equilibrium has been established.   When a liquid is introduced to a closed container, the molecules with higher kinetic energy at the surface of the liquid can escape and go to gas phase. At the same time, the molecules in the gas phase with lower kinetic energy could be trapped back into the liquid phase due to their intermolecular interactions with the liquid molecules at the surface. This is a dynamic process. When equilibrium is established, the number of molecules escaping to the gas phase equals to the number of molecules entering the liquid phase and the vapour pressure remains constant.

Vapour pressure is only dependent on the nature of the substance and temperature. 

The stronger the intermolecular forces in a certain substance, the lower the vapour pressure.

The higher the temperature, the higher the vapour pressure. At higher temperature, the molecules has higher kinetic energy, making it easier for them to escape and stay in the gas phase.

The temperature at which the vapour pressure reaches the atmosphere pressure is called the boiling point. when the atmosphere pressure is 1atm, the boiling point is called normal boiling point. At higher elevations, the atmosphere pressure is lower, thus the boiling points at higher elevations is lower than at sea level for a specific substance.

When two or more volatile substance are mixed together, the total vapour pressure is the sum of the product of each individual ingredients' mole fraction and vapour pressure.

When non volatile substances are dissolved in a solution, the solute molecules at the surface of the solution interfere with the vaporization process, resulting in lower vapour pressure, which in turn elevate the boiling point of the solution. The magnitude of boiling point elevation is proportional to the molality of the solution as well as the particle numbers of the solute when it dissociate in solution. It has been stated in the new curriculum framework that colligative properties is no longer covered by the AP chemistry exams. It does not hurt to know it though.



Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

Regards!




Friday, August 9, 2013

Physical properties (melting point and boiling point) trend prediction in AP chemistry

Physical properties trend prediction such as melting point and boiling point appears every year in AP chemistry exam. Physical properties are determined by the chemical bonding at atomic level. When you encounter such question, first thing you need to do is to figure out what kind of chemical bonding you are dealing with. The stronger the bond is, the higher the melting point or boiling point it will be.

There are four types of chemical bonding as follows:

1. Ionic bond. Ionic bond exists in compounds typically formed between a metal and a non metal, where electron transfer occurs. Normally, if the electronegativity difference between the two elements is bigger than 1.7, the bond is considered to be ionic. Ionic compounds contain lattice structures. The strength of ionic bond is determined by the static interaction between the cation and anion. The higher charge the cation and anion carry, and the smaller the distance between the cation and the anion, the stronger the bond becomes. For example, in NaF and MgO, the ionic radii of sodium and magnesium, fluoride and oxygen are comparable. But since magnesium and oxygen ions carry 2 positive and negative charges while sodium and fluoride have only   1 charge. As a result, MgO has stronger bonding and higher melting and boiling points. In LiF and NaCl, all the ions carry one charge, but since lithium ion and fluoride ion have smaller radii than sodium ion and chloride ion, LiF has stronger bond than NaCl. LiF is expected to have higher melting and boiling points than NaCl.

2. Covalent bonds. Covalent bond exists in compounds formed between nonmetals, where electron sharing occurs. If the electronegativity difference is between 0 and 0.4, the bond is considered non polar and with a electronegativity difference between 0.4 and 1.7, the bond is considered polar covalent. Apart from covalent network crystals, the smallest unit of covalently bonded compounds is molecule. Inside the molecule, covalent bonds exist among the atoms, but what bind the molecule together is intermolecular forces. Physical properties of the compounds are determined by the strength of their intermolecular forces. There are three types of intermolecular forces as follows:

  • Hydrogen bonding. Hydrogen bonding is the strongest form of intermolecular force. It occurs where F or O or N atoms are covalently bonded to a H atom. Due to the high electronegativity of F, O, N, They tend to attract strongly the H atoms from their neighbouring molecules. For example, HF, water and ammonia have exceptionally higher boiling points. It is worthwhile to point out that hydrogen bonding exists when F, O or N are directly bonded to H. In molecules such as CH3F where F and H are all directly bonded to C and No bond is between H and F, there is no hydrogen bonding.
  • Dipole dipole interactions. Dipole dipole interactions occur in polar covalent bonds. The dipoles formed due to the unbalanced partial charges in the molecule tends to align the molecules so that the positive end of one molecule is close to the negative end of another molecule. Dipole dipole interaction is the second strongest form of intermolecular force. Here one thing to keep in mind is that polar covalent compounds contains covalent bond, but compounds contains polar covalent bond are not necessarily polar covalent molecules. It depends on their molecular geometry. If the molecule contains polar covalent bonds but is symmetrical, all the bond polarities will cancel and the overall molecule is non polar.
  • London dispersion force. London dispersion force is the weakest interaction between molecules. It occurs in all the covalent molecules. But due to its weak strength, if the molecule is polar, we do not need to consider London dispersion force, because the dipole dipole interactions are much stronger. In non polar molecules, there is only London dispersion forces. London dispersion forces are formed due to the instant induced dipoles in the molecules. More electrons in the molecule will increase the chance to induce instant dipoles, making the London dispersion force stronger. As a result, the higher the molecular weight, the stronger the London dispersion force, the higher the melting points and boiling points in non polar covalent compounds. For example in the halogen series, F2, Cl2, Br2 and I2, fluorine and chlorine exist as gas, bromine is a liquid and iodine is a solid at room temperature.
As mentioned earlier, some covalent compounds exist as network covalent crystals, the whole compound exist as a giant molecule. To melt or boil, instead of breaking intermolecular forces, covalent bonds has be be broken. Thus, this type of compounds have super high melting and boiling points. Typical compounds belonging to this class are diamond, silicon dioxide (SiO2, the major ingredient of sand), SiC etc.

3. Metallic bond. Metallic bond exist in metals in their elemental form, where all the metal ions take the lattice structure and the electrons move freely among the metal ions. The strength of the metallic bonds depends on the distance of the metal ions, the smaller the radium of the metal, the stronger the metallic bond. Foe example, going down from the top to the bottom of the alkali and alkali earth groups, the metallic bond becomes weaker.

Hope the stuff above helps you to predict physical properties trends in AP chemistry exams.


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Wednesday, August 7, 2013

Keys for Chemical Reaction Kinetics in AP Chemistry Exam

While chemical equilibrium investigate a chemical reaction at equilibrium, kinetics is the part of chemistry that studies the chemical reaction while it is in progress. Here are some key points in understanding kinetics.

1. Rate of a chemical reaction is defined as changes in concentration or pressure of a substance per unit of time. It can be measured as the decrease in concentration of the reactants or increase in concentration of the product per unit of time.

2. The rate of the reaction is obtained by determining the slope of the kinetic curve (concentration vs time curve) at the desired point in time. This can be done as follows:

  1. Select the desired point on the curve and draw a tangent to it.
  2. Select two points on the tangent, and determine the concentrations c and time t, corresponding to those two points.
3. For a general chemical reaction: 

the rate of the reaction is:
Note here that for each individual component (reactant or product), the rate is proportional to their individual coefficients in the chemical reaction. 

4. Collision theory states that for a chemical reaction to successfully to happen, reactant molecules have to collide. The reaction rate is a function of the successful collisions per unit of time. Successful collisions have

  • Sufficient energy (greater than the activation energy) to break the existing bond and form the new bond.
  • The correct spatial orientation of the molecules with respect to each other.
5. Conditions affecting reaction rate include:
  • Nature of the reactants. The stronger the bonds in the reactants, the more difficult to break them and the slower the reaction will be. 
  • Temperature: the higher the temperature, the faster the reaction.
  • Catalyst: Increase the rate of the reaction by taking an alternate reaction path and lowering the activation energy.
  • Concentration of the reactants: The higher the concentration, the faster the reaction goes. This is not always true though. It is true only if the substance is part of the rate law.
  • Surface area: in a heterogeneous reaction, the bigger the surface area, the faster the reaction goes.
6. The activation energy is the energy required for the reaction to take place with a unit of kJ/mol. Activation energy depends on the nature of the reaction. The lower the activation energy the faster the reaction goes. Activation energy is independent of temperature and concentration.

7. Arrhenius equation is no longer required by AP chemistry exams, but it is a good tool to understand how temperature affects reaction rate. The higher the temperature, the higher the rate constant in the rate law, which contributes to higher reaction rate.


8. Given the reaction: 

The rate of the reaction is: 
m and N describe the orders of the reaction with respect to reactants A and B. The overall order of the reaction is the sum of the orders with respect to each reactant. Note here that m and n has nothing to do with the stoichiometric coefficient in the balanced reaction equation. 
  •  If the reaction is Zero order,  the reaction rate is independent of the concentration of any reactant. The unit for rate constant: mols.L-1.sec-1Rate=k;      [A]0-[A]=kt;               t1/2=[A]/2k;  [A] vs t forms a linear plot.
  • If the reaction is First order, The reaction rate is proportional to the concentration of one of the reactants. Rate=k[A];      ln[A]0-ln[A]=kt;           t1/2=0.693/k;  ln[A] vs t forms a linear plot.
  • If the reaction is Second order, the reaction rate is proportional to the square of the concentration of one of the reactants, or it might be proportional to theproduct of two different reactant concentrations.   
      1/[A]-1/[A]
    0=kt;      t1/2=1/k[A0];   1/[A] vs t. forms a linear plot.
8. Reaction mechanism refers to  the sequence of bond breaking and bond
formation steps that lead to an overall chemical reaction. Each
individual step is called an elementary step.
  • The rate of reaction for an elementary reaction is based on the stoichiometric coefficient. If one molecule is involved, it is called unimolecular. If two molecules are involved, it is called bimolecular.
  • The slowest elementary step is called the rate-determining (rate limiting) step. The overall reaction rate and order will be the same as the rate=determining step.
9. Catalyst is a substance that increases the rate of a chemical reaction without itself being consumed.Catalyst works by lowering the activation energy. By lowering the activation energy, both the forward the reverse reactions will be
accelerated to the same extent. As a result, catalyst is not able to shift the chemical equilibrium under any circumstance.





Thank you very much for reading my post, I am available to discuss any topics in AP chemistry. Please give me a call at 604-808-6827 or email me at vancouverchemistrytutor@gmail.com. My website is located at www.vancouverchemistrytutor.com.

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