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-1. Rate=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.

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