A STUDY OF REACTION RATES CHEMICAL KINETICS A STUDY OF REACTION RATES

What is Kinetics? The study of the rate at which a chemical reaction takes place. It provides a clue to how a chemical reaction takes place – this is the mechanism or pathway of the reaction.

How do you measure reaction rates? What happens to the [reactants] over time? What happens to the [products] over time? H2 + I2  2 HI

Calculating reaction rate Br2 (aq) + HCOOH (aq)  2 Br- + 2 H+ + CO2 (g)

Reactant concentration vs time

What happens to the rate over time? Since the concentration of the reactants decrease over time, the reaction rate decreases as well.

Instantaneous rates

GRAPH OF RATE VS [REACTANT]

Rate Expression rate = k · [A]a · [B]b For a given reaction: a A + b B  products rate = k · [A]a ,if B is in excess rate = k · [B]b ,if A is in excess _____________________________ By combining the two expression, we get: rate = k · [A]a · [B]b The power to which the concentration of A or B is raised in the rate expression describes the order of the reaction.

Concentration vs. Reaction Rates rate = k[A]0 rate = k[A]1 rate = k[A]2 The reaction order can ONLY be determined by experimentation. The reaction order for a reaction is simply the sum of all the reaction orders for each concentration.

Concentration vs. Time As you can see from the graphs below, the relationship between concentration and time is not linear except for the zero order. By performing a few mathematical manipulations, we can transform the 1st and 2nd order equations into a linear expression. 11

What initiates reactions? In order for two particles to react chemically, they must collide. It must be an “effective collision.” sufficient amount of energy proper orientation in space

What hinders reactions? All reactions have an activation energy. Activation energy is the minimum energy necessary get the initiate a chemical reaction

What is the activated complex? The activated complex is a transition state that is unstable and has more PE.

Why does the PE increase? Isomerization of isonitrile: CH3NC BTW, why is CH3CN more stable than CH3NC? CH3NC  CH3CN

What factors affect rates? Surface Area/ Contact Area (Opportunity for collisions) Concentration (Increase Frequency) Catalyst (Effective Collisions) Nature of reaction (Increase Frequency) Temperature (Increase Frequency)

How does surface area affect rate? More surface area exposes more reactants to collisions. Gases react quickly. Solids react slowly unless ground into very fine powder. Fireworks take advantage of this concept.

Why does concentration affect rates? At higher concentrations, there are more molecules in a smaller area which increases the frequency of collisions. If there are more collisions, the chance of collisions with proper orientation increases. How do you increase the concentration of a gas? How do you increase the concentration of a solution?

How does a catalyst affect rate? A catalyst helps to orient the reactants for a proper collision. It provides a surface for the reactants to react. A catalyst does not get consumed in the reaction.

How does lowering Ea affect rate?

What does a catalytic converter do? It reduces the harmful emissions (CO, CxHy, NOx) into less harmful emissions (N2, H2O, CO2).

How do type of reactants affect rate? Ions react faster than molecule because of the attraction of opposite ions. Molecules with more bonds take longer to react since more bonds must be broken. Molecules with stronger bonds take more energy to break and therefore, take longer to react

Why does temperature affect rates? At higher temperatures, more molecules have energy sufficient to react. A 10°C increase in temperature usually doubles the reaction rate.

Determining Rate Order Determine the rate orders given the initial concentrations of reactants for the reaction: NO(g) + ½ Cl2(g)  NOCl(g) [NO(g)] [Cl2(g)] Initial Rate (M/s) ___________________________________ 0.250  0.250  1.43 x 10-6 0.250  0.500  2.86 x 10-6 0.500  0.500  11.4 x 10-6 The order of reaction with respect to NO(g) is: The order of reaction with respect to Cl2(g) is: The overall reaction rate = k [NO(g)]m ·[Cl2(g)]n

Why aren’t all reactants the same? Dumplings Analogy – Imagine that in a kitchen that several tasks needed to be carried out to make dumplings. Some tasks take more time…

Using data to determine rate order

Rate Order ZERO order - rate is independent of the concentration of reactant. Concentration doubles, rate remains constant. FIRST order - rate is directly proportional to the concentration of reactant. Concentration doubles, rate doubles. SECOND order - the rate is proportional to the square of the concentration of reactant. Concentration doubles, rate quadruples. 27

Concentration vs. Time As you can see from the graphs below, the relationship between concentration and time is not linear except for the zero order. By performing a few mathematical manipulations, we can transform the 1st and 2nd order equations into a linear expression.

Rate determining step and rate law Molecularity Elementary Step Rate Law Unimolecular A  rate = k[A] Bimolecular A + B  rate = k[A][B] A + A  rate = k[A]2 Termolecular A + A + B  rate = k[A]2[B] A + A + A  rate = k[A]3 A + B + C  rate = k[A][B][C] 29

PRACTICE PROBLEMS What is the reaction order of each reactant on the initial reaction rate?

Reaction Mechanism A chemical equation does not tell us how reactants become products - it is a summary of the overall process.

Rate Determining Step In a multistep reaction, it is the slowest step. It therefore determines the rate of reaction. Think of the dumpling maker!     

Rate Determining Step Step 1 or Step 2?

Intermediates

PRACTICE PROBLEMS Earlier we calculated the rate law to be: Rate = k [A]2 [B]0 or Rate = k [A]2 Experiments show that this reaction occurs in two steps: 2 A  C C + B  R +S slow step fast step Does the reaction mechanism and the rate law validate one another? Explain. What is C?

Half-life of Zero Order Reaction t½ =  [Ao] / 2k (not given on RT)

Zero Order Reaction Linear Plot [A] vs. t Slope = - k y-intercept = [A]0 (not given on RT)

Half-life of First Order Reaction t½ = 0.693 / k (given on RT)

First Order Reaction Linear Plot ln[A] vs. t Slope = - k y-intercept = ln[A]0 (given on RT but slightly different)

Half-life of Second Order Reaction k [Ao] (not given on RT)

Second Order Reaction Linear Plot 1/[A] vs. t Slope = k y-intercept = 1/ [A]0 (given on RT but slightly different)

Graph of rate vs. temperature Arrhenius equation shows the relationship between temperature and reaction rate. k = the rate coefficient A = a constant (frequency factor) Ea = the activation energy R = the gas constant T = the temperature (K) (not given on RT) 42

Arrhenius equation on the AP exam By taking the natural log of the Arrhenius equation we arrive at: 43

Possible AP problem ln(k1/k2)=-Ea/R(1/T1-1/T2) Find the activation energy (in kJ/mol) of the reaction if the rate constant at 600K is 3.4 M-1s-1 and 31.0 at 750K. ln(k1/k2)=-Ea/R(1/T1-1/T2) 44

Pathway for the Kreb Cycle