Reaction Energy and Reaction Kinetics
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Transcript of Reaction Energy and Reaction Kinetics
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Reaction Energy and Reaction
Kinetics
Reaction Rate
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Objectives1. Define the rate of reaction and describe how it can
be determined.2. List and explain factors that affect the rate of
reaction.3. Define a catalyst and describe how it can affect the
rate of reaction.
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Reaction Rate
Reaction rate – change in concentration of reactants per unit time as a reaction occurs.
Chemical kinetics – study of reaction rates and reaction mechanisms.
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Rate-Influencing FactorsNature of the Reactants
H2(g) + Cl2(g) 2 HCl(g) (fast)3 H2(g) + N2(g) 2 NH3(g) (slow)
4 Na(s) + O2(g) 2 Na2O(s) (fast)4 Fe(s) + 3 O2(g) 2 Fe2O3(s) (slow)
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Rate-Influencing Factors
Surface Area
• Heterogeneous reactions – reactions with reactants in two different phases
• Heterogeneous reactions depend on contact between the two phases.
• Increases in surface area result in the increase in the rate of heterogeneous reactions.
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Rate-Influencing FactorsTemperature
An increase in temperature causes:• An increase in collision energy• An increase in collision frequency
Both result in increased reaction rates
• In general, a 10oC increase in temperature causes the reaction rate to double.
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Rate-Influencing factorsConcentration
• Increasing the concentration of the reactants may cause an increase in the rate of reaction for homogeneous reactions.
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Rate-Influencing Factors
Presence of a Catalyst
• Catalysts lower the activation energy for a reaction and, thus, increase the reaction rate.
• Homogeneous vs. Heterogeneous catalysts – same or different phase as reactants.
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Rate Laws for ReactionsObjectives:1. Explain the rate law for a chemical reaction.2. Determine the rate law for a particular reaction given
appropriate kinetics data.3. Discuss the relationship between the rate law and the
reaction mechanism.
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Rate LawRate Law – an equation that relates reaction rate and
concentration of reactants
R = k[A]n[B]m…
R = reaction ratek = rate constant that depends on temperaturen,m = must be determined experimentally
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Rate Laws for ReactionsExample:
2 H2(g) + 2 NO(g) N2(g) +2 H2O(g)
• Doubling the amount of H2 , while keeping the amount of NO constant, causes the reaction rate to double. So…
R [H2]• When the amount of NO is doubled, while keeping the
amount of H2 constant, causes the reaction rate to increase fourfold. So…
R [NO]2
Overall rate law: R = k[H2][NO]2
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Rate Laws for Reactions
For reactions that occur in a single step:
A + B 2C
R = k[A][B]
For the reverse reaction…. 2C A + B
R = k[C]2
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Rate Laws for ReactionsFor multi-step reactions, the rate law depends on the
rate limiting step.
Example:NO2(g) + CO(g) NO(g) + CO2(g)
Step 1: NO2 + NO2 NO3 + NO (slow)Step 2: NO3 + CO NO2 + CO2 (fast)
Rate Law: R = k[NO2]2
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Sample Problems1. The rate law of a reaction is found to be R=k[X]3. By
what factor does the rate increase if the concentration of X is tripled?
The rate will increase by a factor of 27.2. The rate of reaction involving two reactants, X and Z, is
found to double when the concentration of X is doubled and to quadruple when the concentration of Z is doubled. Write the rate law for this reaction.
R = k[X][Z]2
3. The rate law for a single-step reaction that forms one product, C, is R = k[A][B]2. Write the balanced reaction of A and B to form C.
A + 2B C
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Sample Problem• A particular reaction is found to have the following
rate law: R = k[A][B]2
How is the rate affected by each of the following changes?
a. The initial concentration of A is cut in half.b. The initial concentration of B is tripled.c. The concentration of A is doubled, but the
concentration of B is cut in half.d. A catalyst is added.
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Sample ProblemA chemical reaction is expressed by the balanced chemical
equation, A + 2B C. Using the data below, answer the following:
a. Determine the rate law for the reactionb. Calculate the value of the specific rate constant.c. If the initial concentrations of both A and B are 0.30 M, at
what initial rate is C formed?
Experiment # Initial [A] Initial [B] Initial rate of formation of C
1 0.20 M 0.20 M 2.0x10-4M/min2 0.20 M 0.40 M 8.0x10-4M/min3 O.40 M 0.40 M 1.6x10-3M/min