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This chapter explores the essential principles of chemical kinetics, focusing on the speed and rate of reactions. Key factors influencing these rates include concentration, temperature, and catalysts. Higher concentrations generally speed up reactions, while increased temperatures elevate rates, as seen in food spoilage at warmer temperatures. Catalysts, like enzymes, enhance reaction rates without being consumed. The chapter also examines reaction orders, emphasizing how they relate to concentration and how to determine rate laws experimentally to understand the kinetics of reactions.
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Chapter 11 Chemical Kinetics
Chemical Kinetics • Speed and rate at which reactions occur
Concentration Greater concentration = faster reaction (most of the time)
Temperature • Increased temperature = increased rate of reaction rate • Food spoils in warm temperatures more than in the refrigerator
Catalyst • Increases the rate of a reaction without being consumed in the reaction • Enzymes are catalysts in many biochemical reactions
Surface Area • Greater surface area = faster reaction
Reaction Rate • Speed of a chemical reaction • Measures how quickly a substance is produced or consumed For A →B • Average rate = Δ(moles of B) Δ Time Measures the rate of appearance of B
Reactions are: • Dependent on concentration • Reaction rates are usually expressed as M/s
As a reaction proceeds: • Note the concentration vs time • Slope of the tangent gives you the instantaneous rate at a given time
For the reaction A → B • The rate of disappearance of A is equal to the rate of appearance of B • But…if it’s not a 1:1 ratio • 2HI (g)→ H2(g) + I2 (g) 2 moles of HI disappear for each 1 mole of H2 and I2 formed So, to equate the rates, you must divide the rate of disappearance of HI by 2 (its coefficient) Rate = -½ Δ[HI] = Δ[H2] = Δ[I2] Δt Δt Δt
Concentration • Rate of reaction depends on concentration of reactants • As concentration of reactants decreases, the rate of reaction slows • As you increase concentration, reaction rate increases
Rate Laws • Rate laws show how rate is dependent on concentration of reactants
For Most Reactions • Rate Laws are: m n Rate = k [reactant 1] [reactant 2] Exponents ‘m’ and ‘n’ are reaction orders and their sum is the overall reaction order
Zero Order Reaction • For a zero order reaction, changing the initial concentration has no effect on the reaction rate (as long as there is some of the reactant present)
First Order Reaction • For a first order reaction, changing the concentration has a proportional change in rate • In other words, doubling the concentration would double the rate
Second Order Reaction • In a second order reaction, doubling the concentration of the reactant increases the rate by a factor of 4 (2 squared) • Tripling the concentration increases the rate by a factor of 9 (3 squared), etc.
NH4(aq) + NO2(aq)¯→ N2(g) +H 2O (l) Exp. Initial Initial observed initial rate # [NH4+] [NO2¯] M/s 1 0.0100 0.2000 5.4 x 10 -7 2 0.0200 0.2000 10.8 x 10 -7 3 0.0400 0.2000 21.5 x 10 -7 4 0.0600 0.2000 32.3 x 10 -7 5 0.2000 0.0202 10.8 x 10 -7 6 0.2000 0.0404 21.6 x 10 -7 7 0.2000 0.0606 32.4 x 10 -7
Rate Law NH4(aq) + NO2(aq)¯→ N2(g) +H 2O (l) The rate law for this reaction would be: Rate = k [NH4+] [NO2¯] The rate law shows how reaction rate is dependent on the concentration of the reactants.
However: • Reaction orders don’t necessarily correspond with the coefficients in an equation….they must be determined experimentally!!!
