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McMurry and Fay Chemistry Chapter 12 A summary of Chapter 12 of the McMurry and Fay textbook required for the course. Great for studying for tests!

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C.Scott Browning

Chapter 12: Chemical Kinematics - Three fundamental questions related to chemical reactions: o What happens? – balanced chemical equation. o To what extent does it happen? – chemical equilibrium. o How fast does it happen? – chemical kinetics. - How does concentration and temperature affect rates of reactions? - How is a mechanism proposed using rate data? 12.1 – Reaction Rates - Reaction rate is determined by Δ[reactant/product]/Δ time. - Δ [] with time can be determined by the Δ pressure as gas molecules are produced. - Reaction rate is either an increase in [product] or a decrease in [reactant] measure in mol/Ls. o Rate of formation = Δ*Product+/Δt. o Molar coefficients show the rate in respect to that of another reactant or product. o Rate is always positive, therefore the rate of decomposition = -Δ*Reactant+/Δt. - The general rate of reaction is the rate of one reactant or product, divided by the coefficients of the other reactants/products. o Eg) For 2N2O 5(g)4NO 2(g) O 2(g),O 2/Δt = -(1/2)(Δ*N 2 5/Δt) = (1/4)(Δ*NO ]2Δt+ - Rate changes as reaction proceeds. - Depend on [reactions] and decreases as reactants are used up. - Instantaneous rate is over a specific time t rather than an interval. 12.2 – Rate Laws and Reaction Order - Rate law represents the dependence of rate on []. m n - If the reaction is aA+bB  products, then Rate = -Δ*A+/Δt = - Δ*B+/Δt = k[A] [B] (because [] affects consumption). o k = rate constant. o m and n show how sensitive the rate is the Δ*reactants+.  in this case, m and n are unrelated to the coefficients. o If m or n is one, the rate depends linearly on [reactant].  Eg) if m=1 and [A] doubles, then the rate doubles as well.  Eg) if m=2 and [A] doubles, then the [A] is quadrupled and the rate increases by a factor of 4. o Rate is proportional to [A] or [B] . - m and n give the respective reaction orders. - m+n gives the overall reaction order. - m+n must be determined experimentally. 12.3 – Experimental Determination of Rate Law - Method of initial rates: used to determine rate law exponents through a series of experiments. - The rate law exponents can be determined by comparing the changes in [] for each reactant and the rates of their reactions. - As products build up, the rates for the reverse reaction increase. o When the forward and reverse reactions are comparable, then rate depends on the [reactants and products]. o Initial rates works because there are no products at the beginning of a reaction. o Therefore rate law only includes reactants. - k depends on temperature but not the [reactants]. o Can be determined by substituting experimental values into rate law equation. o Units of k depend on the [] terms and their rate law exponents. 12.4 – Integrated Rate Law for a First Order Reaction - Integrated rate laws shows how [reactants and products] vary over time. - First-order reaction is one whose rate depends on the [one reactant] raised to an exponent 1. o i.e., Rate = -Δ[A]/Δt = k[A] - integrated rate law of a first-order reaction: ln[A] /tA] =0-kt o [A]t/[A]0is the fraction of A that remains at a given time. o Can be written as: ln[A] =t-kt + ln[A] ,0therefore k = -slope - The plot of [A] vs time gives an exponential decay; ln[A] vs time gives linear decay for first order. 12.5 – Half-Life of a First Order Reaction - Half-life (1/2 is the time required for half the [initial] to remain. - When t= t ,1/2] /[t] = 0. Therefore t 1/2for a first-order reaction is = 0.693/k. o Half-life of first-order reaction is constant because it only depends on k.  i.e., each half-life is an equal amount of time from each other.  [] decreases by a factor of 2 each time. 12.6 – Second-Order Reactions - Rate can depend on one reactant with an exponent of 2 or two reactants with exponent of 1. - Integrated rate law: (1/[A] )t= kt + (1/[A] 0. - The plot of [A] vs time gives an exponential decay; 1/[A] vs time gives linear growth for second order. - k = slope
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