Wize University Chemistry Textbook > Kinetics

Rate of Reaction

Reaction Rates

The reaction rate is a measure of the change in concentration of a reactant or product over time!
eg, in physics we know velocity = Δd/t, in chemistry, a reaction rate = Δ[ ]/t
The units of rate would be M/s where M = mol/L
A general form of a reaction rate is shown below:

$aA+bB \to cC+dD$

$rate=-\frac{1}{a}\frac{d[A]}{dt}=-\frac{1}{b}\frac{d[B]}{dt}=\frac{1}{c}\frac{d[C]}{dt}=\frac{1}{d}\frac{d[D]}{dt}$

As the reaction proceeds are we getting more/less of the reactant? ________ More/less of the product? ________
The - sign indicates that the concentration of reactants are decreasing (losing reactants as the reaction proceeds)
The + sign indicates that the concentration of products is increasing (gaining product as the reaction proceeds )

**Rates must be determined through experimentation.

Measuring Reaction Rates

Consider the reaction: $\small A\rightarrow B$
The reaction rate can be measured using any substance in the reaction. Reaction rate units are always M/s (M stands for concentration, mol/L)

$Rate=-\cfrac{\Delta\left[A\right]}{\Delta t}=\cfrac{\Delta B}{\Delta t}$

Instantaneous rate: the rate of reaction at an instance in time (the slope of the tangent line to a point in a concentration vs time graph)

Image illustrating Measuring Reaction Rates

Initial rate – instantaneous rate of a chemical reaction at t = 0, immediately after the reaction has begun

Average rate: the rate of reaction over a period of time (the slope of the line connecting 2 points in a concentration vs time graph)

Practice: Rates of Reaction

If N2 is consumed at 2 M/s, what is the rate of production of NH3 and rate of consumption of H2 in the reaction?

$N_2\ \ +\ 3H_2\ \rightarrow\ 2\ NH_3$

Rate of production of NH3 (ans in M/s)Rate of consumption of H2 (ans in M/s)

I don't know

In the following reaction the average rate of formation of NO(g) is 1.12 mol/Ls. If 2 minutes passes, what is the concentration of O2(g) that is consumed?

4 NH3(g) + 5O2(g) → 4 NO(g) + 6 H2O(g)

1.4 M/s

1.68 × 102 M

2.7 × 102 M

2.8 M/s

I don't know

Extra Practice

Elements: Radioactive Decay in Smoke Detectors

Americium-241 is a synthetic isotope made in nuclear reactors around the world. Chances are you have been in close proximity to this radioactive isotope thousands of time in your life because it is used in household smoke detectors. $^{241}\rm Am$ is an alpha emitter whose decay obeys a first order rate-law (as do all nuclear decays) with a half-life of 432.6 years. What is the rate constant of its decay? How much $^{241}\rm Am$ would remain of a 25mg sample if it were left for 300 years.

Given the following data for the $NH_4^++NO_2^-→N_2+2H_2O$ reaction, the rate law for the reaction is

$\def\arraystretch{1.5} \begin{array}{c} Trial &[NH_4^+] &[NO_2^–]& Rate \\ \hline 1 & 0.010 M & 0.030 M &0.020 M/s \\ 2& 0.015& 0.030 &0.030 \\ 3&0.010 & 0.015 &0.005 \\ \end{array}$

Rate Laws: Relative Rates of Reactions

Consider the balanced chemical equation for the reaction

$2CO(g)+O_2(g)⟶2CO_2(g)\qquad Rate=k[CO]^2$

If the rate of the reaction is 3.2 M/s which of the following is TRUE.

For the following reaction:

$A_{(g)}+2B_{(g)} \to C_{(g)}$

When [B] is decreasing at 0.5 M/s, how fast is [A] decreasing?

The Rate Law

Express the rate of reaction in terms of the change in concentration of each reactant and product:

$A_{(g)}+2B_{(g)} \to C_{(g)}$

Given the following data for the $NH_4^++NO_2^-→N_2+2H_2O$ reaction, the rate law for the reaction is

$\def\arraystretch{1.5} \begin{array}{c} Trial &[NH_4^+] &[NO_2^–]& Rate \\ \hline 1 & 0.010 M & 0.030 M &0.020 M/s \\ 2& 0.015& 0.030 &0.030 \\ 3&0.010 & 0.015 &0.005 \\ \end{array}$

Rate Laws: Relative Rates of Reactions

Consider the balanced chemical equation for the reaction

$2CO(g)+O_2(g)⟶2CO_2(g)\qquad Rate=k[CO]^2$

If the rate of the reaction is 3.2 M/s which of the following is TRUE.

Rate of Reaction

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Reaction Rates

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Measuring Reaction Rates

Practice: Rates of Reaction

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Extra Practice