Chapter 18 - Rates of reactions

Question 1

What is the equation for rate?

Answer 1

Δconcentration
Rate = ————————-
time

Question 2

For a reaction A, [A] is shorthand for what?

Answer 2

[A] = concentration of A

Question 3

What is the unit for rate?

Answer 3

-3 -1

moldm s

Question 4

Show the relationship between order n and rate.

Answer 4

n

rate ∝ [A]

Question 5

What is the overall order?

Answer 5

The sum of the individual orders of the reactants.

Question 6

Give the rate equation for reaction A + B → C with orders m for A and n for B.

Answer 6

rate = k [A]^m[B]^n

Question 7

What is half-life?

Answer 7

The time taken for any specified property (e.g. concentration) to halve.

Question 8

What is shown by the gradient of a concentration-time graph?

Answer 8

Rate

Question 9

How can you calculate the rate constant from rate?

Answer 9

rate
k = ——–
[A]

for reaction A

Question 10

How can you calculate the rate constant from half-life?

Answer 10

ln 2
k = ————
half-life

Question 11

What is the rate-determining step?

Answer 11

The slowest step in the reaction mechanism of a multi-step reaction.

Question 12

The rate equation contains the reactants in the ___________ ______.

Explain why.

Answer 12

The rate equation contains the reactants in the rate-determining step.

The rate determining step is so much slower than the other steps that when you measure the rate of reaction you are measuring the rate of the rate-determining step.

Question 13

What is the effect of increasing temperature on the rate constant?

Answer 13

The rate constant is proportional to the rate of reaction.

An increasing temperature increases the rate, increasing temperature therefore increases the rate constant.

Question 14

What is the Arrhenius equation?

Answer 14

-Ea / RT

k = Ae

Question 15

What is the Arrhenius equation in logarithmic/graphical form?

Answer 15

Ea 1
ln k = - —— —- + ln A
R T

Question 16

Give 4 examples of rate of reaction experiments.

Answer 16

• Gas evolving experiments
• Mass change
• Colorimetry (colour change)
• pH change

Question 17

What is a zero order reaction?

Answer 17

rate ∝ [A]^0

Rate is not affected by concentration of reactant A. Constant rate.

Question 18

What is a first order reaction?

Answer 18

rate ∝ [A]^1

Rate is directly proprtional to the concentration of A.

e. g. if [A] is doubled, rate is doubled
e. g. if [A] is halved, rate is halved

Question 19

What is a second order reaction?

Answer 19

rate ∝ [A]^2

Rate is proportional to the square of the concentration of A.

e. g. if [A] is doubled, rate is quadrupled
e. g. if [A] is halved, rate is quartered

Question 20

Concentration-time graphs can be plotted from the results of _______ _________ experiments.

Answer 20

Concentration-time graphs can be plotted from the results of continuous monitoring experiments.

Question 21

Rate-concentration graphs can be plotted from the results of _____ _____ experiments.

Answer 21

Rate-concentration graphs can be plotted from the results of initial rates experiments.

Question 22

What is an intermediate?

What is an overall equation?

Answer 22

A species which is formed in one step but used up in another, it is not seen as a reactant or product in the overall equation.

The overall equation is the sum of the species involved in each step.

Question 23

What is the unit of Ea in the Arrhenius equation?

Answer 23

Jmol-1

Question 24

Rate-concentration graphs can be plotted from the results of…

Answer 24

…initial rates experiments.

Question 25

What is a reaction mechanism?

Answer 25

The series of steps that make up a reaction.