5.1.1: How fast?

Question 1

Define rate of reaction

Answer 1

Change in concentration of a reactant or

a product per unit time

Question 2

At a given instant, how could you calculate the rate of reaction?

Answer 2

Rate of reaction = change in concentration of reactants or products / time

Question 3

If the order is zero with respect to a reactant, what does that mean?

Answer 3

Changing the concentration of the reactant has no effect on the rate

Question 4

What is the effect on rate in a first order reaction?

Answer 4

Rate is directly proportional to the concentration

Question 5

What is the effect on rate in a second order reaction?

Answer 5

Change in rate = change in concentration squared

Question 6

Write a generic rate equation and state what each term means

Answer 6

Rate = k [X]x [Y]y;
k = rate constant for the reaction
[X] and [Y] are concentrations of species X and Y respectively
x and y are the orders of reaction with respect to X and Y

Question 7

Do zero order reactants appear on rate equation? Why?

Answer 7

No, because they don’t have an effect on

the rate

Question 8

How is overall order of a reaction calculated?

Answer 8

Sum of individual orders

Question 9

How would you calculate the units of the rate constant?

Answer 9

Units of rate are mol dm-3s-1 and units of concentration are mol dm-3
Rearrange rate equation to get k=
Sub in units and cancel them out

Question 10

When the overall order is 3, what would be the units for rate constant?

Answer 10

dm6 mol-2 s-1

Question 11

How could you measure the rate of reaction experimentally (different methods)?

Answer 11

Use a colorimeter at suitable intervals if there is a
colour change.
If gas is evolved, use a gas syringe to collect
volume of gas evolved, or measure the change in
mass of the reaction mixture.

Question 12

What does half life mean?

Answer 12

The time taken for concentration of a reactant to decrease by half

Question 13

What is the symbol for half life?

Answer 13

t1/2

Question 14

What is the relationship between first order reactions and half life?

Answer 14

First order reactants have constant half lives

Question 15

What is the equation that is used to determine rate constant using half life in a first order reaction?

Answer 15

k = ln2 / t1/2

Question 16

How would you draw a rate concentration graph?

Answer 16

Plot [A] against time, draw tangents at different values → draw a secondary graph of rate against [A]

Question 17

What is the relationship between rate and time?

Answer 17

Rate ∝ 1/t

Question 18

How to determine the rate constant from a rate concentration graph of first order?

Answer 18

k = rate / concentration

Question 19

What is rate determining step?

Answer 19

The slowest step in a reaction with multiple steps

Question 20

How does the rate determining step link to the species involved in the rate equation?

Answer 20

Any species involved in the rate determining step
appear in the rate equation. Species only involved
after the rate determining step do not appear in the
rate equation

Question 21

For a reactant in the rate equation, what indicates how many molecules of that reactant are involved in the rate determining step?

Answer 21

The order of the reactant

Question 22

The rate equation of a reaction is rate = k[NO]2

How many molecules of NO will be present in the rate determining equation?

Answer 22

2

Question 23

What affects the value of the rate constant for a given reaction?

Answer 23

Temperature, nothing else

Question 24

What is the effect of a 10℃ temperature increase on the rate of reaction, roughly?

Answer 24

Doubles rate of reaction

Question 25

What is the Arrhenius equation? What does each term mean?

Answer 25

k = Ae(to the power of(−E/RT))
k= rate constant for reaction
A= pre exponential factor (number of collisions between reactant molecules)
e= mathematical quantity
R= gas constant
T= temperature in kelvin
E= activation energy for reaction in joules

Question 26

How can you convert the Arrhenius equation into a useful form for plotting a graph?

Answer 26

ln k= -Ea/RT + ln A
Graph of lnk against 1/T is a straight line:
gradient = -EA/R and y intercept is lnA