Rates -

Question 1

General form of a rate equation

Answer 1

The rate of a chemical reaction is related to the
concentration of reactants by a rate equation of the form:
Rate = k[A]^m [B]^n

where m and n are the orders of reaction with respect to
reactants A and B and k is the rate constant

overall order of reaction is (m+n) e.g. 2nd, 3rd

The orders m and n are restricted to the values 0, 1, and
2

Question 2

rate ∝ [A] [B]
what happens if [A] is doubled
What happens if [A] AND [B] is doubled

Answer 2

rate ∝ [A][]/B] (determined experimentally
If [A] doubled, rate is doubled

If [A] and [B] is doubled, the rate quadruples

Question 3

rate ∝ [A] [B]^2
WHAT HAPPENS IF [A] is doubled
If [B] is doubled
If [A] AND [B] is doubled

Answer 3

If [A] is doubled, rate doubles
If [B] is doubled, rate quadruples
If [A] and [B] double, rate increases by a factor of 8

Question 4

What do the units of the rate constant depend on

Answer 4

They depend on the species and their order of reaction in the rate equation

rate = k[A]
k = rate / [A]

rate is in moldm-3s-1

k = mold-3s-1 / moldm-3 = s-1

sometimes have to square moldm-3 if [A]^2
Someitmes no units

Units of K vary

Question 5

How does temperature affect the rate constant

Answer 5

The rate constant k varies with temperature

Question 6

Which equation shows how the rate constant varies with temperature

Answer 6

The rate constant k varies with temperature as shown by
the equation:

k = Ae^–Ea/RT

where A is a constant, known as the Arrhenius constant,

A - value varies depending on reaction
Units same as k

Ea is the activation energy in Jmol-1

and T is the temperature in K.

R = gas constant = 8.31 Jmol-1K-1

Question 7

Define order of reaction

Question 8

Define rate constant

Question 9

explain the qualitative effect of changes in
temperature on the rate constant k

Question 10

How else can k = Ae^–Ea/RT be rearranged

Answer 10

ln k = –Ea /RT + ln A

Question 11

ln k = –Ea /RT + ln A significance

Answer 11

know how to use this rearranged equation with
experimental data to plot a straight line graph with
slope –Ea/R

Question 12

How is the rate equation determined

Answer 12

The rate equation is an experimentally determined
relationship.

Question 13

The orders with respect to reactants can provide
information about the mechanism of a reaction.

Question 14

Do catalysts appear in the rate equation

Answer 14

Catalysts which do not appear in the chemical equation may appear in the rate equation.

(Since catalysts affect rate)
E.g. P + Q -> R
T is a catalyst
rate = k[P]^2 [T]

Question 15

Do all species in a chemical equation appear in the rate equation

Answer 15

Species that appear in the chemical equation do not necessarily appear in the rate equation

Question 16

rate graphs conc./time

Answer 16

rate = Δ [A] / Δ t

Δ y / Δ x

draw tangents if curve
just draw a big triangle is straight line

Question 17

How to determine order of reaction from a graph

Answer 17

Using a rate/conc. graph

Shows how rate varies with a changing concentration

straight horizontal line = 0 order
straight line through the origin = 1st order
Curve = greater than 1st order e.g. 2nd order

Question 18

Explain how you can determine the rate equation

how can you find the orders of a reaction with respect to reactants and consequently the rate equation

Answer 18

the orders of a reaction with respect to reactants and consequently the rate equation ARE FOUND EXPERIMENTALLY

Initial rate of reaction is the rate right at the start of the reaction
YOU CAN FIND INITIAL RATE OF REACTION FROM A CONC.-TIME GRAPH BY CALCULATING THE GRADIENT OF THE TANGENT AT TIME = 0

you can find the order of reaction by using the initial rate method

The rate at the start of a reaction between reactants of known concentration, is measured

The concentration of the reactants is then changed, one at a time, and effect on rate is measured

A comparison of the initial concentration of initial rates for pairs of experiments allows the order with respect to each reactant to be found

Question 19

How is the overall rate of reaction determined

Answer 19

Mechanisms can have one step or a series of steps.
in a series of steps, each step can have a different rate

the overall rate is determined by the step with the slowest rate - the rate determining step

Question 19

Orders of reaction and RDS

Answer 19

The order of a reaction with respect to a reactant shows the number of molecules of that reactant that are involved in the Rate determining step

So if a reaction is second order with respect to X, there will be two molecules of X in the RDS

IF a reactant appears in the rate equation, it must affect rate
So this reactant, or something derived from it, must be in the rate determining step

If a reactant doesnt appear in the rate equation, then it wont be involved in the rate determining step (and neither will anything be derived from it.)

not in rate equation, it CANNOT BE INVOLVED in the Rate determining step

Question 20

Cl ‘ + O3 -> CLO’ + O2 - slow
CLO ‘ + O ‘ -> CL’ + O2 - fast

determine the rate equation and the overall order of the reaction

Answer 20

rate = k [CL’] [O3]

1st reaction - slow (rate detemining step)
Cl’ and O3 must both be in the rate equation, so the rate equation will be rate = k[Cl’]^m [O3]^n

there is one molecule of CL’ radical and one O3 molecule in the RDS, so the orders, m and n, are both 1

Reaction step may involve breaking strong carbon-bromine bonds, so this will be a slow change

If the OH- conc. is high, the positive ion has a good chance of colliding with one. So this step will be fast