Rate Of Reaction

Question 1

Rate of reaction def

Answer 1

Change in concentration over time

Question 2

Order def

Answer 2

The power to which a concentration is raised

Question 3

Overall order def

Answer 3

The sum of powers in the rate equation

Question 4

Rate constant def

Answer 4

Proportionality constant

Question 5

Half-life def

Answer 5

Time taken for concentration of substance to reduce by half

Question 6

Rate-determining step def

Answer 6

Slowest step in a reaction mechanism

Question 7

How is rate measured

Answer 7

Amount of product produced / time:

• Moles per decimeters(cubed) / seconds(time)
• mol dm3 s-1

Question 8

What is the rate equation?

Answer 8

Rate = k[A]^a(x)[B]^b

• rate = mol dm-3 s-1
• a = order of reaction with respect to a
• b = order of reaction with respect to b
• k = rate constant
• A and B = concentrations in mol dm-3

Question 9

Zero order meaning

Answer 9

The concentration of that reagent does not affect the rate

Question 10

First order meaning

Answer 10

The concentration of that reagent is proportional to the rate

Question 11

Second order meaning

Answer 11

The rate of that reaction is proportional to the square of the concentration of A

Question 12

What change in the reaction conditions would cause the rate constant to change?

Answer 12

Change in temperature

• Increase the temperature = increase the rate of reaction
• Decrease the temperature : decrease rate of reaction

Question 13

Equation to work out half life

Answer 13

Kt1/2 = ln(2)

Question 14

How to work out k in rate of reaction from half life

Answer 14

K = ln2/half life

Half life - t1/2

Question 15

Reaction Mechanism def

Answer 15

It is the order in which bonds are made and broken for a reaction to happen
This results in a series of steps

Question 16

What is an intermediate in a reaction mechanism?

Answer 16

It is a substance that is made in the rate-determining step(slowest step) and is then used up in the fast reaction

Question 17

What does the rate equation only include?

Answer 17

Only includes reacting species in the rate-determining step - the slowest step

Question 18

Explain why it is unlikely that a reaction will occur in one step

Answer 18

• Unlikely that all the reactants will form exactly all the products in one reaction
• Number of particles in the rate-determining step don’t match those in the reaction

Question 19

What is the Arrhenius equation?

How do you rearrange this to find Ea on a graph

Answer 19

K = Ae^(-Ea/RT)

K: rate constant 
A: frequency factor or pre-exponential factor
e: mathematical quantity, e
Ea: Activation energy
R: the gas constant (8.31 J K-1)
T: Temperature (Kelvin)

Rearrangement:

lnK = -(Ea/R) x ((1/T) + lnA

Y = Mx + c

Question 20

How do you find the initial rate of reaction using a graph of data?

Answer 20

• Draw a tangent at t = 0
• Calculate the gradient of the tangent
• The gradient of the tangent is the initial rate of reaction

Question 21

Explain the purpose of the sodium thiosulfate solution in reaction between sodium thiosulfate and iodine

Answer 21

• Reacts rapidly with iodine as it is formed and reduces it
• Prevents iodine starch solution/blue black colour forming until a fixed amount of iodine has reacted
• Prevents iodine starch blue/black colour forming until all the thiosulfate has reacted

Question 22

Outline a method, not involving sampling the mixture, which would be suitable for measuring the iodine concentrations in this experiment between sodium thiosulfate and potassium iodide.
(3 Marks)

Answer 22

1. Use a colorimeter to measure absorbance/wavelength of light in iodine solution
2. Measure absorbance of light/colour at set intervals/ periods, e.g. every 10 seconds
3. Use a calibration curve to convert measured absorbance into concentration

Question 23

What affects the value of Kc, the equilibrium constant?

Answer 23

Only temperature affects the equilibrium constant, Kc

Question 24

What affects the value of the rate constant, k?

Answer 24

Rate of reaction affects the rate constant, k

• increase rate of reaction increases the value of rate constant
• decreasing rate of reaction decreases value of the rate constant

Question 25

Outline a titremetric method that could be used to measure the change in concentration over time of a compound A in the reaction:
A + B (+ catalyst) —> C + D
(3 Marks)

Answer 25

1. Take samples of the reaction mixture at various times
2. Remove the catalyst by quenching with the ice
3. Titrate the mixture with a standard solution of acid/ of known concentration.

Question 26

In a reaction mixture, how could you show that equilibrium had been established?
Information on exo/endothermic is not given.
(2 Marks)

Answer 26

Use a colorimeter to absorb wavelengths of light of mixture

Colour/wavelengths of light absorbed will not change.

Question 27

in an experiment to determine rate, explain why an excess of a substance is used in a reaction.
(1 Mark)

Answer 27

• to keep the concentration of the substance (effectively) constant
• to keep the order of reaction for the substance as zero
• to endure equilibrium is far to the right

Question 28

A student investigates the decomposition of H2O2(aq) by measuring the volume of oxygen gas produced over time. All gas volumes are measured at room temperature and pressure.
The student uses 25.0 cm3 of 2.30 mol dm−3 H2O2.
From the results, the student determines the concentration of H2O2(aq) at each time. The student then plots a concentration–time graph.
Suggest a different experimental method that would allow the rate of this reaction to be followed over time.
(1 Mark)

Answer 28

Measure mass loss of the substance