Module 5 - Physical Chemistry and Transition Elements

Question 1

Define rate of reaction.

Answer 1

Rate of reaction - change in conc of a reactant or product in a given time.

Question 2

Define order of reaction.

Answer 2

Order of reaction - power to which conc of a reactant is raised in the rate equation.

Question 3

Define rate constant.

Answer 3

Rate constant - constant that links the rate of reaction with the concentrations of reactants raised to powers of their orders in the rate equation.

Question 4

Define overall order.

Answer 4

Overall order - sum of orders with respect to each reactant.

Question 5

Define half-life.

Answer 5

Half-life - time taken for concentration of a reactant to decrease by half.

Question 6

Define rate-determining step.

Answer 6

Rate-determining step - slowest step in the reaction mechanism of a multi-step reaction.

Question 7

Define initial rate.

Answer 7

Initial rate - instantaneous rate at the start of the reaction, when t=0.

Question 8

What is the special feature of first order reactions?

Answer 8

Constant half-life.

Question 9

What happens to half-life of zero order reaction when reactant conc is decreased?

Answer 9

Half-life increases.

Question 10

What happens to half-life of first order reaction when reactant conc is decreased?

Answer 10

Half-life remains constant.

Question 11

What happens to half-life of second order reaction when reactant conc is decreased?

Answer 11

Half-life increases.

Question 12

How do you calculate the rate constant using the half life of a first order reaction?

Answer 12

k = 0.693 / half-life.

Question 13

How do you calculate k from a zero order conc-time graph?

Answer 13

Zero order - k = -slope of conc-time graph.

Question 14

How do you calculate k from a zero order rate-conc graph?

Answer 14

Zero order - k = rate at any concentration.

Question 15

How do you calculate k from a first order conc-time graph?

Answer 15

First order - k = slope of ln(conc) vs time.

Question 16

How do you calculate k from a first order rate-conc graph?

Answer 16

First order - k = slope of rate vs conc graph.

Question 17

How do you calculate k from a second order conc-time graph?

Answer 17

Second order - k = slope of 1/concentration vs time.

Question 18

How do you calculate k from a second order rate-conc graph?

Answer 18

Second order - k = slope of rate vs concentration.

Question 19

Outline method for using colorimeter to monitor concentration change.

Answer 19

Make known conc solutions of X. Measure absorbance of each using colorimeter. Plot absorbance against conc to make calibration curve. Measure absorbance due to X during experiment. Use calibration curve to convert absorbance to concentration.

Question 20

When can you use ‘loss of mass method’ for monitoring concentration?

Answer 20

If a relatively heavy gas is formed, eg CO2, and not hydrogen because the mass change would be too small to measure.

Question 21

When shouldn’t you use a gas syringe?

Answer 21

If a lot of gas is evolved.

Question 22

When shouldn’t you use ‘mass loss method’?

Answer 22

If the gas is toxic.

Question 23

When shouldn’t you use the inverted cylinder method?

Answer 23

If gas reacts/is soluble in water.

Question 24

How do you monitor the progress of a reaction when brown iodine is produced?

Answer 24

Use a colorimeter to measure the increase in absorbance of light.

Question 25

How do you monitor the progress of a reaction when a gas is produced?

Answer 25

Use a gas syringe or a measuring cylinder over water to measure the volume of gas produced.

Question 26

What is the continuous monitoring method?

Answer 26

Measuring concentration of reactants over time to plot conc-time graph from which we can calculate initial rate/any rate.

Question 27

Describe the clock method.

Answer 27

Measure time taken for an obvious change from start of reaction. Average rate = initial rate. Initial rate is proportional to 1/t. Repeat clock reaction several times with different conc. Plot 1/t against concentration.

Question 28

How do you improve accuracy of clock method?

Answer 28

Measure average rate during the first part of reaction; the shorter the time period, the less the rate changes.

Question 29

How does the rate equation relate to the rate-determining step?

Answer 29

Rate equation tells us which species react in the rate-determining step and in what proportions (orders = moles).

Question 30

What is the most important rule when suggesting mechanisms?

Answer 30

Never have 3 or more particles colliding in one step.

Question 31

Define oxidising agent.

Answer 31

Oxidising agent - reagent which oxidises (takes electrons from) another species.

Question 32

Define reducing agent.

Answer 32

Reducing agent - reagent which reduces (adds electrons to) another species.

Question 33

When shouldn't you use the inverted cylinder method?

Answer 33

If gas reacts/is soluble in water.