Module 5: Section 1

Question 1

Describe reaction rate

Answer 1

Change in amount of reactants or products per unit time

Question 2

Give 4 ways to follow the rate of reaction

Answer 2

1. Measure the vol. of gas evolved
2. Measure the loss in mass as a gas is evolved.
3. Use colorimetry to measure colour change of a reaction.
4. Measure pH of a reaction.

Question 3

Give 2 ways you can use experimental data to work out the order.

Answer 3

1. Continuously monitor the change in concentration of A against time to construct a rate-conc graph.
2. Use an initial rates method to find out how the initial rate changes as you vary the conc of A

Question 4

Describe the Conc-time graphs for a reaction with:

1. Zero order
2. First order

Answer 4

1. Diagonal line decreasing in conc over time

2. Curved decreasing line

Question 5

Describe the Rate-conc graphs for a reaction with:

1. Zero order
2. First order
3. Second order.

Answer 5

1. Straight horizontal line at top of y-axis (rate)
2. Straight diagonal line increasing evenly
3. Curved increasing line.

Question 6

If the rate constant (K) is large, what does this tell you about the reaction?

Answer 6

The larger the rate constant, the faster the reaction.

Question 7

Define half life.

Answer 7

Time it takes for half of the reactant to be used up.

Question 8

In a first order reaction, how is half life affected by changes in concentration?

Answer 8

The half-life is independent of the concentration, so half-life stays the same.

Question 9

Give the equation linking half-life and the rate constant.

Answer 9

K = ln2 / half life

Question 10

How does changing the temperature change the rate constant?

Answer 10

The rate constant applies to a particular reaction at a certain temperature. At a higher temperature, the reaction will have a higher rate constant.

Question 11

Give the Arrhenius equation and state what all the variables stand for.

Answer 11

K = A e ^ -Ea / RT

K = rate constant
Ea = activation energy
T = temperature (K)
R = gas constant (8.314)
A = pre-exponential factor

Question 12

State Arrhenius equation in logarithmic form.

Answer 12

lnK = - Ea / RT + lnA

Question 13

On a graph where you create an Arrhenius plot, what are the axis labels?
What is the gradient and y-intercept?

Answer 13

Y axis = lnK
X axis = 1/T

Gradient = -Ea / RT
Y-intercept = lnA

Question 14

Give 2 features of a dynamic equilibrium.

Answer 14

1. Rate of the forward reaction is the same as the rate of the reverse reaction.
2. There is no overall change in the concentrations of the reactants and products.

Question 15

What condition are necessary for a dynamic equilibrium to be set up?

Answer 15

Dynamic equilibrium can only happen in a closed system at a constant pressure.

Question 16

Define total pressure of a gas mixture.

Answer 16

Total pressure of a gas mixture is the sum of all the partial pressures of the individual gases.

Question 17

How do you calculate partial pressures?

Answer 17

1. Calculate mole fraction;
   no of mols of gas X / total no of mols of gas
2. Calculate partial pressure;
   mole fraction x total pressure of the mixture.

Question 18

The equilibrium constant Kp is always measured in what?

Answer 18

Partial pressures.

Kc is always measured in concentrations.

Question 19

When writing Kp, what dont you include?

Answer 19

You do not include any solids or liquids for a heterogeneous equilibrium.

Question 20

State La Chatelier’s principle.

Answer 20

If a system in dynamic equilibrium is subject to an external change in conditions, it will move to minimise the change.

Question 21

What does a large Kc value tell you?

Answer 21

The larger the value of Kc, the further to the right the equilibrium lies and the more products there will be, relative to reactants.

Question 22

What does a small Kc value tell you?

Answer 22

The smaller the value of Kc, the further to the left the equilibrium lies and the more reactants there will be relative to products.

Question 23

Only one condition causes the equilibrium constant to change, what is it?

Answer 23

Temperature.

Changing the temperature alters the position of equilibrium and the value of the equilibrium constant.

Question 24

What affect do catalysts have on the position of equilibrium?

Answer 24

Catalysts have no affect on the position of equilibrium, they only cause equilibrium to be approached faster.

Question 25

Define a Bronsted-Lowry acid.

Answer 25

They are proton donors.

Question 26

Define a Bronsted-Lowry base.

Answer 26

They are proton acceptors.

Question 27

Describe a monobasic acid, and give an example.

Answer 27

Acids that can only donate one proton.

HCl, HNO3

Question 28

Describe a dibasic acid, and give an example.

Answer 28

Acids that can donate 2 protons.

H2SO4

Question 29

Describe a tribasic acid, and give an example.

Answer 29

Acids that can donate 3 protons.

H3PO4

Question 30

What is formed when you react Metals and Acids.

Answer 30

Metal + Acid —> Salt + Hydrogen

Question 31

What is formed when you react Carbonates and Acids.

Answer 31

Carbonate + Acid —> Carbon dioxide + Water

Question 32

How do you calculate pH of strong acids?

Answer 32

Strong acids fully ionise, so the [H+] is the same as the acid concentration.

Question 33

Give the Kw expression.

Answer 33

H2O(l) —> OH-(aq) + H+(aq)

Question 34

How does changing the concentration of [H+] or [OH-] affect the value of Kw and why

Answer 34

It has no effect on the value of Kw, as the equilibrium will shift, changing the concentrations of the other substances to keep the value of Kw the same.

Question 35

How does changing the temperature of the solution affect the value of Kw and why.

Answer 35

Value of Kw changes as dissociation of water is an endothermic process;
For Example, if it is warmed, the equilibrium moves to the right and Kw increases.

Question 36

How do you calculate pH of strong bases?

Answer 36

Use the Kw expression.
Strong bases fully ionise in water and donate one mole of OH- ions per mole of base. This means [OH-] is the same as the concentration of the base.

Question 37

How do you calculate the pH of weak acids?

Answer 37

Set up a Ka expression;
Ka = [H+] [A-] / [HA]

You can assume that all the H+ ions in the solution come from the acid, so;
Ka = [H+]^2 / [HA]

Question 38

How do you calculate pKa from Ka

Answer 38

pKa = -log10 Ka

Question 39

How do you calculate Ka from pKa

Answer 39

Ka = 10^-pKa

Question 40

Define a buffer.

Answer 40

A buffer is a solution that minimises changes in pH when small amounts of acid or base are added.

Question 41

How does a buffer minimise pH changes if a small amount of acid is added.

Answer 41

If you add a small amount of acid, the [H+] increases. H+ react with CH3COO- ions to form CH3COOH. Equilibrium shifts to the left, reducing the [H+] to its original value, minimising changes in pH.

Question 42

How does a buffer minimise pH changes if a small amount of alkali is added?

Answer 42

If a small amount of alkali is added, the [OH-] increases. It reacts with H+ to form water, removing the H+ ions.
OH- + H+ —> H2O
More CH3COOH dissociates to form H+ ions and so equilibrium shifts to the right. [H+] increases until its close to its original value, minimising changes in pH.

Question 43

How can you measure pH of a solution?

Answer 43

Use a pH meter.

Question 44

What does a small pKa value tell you?

Answer 44

small pKa = strong acid