Module 3.2 - Physical Chemistry

Question 1

What is enthalpy change?

Answer 1

Enthalpy change is the heat energy transferred in a reaction at constant pressure. The units of enthalpy change are kj/mol-1.

Question 2

Describe the temperature/energy and enthalpy change for exothermic reactions?

Answer 2

Exothermic reactions give out energy and the temperature often goes up. Enthalpy change is negative.

Question 3

Describe the temperature/energy and enthalpy change for endothermic reactions?

Answer 3

Endothermic reactions absorb energy and the temperature falls. Enthalpy change is positive.

Question 4

Give 2 examples of exothermic reactions?

Answer 4

Oxidation is usually exothermic -
>The combustion of a fuel like methane.
>The oxidation of carbohydrates, such as glucose in respiration.

Question 5

Give 2 examples of endothermic reactions?

Answer 5

> The thermal decomposition of calcium carbonate.

>Photosynthesis.

Question 6

What is activation energy?

Answer 6

The activation energy (Ea) is the minimum amount of energy needed to begin breaking reactant bonds and starts a chemical reaction.

Question 7

Why is it important to specify the conditions of enthalpy changes in data books?

Answer 7

Enthalpy changes you find in data books are usually standard enthalpy changes under standard conditions (298K and 100kPa). This is important because enthalpy changes are affected by temperature and pressure.

Question 8

What is the standard enthalpy change of reaction?

Answer 8

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions.

Question 9

What is the standard enthalpy change of formation?

Answer 9

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.

Question 10

What is the standard enthalpy change of combustion?

Answer 10

The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions.

Question 11

What is the standard enthalpy change of neutralisation?

Answer 11

The enthalpy change when an acid and an alkali react together, under standard conditions, to form 1 mole of water.

Question 12

Is bond breaking an endothermic or exothermic process?

Answer 12

Endothermic.

Question 13

Describe the enthalpy change for an exothermic reaction?

Answer 13

More energy is used to make bonds than break bonds ans so therefore, the enthalpy change is overall negative.

Question 14

What is bond dissociation enthalpy?

Answer 14

The amount of energy you need to break bonds per mole. Always involve bond breaking in gaseous compounds which makes comparisons fair.

Question 15

What do you get when you look up an average bond enthalpy?

Answer 15

The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds.

Question 16

What are the two factors that you need to know to measure enthalpy change?

Answer 16

> The number of moles of whats reacting.

>The change in temperature.

Question 17

What does the equation q=mcΔt mean?

Answer 17

q  = Heat lost or gained (in joules)
m = mass of water in the caolrimeter, or solution in the insulated container.
c = specific heat capacity of water.
Δt  = The change in temperature of the water or solution (in K).

Question 18

How do you find the enthalpy of combustion of a flammable liquid?

Answer 18

1)Set up the equipment using a fuel reactant and water with a stirrer and a thermometer as well as tripod, gauze, heatproof mat and insulation around the water.
2)As the fuel burns it heats the water and you can work out the heat absorbed by the water if you know the mass of the water, temperature change of the water and specific heat capacity of water.
>Ideally all the heat given out by the fuel as it burns would be absorbed by the water - allowing you to work out the enthalpy change of combustion.

Question 19

What is Hess’s law?

Answer 19

The total enthalpy change of a reaction is always the same, no matter which route is taken.

Question 20

What is the equation for standard enthalpy change using Hess’s law?

Answer 20

Standard enthalpy change = the sum of the enthalpy change of the products - the sum of the enthalpy change of the reactants.

Question 21

In terms of energy what is the equation for the enthalpy change of reaction?

Answer 21

Enthalpy change of reaction = Total energy absorbed - total energy released.

Question 22

What are the two conditions particles need in order for a reaction/successful collision to take place?

Answer 22

> Collide at the correct orientation, need to be in the right direction.
Collide with at least a certain minimum amount of kinetic (movement) energy.

Question 23

What is the minimum amount of energy particles need to break the bonds and start the reaction called?

Answer 23

Activation energy.

Question 24

What does a Boltzmann distribution curve show?

Answer 24

It shows the differing kinetic energies of a set number of particles.

Question 25

What happens to a reaction and the Boltsmann distribution curve if you increase the temperature?

Answer 25

> If you increase the temperature, the particles on average, have more kinetic energy and will move faster.
So, a greater proportion of molecules will have at least activation energy and be able to react so pushes the Boltsmann distribution curve over to the right.

Question 26

How does concentration affect reaction rate?

Answer 26

If you increase the concentration of reactants in a solution, the particles will be closer together, on average.

Question 27

How does increasing pressure speed up reactions?

Answer 27

If any of your reactants are gases, increasing the pressure will increase the rate of reaction for the particles will be closer together increasing the chance of successful collisions.

Question 28

How do catalysts speed up reactions?

Answer 28

Catalysts lower the activation energy by providing a different way for the bonds to be broken and remade. If the activation energy’s lower, more particles will have enough energy to react.

Question 29

What is a catalyst?

Answer 29

A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is chemically unchanged at the end of the reaction.

Question 30

Give an example of a specific catalyst used in a specific process?

Answer 30

An iron catalyst is used in the Haber process to make ammonia.

Question 31

What is a heterogenous catalyst?

Answer 31

A heterogenous catalyst is one that is in a different phase from the reactants .

Question 32

What is a homogenous catalyst?

Answer 32

Homogenous catalysts are in the same physical state as the reactants.

Question 33

How does a homogenous catalyst work?

Answer 33

Works by forming an intermediate species where the reactants combine with the catalyst to make an intermediate species, which then reacts to form the products and reform the catalyst.

Question 34

How can you increase the rate of reaction of a reaction with a heterogenous catalyst?

Answer 34

The reaction happens on the surface of the heterogenous catalyst. So, increasing the surface area of the catalyst increases the number of molecules that can react at the same time, increasing the rate of reaction.

Question 35

What are 4 benefits of using a catalyst in industry?

Answer 35

> The temperature doesn’t have to be really high in order for the reaction to happen - conditions.
To reduce cost
Help produce more product.
Can change the properties of a product to make it more useful.

Question 36

Give 3 ways how catalysts be used to aid environmental sustainability?

Answer 36

> Using catalysts means that lower temperatures and pressures can be used so energy is saved and less greenhouse gases are released.
Can reduce waster by allowing a different reaction with a better atom economy.
Catalytic converters on cars reduce then pollution released into the atmosphere by speeding up the reaction.

Question 37

What is the equation for rate of reaction?

Answer 37

Rate of reaction = amount of reactant used or product formed / time.

Question 38

What is the equation for calculating the gradient of a line?

Answer 38

Gradient = change in y / change in x

Question 39

What are two ways you can investigate reaction rates?

Answer 39

> Change in mass

>Volume of gas give off

Question 40

What is a dynamic equilibrium?

Answer 40

Where the concentrations of reactants and products stay constant and the forward and backwards reaction are at the same rate in a closed system.

Question 41

What is Le Chatelier’s principle?

Answer 41

The principle of if you change the conditions in a dynamic equilibrium, the position of the equilibrium will react to counteract the change.

Question 42

What happens if you increase the concentration of the reactant in a dynamic equilibrium in terms of Le Chatelier’s principle?

Answer 42

The equilibrium tries to get rid of the extra reactant by making more product, so the equilibrium is shifted to the side of the products.

Question 43

What happens in gases if you decrease the pressure in a dynamic equilibrium in terms of Le Chatelier’s principle?

Answer 43

Decreasing the pressure shifts the equilibrium to the side with more gas molecules which raises the pressure again.

Question 44

What happens if you increase the temperature in a dynamic equilibrium in terms of Le Chatelier’s principle?

Answer 44

Increasing the temperature means adding heat and the equilibrium shifts in the endothermic direction to try to replace the heat.

Question 45

What is the effect of catalysts on the position of the equilibrium?

Answer 45

They have NO EFFECT on the position of the equilibrium. They do speed up the forward and reverse reactions by the same amount. They can’t increase yield -but they do mean equilibrium is reached faster.

Question 46

In terms of the reaction between ethene and steam to form ethanol what is the compromise temperature used in industry and why?

Answer 46

> It’s an exothermic reaction so lower temperatures favour the forward reaction, meaning more ethanol is produced and you get a better yield.
But lower temperatures mean a slower rate of reaction, so 300 degrees is usually a favoured compromise.

Question 47

What could be an industrial/experimental issue with increasing the pressure to a high level?

Answer 47

High pressures are expensive to produce, you need stronger pipes and containers to withstand high pressure. In this process, increasing the pressure can also cause side reactions to occur.

Question 48

What is the the general equation for calculating the equilibrium constant (Kc) with the original equation?

Answer 48

aA + bB -> dD + eE, Kc= [D]^d [E]^e / [A]^a [B]^b

Question 49

How do you estimate the position of the equilibrium using the value of Kc?

Answer 49

> The larger the value of Kc, the more products there are at equilibrium, so the further the equilibrium lies to the right.
Similarly, the smaller the value of Kc, the more reactants there are at equilibrium, so the further the equilibrium lies to the left.

Question 50

Describe an experiment you could do to investigate the equilibrium position with changing temperature?

Answer 50

Using the reaction of 2NO2 (brown) <>N2O4 (colourless)

1) Place 2 sealed tubes containing the equilibrium mixture in water baths - one in a warm water bath and the other in a cool water bath and observe the colours of the mixtures.
2) The tube in the warm water bath will change to a darker brown colour as the endothermic reaction speeds up (which is the backwards reaction) and speeds up to absorb the extra heat pushing the equilibrium to the left.
3) The tube in the cool water bath will lose colour as the exothermic reaction speeds up to try and replace the lost heat, pushing equilibrium to the right.

Question 51

How to investigate the equilibrium position with changing concentration?

Answer 51

> Add equal amounts of equilibrium mixture to 4 test tubes and use test tube 1 as a ‘control’ which has an initial reddish colour.
Then add reactants or products to each test tube and observe changes (colour).
By adding more reactants the forward reaction speeds up to produce more product so equilibrium moves to the right (vice versa).