Chapter 9

Question 1

What is enthalpy change?

Answer 1

A measure of the heat energy in the a chemical system

Question 2

How is enthalpy change calculated?

Answer 2

The difference in the enthalpies of products - the difference in enthalpies of the reactants.

Question 3

If the energy transfer goes from the system to the surroundings, what sort reaction is it?

Answer 3

An exothermic reaction

Question 4

If energy change goes from the surroundings to the system, what is the reaction?

Answer 4

Endothermic

Question 5

What is the standard conditions for standard enthalpy changes? (Pressure, temperature, concentration, state)

Answer 5

101kPa
298K
1moldm^3
State - physical state of a substance under standard conditions

Question 6

What is the definition of the standard enthalpy change of formation?

Answer 6

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states

Question 7

What is the standard enthalpy change of combustion?

Answer 7

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all the reactants and products in their standard states.

Question 8

What is the definition of the enthalpy change of neutralisation?

Answer 8

The energy change that accompanies the reaction of an acid by a base to form one mole of H2O, under standard conditions, with all reactants and products in their standard states

Question 9

What equations are used to calculate the enthalpy change?

Answer 9

q=mc(delta)T
Mol=mass/Mr

Question 10

What could cause inaccuracies in the experimentally enthalpy change of combustion?

Answer 10

• heat lost to the surroundings other than water
• incomplete combustion may have taken place (seen as soot)
• the alcohol used in the experiment may have evaporated as it is volatile
• The condition of the experiment are unlikely to be identical to standard conditions

Question 11

How could you limit the inaccuracy of the experimental value of the enthalpy of combustion?

Answer 11

Use of drought screens and an input of oxygen gas could minimise hear loss and incomplete combustion

Question 12

What is the definition of the standard enthalpy change of reaction?

Answer 12

The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states

Question 13

In an exothermic enthalpy profile diagram, are the reactants higher or lower than the products?

Answer 13

The reactants are higher than the products and they have more energy to begin with and then they lose this energy to the surroundings

Question 14

For an endothermic enthalpy profile diagram, are the reactants higher or lower than the products?

Answer 14

The reactants are lower than the products as the products lose energy to the system which gains the energy

Question 15

What is the activation energy?

Answer 15

The minimum energy required for a reaction to take place

Question 16

How can you determine the enthalpy change of reaction (and also neutralisation)?

Answer 16

Carry out a reaction in a polystyrene cup, and measure the temperature of the solution within the cup as it is the immediate surroundings

Question 17

How can a cooling curve be corrected for heat loss when finding the correct initial temperature?

Answer 17

Extrapolate the curve back to the time when the reactant was added and the reaction began to take place

Question 19

What is the average bond enthalpy?

Answer 19

The energy required to break one mole of a specific type of bond in a gaseous molecule

Question 20

Why do bond enthalpies always have a positive enthalpy value?

Answer 20

They are always endothermic

Question 21

What are the limitations of the average bond enthalpy?

Answer 21

They are calculated using actual bond enthalpies and actual bond enthalpies can vary depending on the chemical environment of the bond

Question 22

What process is bond breaking and why?

Answer 22

Endothermic as energy is required to break bonds

Question 23

What process is bond making and why?

Answer 23

Exothermic as energy is released when bonds form

Question 24

How do you calculate the enthalpy change from average bond enthalpies?

Answer 24

Energy to break bonds - energy to form new bonds

Question 25

What are the limitations of using average bond enthalpies in an enthalpy change calculation?

Answer 25

The actual energy would be slightly different due to the bonds being in different environments. All species must be in a gaseous state in the calculations using average bond enthalpies

Question 26

Why is Hess’ Law used?

Answer 26

To determine enthalpy changes indirectly as enthalpy changes of some reactions are too difficult to determine directly

Question 28

What does Hess’ Law state?

Answer 28

If a reaction can take place by two routes, and the starting and finishing conditions are the same, then the total enthalpy change is the same for each route

Question 29

Which way do the arrows in a Hess’ cycle point for the determination of enthalpy change of formation?

Answer 29

Upwards

Question 30

Which way do the arrows point in a Hess’ cycle for the determination of the enthalpy change of combustion?

Answer 30

Downwards