C9-10. Enthalpy Changes

Question 1

How do we define enthalpy?

Answer 1

Enthalpy is a measure of the energy stored in a system.

Question 2

How can we observe enthalpy change?

Answer 2

• Enthalpy changes occur when energy is given out or taken into account system during reaction. It cannot be directly measured.
• We observe it as a temperature change, using a thermometer or temperature probe.

Question 3

How can we calculate enthalpy change?

Answer 3

• We use Q=mcT to find the total energy change in the system.
• This is then converted to kJ, before being divided by the number of moles of the subject of the calculation to calculate delta-H.

Question 4

What are the standard conditions for measuring enthalpy changes?

Answer 4

• 100 kPa
• 298 K
• 1 mol dm-3
• Standard states of all reactants and products

Question 5

Define standard enthalpy change of combustion.

Answer 5

The enthalpy change when 1 mole of a substance undergoes complete combustion under standard conditions.

Question 6

Define standard enthalpy change of reaction.

Answer 6

The enthalpy change of a reaction in the molar quantities shown in the chemical formula under standard conditions.

Question 7

Define standard enthalpy change of formation.

Answer 7

The enthalpy change when 1 mole of a substance is formed from its constituent elements under standard conditions.

Question 8

Define standard enthalpy change of neutralisation.

Answer 8

The enthalpy change when 1 mole of water is produced during a reaction between an acid and base under standard conditions.

Question 9

What is an exothermic reaction?

Answer 9

Exothermic reactions occur when the products formed have less energy than the reactants used, since energy is transferred to the surroundings.

Question 10

What is an endothermic reaction?

Answer 10

Exothermic reactions occur when the products formed have greater energy than the reactants used, since energy is transferred from the surroundings.

Question 11

How do we use cooling curves?

Answer 11

• Plot 2 lines of best fit: one for the starting temperature, and one for the temperature change.
• Extrapolate the latter to the time at which the reaction was initiated.
• Find the difference between this point and the initial temperature.

Question 12

How can we practically measure enthalpy changes?

Answer 12

• We use an insulated reaction vessel containing reacting substances and a thermometer.
• We record temperature every minute until it is stable, before initiating the reaction and continuing to record temperature.
• We then use cooling curves to determine max. delta-H.

Question 13

Why do we need to use cooling curves?

Answer 13

They allow us to estimate the enthalpy change at the instant of the reaction, at which point negligible energy has been lost to the surroundings.

Question 14

What sources of error do cooling curves let us account for?

Answer 14

• Evaporation of the reacting substance
• Loss of thermal energy to the surroundings

Question 15

What can often affect measured/estimated enthalpy change?

Answer 15

• Evaporation of reacting substances
• Loss of thermal energy to the surroundings through lack of insulation
• Other reactions occurring with different enthalpies such as incomplete combustion

Question 16

What is the activation energy of a reaction?

Answer 16

The minimum energy required to enable to a reaction to take place. This is attained when the energy input is sufficient to break the bonds in the reactants.

Question 17

Is bond breaking exo- or endo-thermic?

Answer 17

Energy is required to break bonds, therefore bond breaking is an endothermic process.

Question 18

Is bond formation exo- or endo-thermic?

Answer 18

Energy is released when bonds are formed, therefore bond formation is an exothermic process.

Question 19

How can we use bond enthalpies to estimate enthalpy change?

Answer 19

(sum of bonds broken) - (sum of bonds formed) = enthalpy change.

Question 20

Why is the use of bond enthalpies of calculate enthalpy change only an estimation?

Answer 20

• The values we are provided are average bond enthalpies.
• These are an average value taking into account the enthalpy changes of the bond in all molecules involving the named bond.
• These vary between molecules.

Question 21

Why is it difficult for us to measure enthalpies accurately or easily?

Answer 21

• The activation energy may be high.
• The reaction rate may be very slow.
• Multiple reactions could be occurring at once, especially during the formation of hydrocarbons.

Question 22

Summarise Hess’ law.

Answer 22

• This states that the overall enthalpy change for a reaction is independent of the route taken, in a closed system.
• I.e. the enthalpy change of the direct route X>Y equals that of the indirect route X>Z>Y.

Question 23

How can we use Hess cycles to determine overall enthalpy changes using enthalpy change of formation?

Answer 23

• Using formation, overall enthalpy change of X>Y would equal that of (Z>Y) - (X>Z).
• We know this because the arrows we draw point upwards from the elements.

Question 24

How can we use Hess cycles to determine overall enthalpy changes using enthalpy change of combustion?

Answer 24

• Using combustion, overall enthalpy change of X>Y would equal that of (X>Z) - (Z>Y).
• We know this because the arrows we draw point downwards towards the elements.

Question 25

How do we use Hess’ law to calculate overall enthalpy change for a reaction?

Answer 25

• If we are given known enthalpy changes of combustion/formation of an indirect reaction pathway, we can combine these algebraically to produce an overall enthalpy change.
• For enthalpy of formation: (Z>Y) - (X>Z) = (X>Y)
• For enthalpy of combustion: (X>Z) - (Z>Y) = (X>Y)

Question 26

What happens to the temperature of the surrounding in…
- exothermic reactions
- endothermic reactions

Answer 26

• Temperature increases. Energy is transferred to the surroundings from the system.
• Temperature decreases. Energy is transferred from the surroundings to the system.

Question 27

Why do reactions involving small activation energies often take place very readily?

Answer 27

The energy required to surpass this activation energy and break bonds is readily available from the surroundings alone.

Question 28

What is the standard enthalpy change of formation of an element from itself?

Answer 28

This is always 0 kJ mol-1, since no change occurs when forming one mole of an element from itself. This is important in Hess cycles, where this idea comes up regularly.

Question 29

What state do all species need to be in when determining enthalpy changes from average bond enthalpies? Why is this?

Answer 29

All species must be in the gaseous state. This is because average bond enthalpy is the energy required to break one mole of a specific bond in a gaseous molecule.

Question 30

Define specific heat capacity.

Answer 30

• This is the energy required to raise the temperature of 1g of a substance by 1 K.
• Note that this differs from the definition of SHC in Physics.
• SHC is given by the letter c.

Question 31

What assumptions can we make when completing calorimetry calculations?

Answer 31

• We can assume that the density of water in a calorimeter is 1 g cm-3.
• We can also assume that the specific heat capacity of water in a calorimeter is 4.18 J g-1 K-1.