Energetics

Question 1

What is enthalpy change?

Answer 1

Change in heat energy during a chemical process at constant pressure

Question 2

What are the standard conditions for enthalpy change measurements?

Answer 2

Pressure = 100kPa
Temperature - 298K
Concentration for all reacting solutions = 1 mol dm-3

Question 3

What kind of process is breaking bonds?

Answer 3

endothermic as it requires energy

Question 4

What kind of process is making bonds?

Answer 4

Exothermic as it releases energy

Question 5

What is activation energy?

Answer 5

the minimum amount of energy needed to start a reaction

Question 6

What does the term mean bond enthalpy mean?

Answer 6

The energy required to break one mole of a covalent bond into gaseous atoms, averaged over a range of different compounds

Question 7

Why are mean bond enthalpies always positive?

Answer 7

Breaking bonds is always endothermic

Question 8

What is the standard enthalpy of formation?

Answer 8

The enthalpy change when one mole of substance is formed from its constituent elements under standard conditions with all reactants and products being in their standard state

Question 9

What is the standard enthalpy of combustion?

Answer 9

the enthalpy change when one mole of a substance is completely burnt in excess oxygen under standard conditions, all reactants and products being in their standard states

Question 10

Why may the enthalpy of combustion of a compound be difficult to measure?

Answer 10

Incomplete combustion may occur

Question 11

What does Hess’s Law state?

Answer 11

Enthalpy change of a chemical reaction is the same, whatever route is taken from reactants to products

Question 12

Give one reason why the bond enthalpy you calculated above is different from the mean bond enthalpy quoted in a data book

Answer 12

The data book value is averaged over a range of different compounds so the mean bond enthalpies are not the same as the actual bond enthalpies

Question 13

What is the assumption made in calorimetry?

Answer 13

The energy transferred to the water from the reacting chemicals (or vice versa for endothermic reaction) is equal to the energy released (or taken in) by the reaction

Question 14

What are the sources of error in combustion calorimetry?

Answer 14

Heat loss to surroundings
Incomplete combustion
Heat can pass through metal calorimeters

Question 15

What are the sources of error in solution calorimetry?

Answer 15

Heat loss to surroundings

Question 16

What improvements can be made to minimise sources of error in combustion calorimetry?

Answer 16

Reduce distance between water and flame
Put a lid on calorimeter
Use polystyrene cup
Put a sleeve around the flame to protect from drafts

Question 17

What improvements can be made to minimise sources of error in solution calorimetry?

Answer 17

Add a lid
Put a sleeve around the flame to protect from drafts

Question 18

How to measure enthalpy change using a cooling curve?

Answer 18

Measure 50cm3 of water using a measuring cylinder
Add to a polystyrene cup
Add known mass of solid
Weigh by difference by using mass before and mass after
Record initial temperature for 4 mins to establish room temp
Add the solid and record temp at regular time intervals until a trend is seen
Plot a graph of temp vs time
Extrapolate back to the point of addition to get max/theoretical temperature change

Question 19

A 50.0 g sample of water was used in this experiment.
Explain how you could measure out this mass of water without using a balance

Answer 19

Water has a known density (of 1.0 g cm–3)
Therefore, a volume of 50.0 cm3 could be measured out

Question 20

Explain how to set up an experiment to calculate the enthalpy change of the combustion of a fuel.

Answer 20

Measure the mass of a spirit burner containing the desired fuel
Measure a known quantity of water into a boiling tube or other suitable container
Insulate the calorimeter as thoroughly as possible (use a lid, flame proof insulation, etc.)
Insert a thermometer into the water and record the initial temperature
Ignite the fuel and measure the temperature change of the water over a defined period of time
Measure again the mass of the spirit burner and fuel and calculate the mass of fuel that combusted
Use q = mcT to calculate the energy released
Divide by the number of moles of fuel combusted to calculate the enthalpy change in J mol-1, then divide by 1000 to calculate the value in kJ mol-1

Question 21

Why do values calculated using mean bond enthalpies differ from those calculated using Hess’s Law?

Answer 21

Mean bond enthalpies are mean values calculated across a range of compounds containing that bond where as Hess’s law find the actual values of the bond enthalpies in that reaction.