Energetics

Question 1

Define the term enthalpy change (1)

Answer 1

1. Change in heat energy a constant pressure

Question 2

Define the term activation energy (1)

Answer 2

1. Minimum energy that is required to start a reaction

Question 3

Define the term mean bond enthalpy (1)

Answer 3

1. The standard enthalpy change when one mole of gaseous molecules each break a covalent bond to form two free radicals averaged over a range of different compounds

Question 4

Define the standard enthalpy of formation (1)

Answer 4

1. The standard enthalpy change when one mole of a compound is formed from it’s elements under standard conditions, with all reactants and products in their standard states

Question 5

State why the enthalpy of formation of Na(s) is zero. (1)

Answer 5

1. Na(s) is an element in it’s standard state

Question 6

State why the enthalpy of liquid Na is not zero. (1)

Answer 6

1. Na (l) is not the standard state of Na

Question 7

Define the standard enthalpy of combustion (1)

Answer 7

1. The standard enthalpy change when one mole of a substance is completely burnt in oxygen, under standard conditions, with all products and reactants in their standard states.

Question 8

Which two equations are required to work out the enthalpy changes? (2)

Answer 8

1. Q = mc X change in temp
2. Enthalpy change = Q/n

Question 9

Steps to measure an enthalpy change using a cooling curve (6)

Answer 9

DATA COLLECTION
1. Record temperature of solution every minute for 3 minutes before adding reactants to establish accurate starting temperature
2. Add reactants at fourth minute
3. Measure temperature every minute until a cooling trend is seen

GRAPH PLOTTING
4. Plot a graph of temperature vs time
5. Draw a line of best fit through cooling curve and extrapolate it back to time of addition
6. To calculate theoretical temperature change accounting for heat loss

Question 10

The enthalpy of combustion determined experimentally is less exothermic
than that calculated using enthalpies of formation.
Give one possible reason for this, other than heat loss. (1)

Answer 10

1. Incomplete combustion