9.2 Measuring Enthalpy Changes

Question 1

How do you calculate an energy change?

Answer 1

q = mc(delta)t

m = mass ( usually in g) 
c = specific heat capacity of surroundings
t = temperature

Question 2

How do you calculate enthalpy change of combustion of a reaction?

Answer 2

1 - work out q. Divide by 1000 to get answer in kJ

2 - calculate amount in moles of alcohol burnt using moles = mass / molar mass

3 - Divide energy in kJ by moles of alcohol

4 - add a minus sign

Question 3

Why is the experimental value often different from the data book value?

Answer 3

Less exothermic than expected :

Heat loss due to the surroundings other then the water ( e.g. beaker, air surrounding flame )

Incomplete combustion of alcohol, where carbon monoxide and carbon is produced instead of carbon dioxide.

Evaporation of alcohol from wick ( therefore the burner must be weighed asap ).

More exothermic than expected:

Non - standard conditions. Conditions for experiment unlikely to be identical to standard conditions.

Question 4

How do you work out enthalpy change of reaction?

Answer 4

1 - calculate q. Divide by 100 to get answer in kJ

2 - calculate amount in moles of liquid that reacted ( or whatever is not in excess ) ( using n = cv)

3 - use balancing numbers to calculate moles of the rest

4 - divide energy in kJ by moles

5 - add a minus sign

Question 5

How do you work out enthalpy change of neutralisation ?

Answer 5

1 - calculate q. Divide by 1000.

2 - calculate amount in moles of acid / base that reacted

3 - divide energy in kJ by moles

4 - add a minus sign