IGCSE chemistry energetics
calculating molar enthalpy
Molar enthalpy = amount of energy transferred for 1 mole of substance
1. A student added 100 cm3 of a 2.0 mol dm-3 of hydrochloric acid to 100 cm3 of 2.0 mol dm-3 sodium
hydroxide solution. The student recorded the temperature rise as 11.5 C.
(a) Write the equation for the reaction.
(b) Calculate the heat given out in the reaction. (Assume the specific heat capacity of the reaction
mixture to be 4.2 J K-1 g-1)
(c) Work out how many moles there are in 100 cm3 of the 2.0 mol dm-3 solutions of acid and alkali.
(d) From the answers above, what is the molar enthalpy of neutralization for the reaction between
HCl and NaOH solutions?
(e) Draw the apparatus the student could have used to perform the experiment.
(f) Consider the sources of experimental error and the effect they would have on the student’s value
for the enthalpy of neutralization.
2. Another student measured the enthalpy of combustion of ethanol, C2H5OH, by using a calorimeter.
The calorimeter contained 200 cm3 of water and its temperature rose by 6.2 C.
(a) Draw the apparatus used in the experiment.
(b) Work out how much heat was given out by the burning ethanol.
(c) The ethanol was weighed before and after the experiment. Its mass decreased by 0.19 g.
How many moles of ethanol were burned in the experiment?
(d) Using the answers above, calculate the molar enthalpy of combustion of ethanol.
(e) The value given for the enthalpy of combustion of ethanol in the data book is - 1367.3 kJ mol-1.
Can you think of any reasons for the difference between this and the value obtained in the
student’s experiment.
3. 50 cm3 of a 1.0 mol dm-3 potassium hydroxide solution was added to 50 cm3 of a 1.0 mol dm-3 nitric
acid solution and a temperature rise of 6.1 C was recorded.
(a) Write an equation for the reaction.
(b) Calculate the molar enthalpy of neutralization.
4. In an experiment to find the molar enthalpy of combustion of hexane a calorimeter was used and
the following raw data was collected. Calculate the molar enthalpy of combustion of hexane.
Volume of water in calorimeter (cm3)
100
Temperature rise of water (C)
40
Mass of hexane + burner at start (g)
66.87
Mass of hexane + burner at end (g)
67.30
Answers
1. (a) HCl (aq) + NaOH (aq)  H2O (l) + NaCl (aq)
(b) heat given out = 200 x 4.2 x 11.5 = 9660 J
(c) moles = 0.1
dm3
x 2 mol dm3 = 0.2 moles
(d) H = 9660/0.2 = 48300 J = -48.3 kJ mol-1
(e) and (f) see lesson notes and handout and power points
2. (a) see lesson notes
(b) heat given out = 200g x 6.2 C x 4.2 J g -1 C = 5208 J
(c) moles = 0.19/46 = 0.004 moles
(d) H = 5208/0.004 = - 1302 kJ mol-1
(e) see handout/power points
3. (a) HNO3 (aq) + KOH (aq)  H2O (l) + KNO3 (aq)
(b) heat given out = 100g x 4.2 J g -1 C x 11.5 C= 2562 J
moles = 0.05
dm3
x 1 mol dm -3 = 0.05 mole
H = 2562 /0.1 = -51.24 kJ mol-1
4. heat given out = 100g x 4.2 J g -1 C x 40 C= 16800 J
moles = 0.43g /86g = 0.005 mole
H = 16800 /0.005 = -3360 kJ mol-1