Revision Cont'd
Francine Taylor-Campbell, Contributor

The Hopewell High School emblem bearing the school motto: 'Pioneers For Excellence'. The school colours are teal and aquamarine. - Claudine Housen/Staff Reporter

Note:

Please remember to read the questions carefully and answer them as clearly as possible, so that the examiner can understand what you are trying to say.

Writing much and being vague will not guarantee the marks, if what is being asked is not answered.

Be relaxed with calculation questions by practising using formulas and memorising them.

Question 1

The following results were obtained by burning methane and using the energy obtained to heat a known mass of water:

Mass of water = 250 g
Initial temperature of water = 26 0C
Final temperature of water = 36 0C
Mass of methane = 0.2 g

Use this information to calculate the heat of combustion in KJ per mole of methane. Assume that there are no heat losses and that the 4.2J of energy are needed to raise the temperature of 1g of water by 10C.

Answer:

Heat change for rxn = mass of soln x heat capacity of soln x temperature change. This is an equation worth remembering!

H = 250g x 4.2 Jg-1C-1 x 10 C = 10.5 KJ

This energy change was given out from the burning of 0.2g of methane.

1 mole of methane (CH4) has a mass of (12 + 4) = 16 g

# mol in 0.2 g = 0.2/16 = 0.0125 mol

Thus 0.0125 mol give out 10.5 KJ of heat

1 mole = 10.5/0.0125 = 840 KJ/mol

Question 2

0.10g of magnesium ribbon and 25 cm³ of 2.0 mol/dm³ sulphuric acid were mixed and the total volume of hydrogen was measured. (See table below)

a) Calculate how many moles of magnesium and of sulphuric acid were used in this experiment.

b) Explain why the reaction stopped.

c) The experiment was repeated using the same mass of magnesium, but 25 cm3 of 3.0 mol/dm3 sulphuric acid. How will the initial rate of formation of hydrogen and the total volume of hydrogen collected compare with the original experiment.

d) Give two other methods of changing the initial rate of reaction.

ANSWERS

a) # mol of Mg = 0.1/24 = 4.17x10-3 mol

# mol of H2SO4 = (25 x 2)/1000 = 5 x 10-2 mol

b) Acid is in excess. The reaction ends when all the magnesium has reacted.

c) If 3.0 mol/dm3 of sulphuric acid is used, the initial rate of formation of hydrogen will increase. As concentration increases, the reaction becomes faster.

The total volume of hydrogen will remain the same, as the mass of magnesium is the same.

d) Increasing the temperature and using powdered magnesium can change the initial rate of production of hydrogen.

Francine Taylor-Campbell is an independent contributor.