Key Concept: Heat energy change is calculated using Q = mcDeltaT and can be converted to molar enthalpy.
Section A: Calculating Q
1. 100 g of water warms by 12 C. Calculate Q. Use c = 4.2 J/g C. [2]
2. A reaction cools 50 g of water by 5 C. Calculate the energy change of the water. [2]
Section B: Molar Enthalpy
3. If 2.0 g of magnesium reacts and releases 24 kJ of energy, calculate DeltaH per mole of Mg. Ar(Mg)=24. [3]
4. State the sign of DeltaH for an exothermic reaction. [1]
Total marks: 8
Mark Scheme
1. Q = 100 x 4.2 x 12 = 5040 J [2]
2. Q = 50 x 4.2 x 5 = 1050 J; energy decreases [2]
3. moles = 2.0/24 = 0.0833; DeltaH = 24 kJ / 0.0833 = 288 kJ/mol (exothermic) [3]
4. Negative [1]