Energetics
5 marks · EasyThe equation below represents the reaction between calcium oxide and water:
Draw an energy profile diagram for the reaction above, clearly labelling the reactants, products, and the direction of energy change. Indicate whether the reaction is exothermic or endothermic.
A student adds calcium oxide to 100.0 g of water in an insulated cup. The temperature rises from 20.0 C to 32.5 C. Calculate the heat energy released to the water. (Assume the specific heat capacity of water is 4.18 J g⁻¹ °C⁻¹.)
Another experiment involves dissolving ammonium nitrate in water, which lowers the temperature of the same mass of water by the same number of degrees as your answer in (b). Explain, in terms of heat transfer, whether this second reaction is exothermic or endothermic.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) An exothermic energy profile diagram: reactants at higher energy than products, arrow down labelled ΔH, with clear labels for reactants and products, and ΔH negative. · must show higher energy for reactants than products, arrow labelled energy given out or ΔH, labels for reactants and products, diagram indicates exothermic
(b) Heat released = 5225 J · must show use q = mcΔT, show working: correct substitution
(c) Endothermic; heat is taken from the water, making it colder. · must show states endothermic, reason: heat absorbed from surroundings
Common mistakes: mislabelled diagram; incorrect formula; wrong direction of heat transfer
Show mark scheme and worked solution
Marking points
D1 (a) · 1 mark · Diagram shows reactants at higher energy than products; energy change arrow shown.
A1 (a) · 1 mark · Diagram is correctly labelled: reactants, products, negative ΔH, and exothermic indicated.
M1 (b) · 1 mark · Uses q = mcΔT with correct values substituted (m = 100.0 g, c = 4.18 J g⁻¹ °C⁻¹, ΔT = 12.5 °C). (follow-through)
A2 (b) · 1 mark · Correct calculation: 5225 J (allow ±10 J for rounding). (follow-through)
B1 (c) · 1 mark · Correctly explains the reaction is endothermic because heat is taken in from the water/surroundings, causing the temperature to fall.
Worked solution
(a) The reaction is exothermic. Draw reactants (CaO + H2O) at higher energy on the left, products (Ca(OH)2) at lower energy on the right. An arrow pointing down (from reactants to products) showing ΔH is negative, and label the energy given out.
(b) Calculate the heat released: Use .
So, the heat released is 5225 J.
(c) When ammonium nitrate dissolves, the temperature falls, so heat is taken from the water into the reaction. This means the reaction is endothermic.