A ceramics workshop makes quicklime for a glaze by heating crushed limestone in a small electric kiln. Calcium carbonate breaks down to calcium oxide and ca...
Question 1 Report
A ceramics workshop makes quicklime for a glaze by heating crushed limestone in a small electric kiln. Calcium carbonate breaks down to calcium oxide and carbon dioxide. The technician has sketched Fig. 1 after recording the energy of the chemicals during heating. The vertical scale is in kJ. The arrow marked P represents the energy taken in as bonds in calcium carbonate are broken.
Fig. 1
(a) Give the balanced chemical equation for the thermal decomposition of calcium carbonate. [2]
(b) Use Fig. 1 to state whether this reaction is exothermic or endothermic. [1]
(c) Describe what happens to the energy store of the kiln heater while the limestone is decomposing. [2]
(d) Explain why a continuous supply of energy is needed, even after the calcium carbonate has reached a high temperature. [3]
(e) Suggest three ways in which the workshop could reduce the energy wasted during this process. [3]
Answer Details
(a) CaCO3 → CaO + CO2. The numbers of calcium, carbon and oxygen atoms are the same on both sides. [2]
(b) The reaction is endothermic. The products are shown at a higher energy level than calcium carbonate, so energy has been taken in overall. [1]
(c) The heater's energy store decreases, because energy is transferred electrically from the heater to the kiln and limestone. [2]
(d) Breaking bonds in calcium carbonate requires energy. Although forming bonds in calcium oxide and carbon dioxide releases energy, more energy is needed for bond breaking than is released by bond formation. The reaction therefore takes in energy overall and continues to need a supply of energy. [3]
(e) Insulate the kiln walls, keep the kiln door closed, and repair gaps in insulation or seals. Other creditworthy measures include recovering heat from hot gases and heating a full kiln rather than a partly empty one. Any three. [3]
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