Energy Changes — practice questions

State the sign of ΔrH for an exothermic reaction and describe what happens to the temperature of the surroundings.

Explain why ΔrH is negative for an exothermic reaction, even though the surroundings get hotter.

The combustion of methane has ΔrH = −890 kJ mol⁻¹. Sketch and describe the enthalpy diagram, state ΔrH for the reverse reaction with justification, and explain why a reaction being exothermic does not guarantee it happens quickly.

The combustion of methane has ΔrH = −890 kJ mol⁻¹. Calculate the energy released when 2.00 mol of methane burns.

Calculate the energy released when 8.00 g of methane (M = 16.0 g mol⁻¹) burns completely. ΔrH = −890 kJ mol⁻¹.

Ethanol burns with ΔrH = −1367 kJ mol⁻¹ (C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O). A camping stove burns 15.0 g of ethanol to heat water. Calculate the energy released, then determine what mass of propane (M = 44.1 g mol⁻¹, ΔrH = −2220 kJ mol⁻¹) would release the same energy. Comment on which is the better fuel by mass.

State whether melting is exothermic or endothermic, and give the sign of ΔH.

Explain why the temperature of a substance stays constant while it is melting, even though heat is still being supplied.

Explain why a burn from steam at 100 °C is more severe than a burn from liquid water at 100 °C, using enthalpy changes. Then explain why boiling water requires much more energy than melting the same mass of ice, in terms of the particles.

State whether breaking bonds is exothermic or endothermic, and whether making bonds is exothermic or endothermic.

Calculate ΔrH for H₂ + Cl₂ → 2HCl using bond enthalpies H–H = 436, Cl–Cl = 243, H–Cl = 432 kJ mol⁻¹, and state whether it is exothermic.

Calculate ΔrH for the complete combustion of ethene (C₂H₄ + 3O₂ → 2CO₂ + 2H₂O) using C–H = 413, C=C = 614, O=O = 498, C=O = 805, O–H = 464 kJ mol⁻¹. The experimentally measured value is −1411 kJ mol⁻¹. Compare your answer with this and explain any difference.