Enthalpy changes and standard conditionsAQA A-Level Chemistry: Mind map

Exam questions on Enthalpy changes and standard conditions

1. An instant cold pack used for sports injuries contains solid ammonium nitrate and a separate bag of water. When the bag is broken and the solid dissolves, the temperature of the mixture falls. An instant hand warmer works in the same way using calcium chloride, and the temperature rises. Both are measured at constant pressure.
   Explain why the temperature of the cold pack falls when the ammonium nitrate dissolves.2 marks
2. Ethanol, C₂H₅OH, is used as a biofuel. Its standard enthalpy of combustion is −1367 kJ mol⁻¹. Standard enthalpy changes refer to 100 kPa and 298 K.
   Calculate the enthalpy change when 2.00 mol of ethanol burn completely under standard conditions, and state what the sign of the value shows.2 marks
3. Hydrogen is a possible clean fuel for vehicles. Under standard conditions, H₂(g) + ½O₂(g) → H₂O(l) has ΔH = −286 kJ mol⁻¹, and the standard enthalpy of combustion of methane, the main component of natural gas, is −890 kJ mol⁻¹. The relative molecular masses are 2.0 for hydrogen and 16.0 for methane.
   Define standard enthalpy of combustion, and explain why the value for hydrogen is also the standard enthalpy of formation of liquid water.3 marks