Question

For each of the following chemical and physical changes carried out at constant pressure, state whether work is done by the system on the surroundings or by the surroundings on the system, or whether the amount of work is negligible. State whether process or reaction is endothermic or exothermic. A. benzene (l) → benzene (g) B. ½ N2(g) + 3/2 H2(g) → NH3(g) ΔH = -92 kJ/mol C. 2NH4NO3(s) → 2N2(g) + 4H2O(g) + O2(g) + heat D. H2(g) + Cl2(g) → 2HCl(g) + heat E. CaCO3(s) + 178 kJ/mol → CaO + CO2 (g)

Answer 1

Answer:

A) Work is done by the system and the reaction is endothermic;

B) Work is done by the surroundings and the reaction is exothermic;

C) Work is done by the system and the reaction is exothermic;

D) Work is negligible and the reaction is endothermic;

E) Work is done by the system and the reaction is endothermic.

Explanation:

When the work is done by the system, the gas volume of the system increases, the gas is expanding. When the gas volume decreases, the work is done by the surroundings. Only gases influence the work because it depends on the pressure.

When the system loses heat, its enthalpy decreases, so ΔH is negative and will call the reaction exothermic when the system gains heat, the enthalpy increases, ΔH is positive and the reaction is called endothermic.

A) First, there are no gases, and the substance formed is a gas, so the gas volume increased. The work is done by the system. The temperature must increase to change the substance from liquid to gas, so the enthalpy must be positive and the reaction is endothermic.

B) First, there is 2 volume of gas (1/2 +3/2) and is formed 1 volume of gas, then the work is done by the surroundings. ΔH is negative, so the reaction is exothermic.

C) First, there is only solid and then gases are formed and heat is lost, so the work is done by the system and the reaction is exothermic.

D) First, there are 2 volumes of gas and is formed 2, so the work is negligible. The reaction loses heat, so it's exothermic.

E) First, there is no gas, and then is formed a volume of gas, so the work is done by the system. The reaction occurs with a gain of heat, so it's endothermic.