Heat and Work: First Law and Thermochemical Equations: ΔH from Energy Flow

Question

1) An automobile engine provides 588 Joules of work to push the pistons and generates 2322 Joules of heat that must be carried away by the cooling system.

Calculate the change in the internal energy of the engine.

 E =  ___ Joules

2) When Na(s) reacts with H₂O(l) according to the following reaction, 184 kJ of energy are evolved for each mole of Na(s) that reacts. Complete the following thermochemical equation.

2Na(s) + 2H₂O(l)2NaOH(aq) + H₂(g)    H = ____ kJ

3) When CO(g) reacts with O₂(g) according to the following reaction, 283 kJ of energy are evolved for each mole of CO(g) that reacts. Complete the following thermochemical equation.

2CO(g) + O₂(g)2CO₂(g)    H = ____ kJ

Answer 1

QUESTION 1

Internal energy of a system may be written as a function of work and heat. Note that the sign of the work depends on whether work is being done ON THE SYSTEM by an outside species or if work is being done ON AN OUTSIDE SPECIES by the system. The sign of heat is dependent upon whether heat is entering the system or leaving it. Therefore:

Change in Internal energy = Heat + Work

In this case, the engine is doing work on an outside system (piston) and the sign of work is negative. Heat is leaving via the cooling system and the sign of heat is negative.

Change in Internal Energy = -588 J - 2322 J = -2910 J

QUESTION 2

The chemical equation may be written as

2Na + 2H₂O --> 2NaOH + H₂ + energy

For each mole of Na used, 184 kJ is release (or "evolved"). Therefore:

dH = 2(-184 kJ/mol) = -368 kJ

(note that it is negative because the reaction is exothermic and the energy is released).

QUESTION 3

Same thing as earlier:

dH = 2(-283 kJ/mol) = -566 kJ