Question

At constant pressure, which of these systems do work on the surroundings?
a. 2A(g) + 3B (g) ------> 4C (g)
b. 2A(g) + 2B (g) -------> 5C(g)
c. 2A (g) + B (g) -----> C(g)
d. A(s) + B(g) -------> 2C (g)

Answer 1

Answer:

The system does work on the surroundings.

Explanation:

The work (w) exerted in a chemical reaction depends on the change in the number of gaseous moles (Δn(g)), where

Δn(g) = n(gas, products) - n(gas, reactants)

These variables are linked through the following expression.

w = -R.T.Δn(g)

where,

R is the ideal gas constant

T is the absolute temperature

• If Δn(g) > 0, w < 0 and the system does work on the surroundings.
• If Δn(g) < 0, w > 0 and the surroundings do work on the system.
• If Δn(g) = 0, w = 0 and no work is done.

At constant pressure, which of these systems do work on the surroundings?

a. 2A(g) + 3B (g) ------> 4C (g)

Δn(g) = 4 -5 = -1. The surroundings do work on the system.

b. 2A(g) + 2B (g) -------> 5C(g)

Δn(g) = 5 -4 = 1. The system does work on the surroundings.

c. 2A (g) + B (g) -----> C(g)

Δn(g) = 1 - 3 = -2. The surroundings do work on the system.

d. A(s) + B(g) -------> 2C (g)

Δn(g) = 2 -2 = 0. No work is done.