Question

In which of the following processes is (delta)H = (delta)E? Why?

(a) Two moles of ammonia gas are cooled from 325 C to 300 C at 1.2atm

(b) One gram of water is vaporized at 100C and 1 atm.

(c) A mole of nitrogen gas reacts with a mole of oxygen gas to form two moles of nitric oxide gas in a 40L container.

(d) Calcium carbonte is heated to form calcium oxide and carbon dioxide in a container with variable volume.

(e) One mole of solid carbon dixide sublimes to the gas phase.

Answer 1

Using the equation delta H = delta E + PV. Delta E & H will essentially be equal when there is no change in volume.

Now let look at each of these processes

(a) Two moles of ammonia gas are cooled from 325 C to 300 C at 1.2atm

We know PV=nRT

since n and P are constant when T decreases V decreases so here delta H delta E

b) One gram of water is vaporized at 100C and 1 atm.

Vapourization of water can be denoted pictographically as

so definitely there will be an increase in volume. so for this also delta H delta E

c) N2(g) + O2(g) 2 NO(g)

two moles of gas give two moles of gas so there is no chnage in pressure of volume so here

delta H = delta E

d) CaCO₃ (s) CaO(s) + CO₂ (g)

here a gas is released on heating so the volume increases so

delta H delta E

e) CO₂(s) CO₂(g)

Here a solid is converted to a gas so volume increases so delta H delta E

So the only example where delta H = delta E is

(c) A mole of nitrogen gas reacts with a mole of oxygen gas to form two moles of nitric oxide gas in a 40L container.