Question

Which of the substances have a standard heat of formation ( Δ H ∘ f ) of zero?
a. Cl 2 (g) at 2 atm
b. Fe at 1200 ∘ C
c. C2H6 (g) at standard conditions
d. O2(g) at 25.0 ∘ C and 1 atm

Answer 1

Answer:

d. $O_{2 (g)}$ at 25.0$^{0}C$ and 1 atm

Explanation:

The standard heat of formation of a substance is the amount of heat absorbed or evolved when 1 mole of the substance is formed from from its constituent elements. It is also known as standard enthalpy of formation.

All elements in their natural states, have a standard heat of formation of zero because there is no temperature change involved in their formation. Example includes oxygen gas, graphite (solid carbon).

a. ΔH°$_{f}$ of  $Cl_{2 (g)}$ at standard pressure of 1 atm is zero, but not at 2 atm

b. ΔH°$_{f}$ of Fe at 25.0$^{0}C$ is zero, but not at 1200$^{0}C$

c.  ΔH°$_{f}$ of $C_{2}H_{6}$$_{(g)}$ is not zero because it's a compound

d. ΔH°$_{f}$ of $O_{2 (g)}$ is zero at standard conditions

Answer 2

From the given substance, the heat of formation of oxygen gas is zero. Thus option d. is correct.

Standard heat of formation, or the standard enthalpy of formation has been the amount of energy required for the formation of 1 mole of the substance. The enthalpy of formation has been zero for the natural elements.

This is because the elements that have been existing in the natural form have not been formed at the standard conditions, thereby the standard enthalpy of formation is zero. However, with the change in the standard conditions, the heat of formation may vary from zero.

For the given substances, the heat of formation will be:

a. $\rm Cl_2$ at 2 atm.

The chlorine gas is a natural element, thus having 0 heat of formation at 25 $\rm ^\circ C$ and 1 atm. Since the given pressure is 2 atm. The heat of formation has been varied from zero.

b. Fe at 1200 $\rm ^\circ C$.

Fe has also been the natural element, having zero heat of formation at 25 $\rm ^\circ C$. The given temperature is 1200 $\rm ^\circ C$, thus the heat of formation varies from zero.

c. $\rm \bold C \bold _2 H_6$ at standard conditions.

Although the compound has been present at the standard conditions, the heat of formation has not been equal to zero, as it is not a naturally occurring compound.

d. $\rm \bold O_2$ at 25 $\rm ^\circ C$ and 1 atm.

The oxygen gas is a naturally occurring element and is present at the standard conditions, Thus the heat of formation of Oxygen gas has been zero.

From the given substance, the heat of formation of oxygen gas is zero. Thus option d. is correct.

For more information about the heat of formation, refer to the link:

https://brainly.com/question/13096727