# Quiz | Thermochemistry

1

Which statement correctly describes an endothermic chemical reaction?

In an endothermic chemical reaction, reactants absorbs heat energy
⇒ the products will have higher potential energy than reactants
⇒ Hproducts > Hreactants
⇒ ∆H = Hproducts – Hreactants > 0

2

What happens to the value of ∆H of a thermochemical reaction if the reaction is reversed?

Hess’s Law: enthalpy changes for chemical equations are additive
⇒ ΔHrxn = ΔHrxn (forward) + ΔHrxn (reverse) = 0
⇒ ΔHrxn (reverse) = - ΔHrxn (forward)

3

∆H = 114 kJ for the reaction: 2 NO2 (g) → 2 NO(g) + O2(g). What is the ∆H for NO(g) + ½ O2(g) → NO2(g)?

Reaction 1:   2 NO2 (g) → 2 NO(g) + O2(g),   Reaction 2:   NO(g) + ½ O2(g) → NO2(g)
Reaction 2 is the reverse of reaction 1 with the reactants and the products divided by 2

Hess’s Law: enthalpy changes for chemical equations are additive
⇒ ΔHrxn (reverse) = - ΔHrxn (forward)
AND   ΔHrxn = n x ΔHrxn (A) when a chemical equation A is multiplied by a factor n
⇒ ΔHrxn (Reaction 2) = - $\frac{1}{2}$ ΔHrxn (Reaction 1) = - $\frac{1}{2}$ x 114 = - 57 kJ

4

Which of the following does not have a standard heat of formation equal to 0 at 298 K and 1.0 atm?

The standard enthalpy of formation of the most stable form of an element is 0.
The most stable element for hydrogen is H2, not H ⇒ ΔH0f of H2 = 0   but   ΔH0f of H  0

5

Knowing that the enthalpy of combustion of propane gas is – 2220.0 kJ.mol-1, which of the following statements is incorrect?

ΔH combustion (C3H8) = – 2220.0 kJ.mol-1 < 0 ⇒ exothermic process
Exothermic process ⇒ q < 0: reaction releases energy as heat
ΔH combustion (2 moles of C3H8) = 2 x ΔH combustion (C3H8) = – 4440.0 kJ

6

If a reaction is carried out at constant pressure, which of the following statements is correct?

ΔHrxn = ΔUrxn + PΔVrxn     (when P = cst, isobaric conditions)
AND   ΔUrxn =  q + w = q - PΔVrxn    with q = heat change
⇒ ΔHrxn = q - PΔVrxn + PΔVrxn = q     at constant pressure