Question

reference tablesduring the quiz. however, as stated in the course outline, and in the orientation session, this quiz is not open book. you cannot access course material while you write the quiz. nor can you google your answers, work as a group, use a cheat sheet, etc.
sch4u -- unit d -- quiz 1
question 1 (1 point)
which of the following equations represents enthalpy of formation of h₂o?
2h₂(g) + o₂(g) → 2h₂o(ℓ) δh = negative
h₂(g) + 1/2 o₂(g) → h₂o(ℓ) δh = negative
2h₂o (l) → 2h₂(g) + o₂(g) δh = positive
h₂(g) + 1/2 o₂(g) → h₂o(g) δh =positive
2h₂o₂ (l) → 2h₂o(l) + o₂(g) δh = positive

Explanation:

Step1: Recall the definition of enthalpy of formation

The enthalpy of formation ($\Delta H_f$) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states.

Step2: Analyze each option

• Option 1: $2H_2(g)+O_2(g)\to2H_2O(l)\ \Delta H =$ negative. This forms 2 moles of $H_2O$, not 1 mole. So, it is not enthalpy of formation.
• Option 2: $H_2(g)+\frac{1}{2}O_2(g)\to H_2O(l)\ \Delta H =$ negative. Here, 1 mole of $H_2O(l)$ is formed from its elements ($H_2(g)$ and $O_2(g)$ which are in their standard states). Combustion (formation of water from hydrogen and oxygen) is an exothermic reaction ($\Delta H<0$).
• Option 3: $2H_2O(l)\to2H_2(g)+O_2(g)\ \Delta H =$ positive. This is the reverse of the formation reaction (decomposition of water), not formation.
• Option 4: $H_2(g)+\frac{1}{2}O_2(g)\to H_2O(g)\ \Delta H =$ positive. Formation of water from hydrogen and oxygen is exothermic ($\Delta H<0$), not endothermic ($\Delta H > 0$).
• Option 5: $2H_2O_2(l)\to2H_2O(l)+O_2(g)\ \Delta H =$ positive. This is a decomposition reaction of $H_2O_2$, not formation of $H_2O$ from its elements.

Answer:

$H_2(g)+\frac{1}{2}O_2(g)\to H_2O(l)\ \Delta H =$ negative (the second option)