Question

analyzing the intermediate reactions
if you reverse reaction 2 and combine the reactions, which substances do not cancel? choose three correct answers.
reaction equation δh (kj/mol)
reaction 1: mg(s) + 2hcl(aq) → mgcl₂(aq) + h₂(g) -450
reaction 2: mgo(s) + 2hcl(aq) → mgcl₂(aq) + h₂o(l) -81
reaction 3: h₂(g) + ¹/₂o₂(g) → h₂o(l) -280
magnesium combustion: mg(s) + ¹/₂o₂(g) → mgo(s) ?
h₂o
mgcl₂
h₂

Explanation:

Step1: Write Reaction 2 in reverse

Reaction 2 (reversed): $\text{MgCl}_2(aq)+\text{H}_2\text{O}(l)
ightarrow\text{MgO}(s) + 2\text{HCl}(aq)$

Step2: Analyze cancellation with Reaction 1

Reaction 1: $\text{Mg}(s)+2\text{HCl}(aq)
ightarrow\text{MgCl}_2(aq)+\text{H}_2(g)$. When combined with reversed - Reaction 2, $\text{HCl}(aq)$ and $\text{MgCl}_2(aq)$ will cancel out.

Step3: Determine non - canceling substances

$\text{H}_2\text{O}$, $\text{H}_2$ and $\text{MgO}$ do not cancel. Since we need to choose three correct answers from the given options, $\text{H}_2\text{O}$, $\text{MgCl}_2$, $\text{H}_2$, $\text{MgCl}_2$ cancels out, while $\text{H}_2\text{O}$ and $\text{H}_2$ do not cancel.

Answer:

H₂O, H₂