Question

which of the following reactions are written correctly? you have two correct answers to choose. refer to figure 6.1.
$\square 2h_{2(g)} + o_{2(g)} \to 2h_{2}o_{(l)} + heat$
$\square heat + 2h_{2(g)} + o_{2(g)} \to 2h_{2}o_{(l)}$
$\square heat + 2hgo_{(s)} \to 2hg_{(l)} + o_{2(g)}$
$\square 2hgo_{(s)} \to 2hg_{(l)} + o_{2(g)} + heat$

Explanation:

Step1: Analyze the first reaction

For the reaction \(2H_{2(g)}+O_{2(g)}\to2H_{2}O_{(l)}+\text{Heat}\), hydrogen and oxygen combine to form water and release heat. This is a combustion reaction (exothermic). The chemical equation is balanced (\(2\times2 + 2\times2= 2\times2 + 2\times1\) for atoms of \(H\) and \(O\) respectively).

Step2: Analyze the second reaction

For the reaction \(\text{Heat}+2H_{2(g)}+O_{2(g)}\to2H_{2}O_{(l)}\), hydrogen and oxygen do not need heat input to combine to form water. Combustion of hydrogen is exothermic (releases heat), so heat should be on the product side, not the reactant side.

Step3: Analyze the third reaction

For the reaction \(\text{Heat}+2HgO_{(s)}\to2Hg_{(l)}+O_{2(g)}\), mercury (II) oxide decomposes when heated. This is a decomposition reaction (endothermic). The chemical equation is balanced (\(2\times1 + 2\times1=2\times1+2\times1\) for atoms of \(Hg\) and \(O\) respectively).

Step4: Analyze the fourth reaction

For the reaction \(2HgO_{(s)}\to2Hg_{(l)}+O_{2(g)}+\text{Heat}\), mercury (II) oxide decomposition is an endothermic reaction (needs heat input), so heat should be on the reactant side, not the product side.

Answer:

\(2H_{2(g)}+O_{2(g)}\to2H_{2}O_{(l)}+\text{Heat}\) and \(\text{Heat}+2HgO_{(s)}\to2Hg_{(l)}+O_{2(g)}\)