Chapter 7, Problem 9STP

Interpretation Introduction

Interpretation:

A balanced chemical equation corresponding to the reaction between magnesium metal and water needs to be presented.

Concept Introduction:

• A chemical reaction is represented in terms of a chemical equation with the reactants on the left and the products on the right.

  $\text{Reactants }\to \text{ Products}$

• A balanced chemical equation is one in which the number of atoms of various elements on the reactants side is equal to that on the product side. It depicts the stoichiometry of a given reaction.

• The equations are balanced in two steps:

1) Balance the atoms by matching the number of atoms of each type on the reactants and product side.

2) Adjust the coefficients to the smallest whole number. A coefficient of '1' is implied and therefore not explicitly written.

Answer to Problem 9STP

  ${\text{Mg (s) + 2H}}_{\text{2}}\text{O (l)}\to {\text{Mg(OH)}}_2{\text{ (s) + H}}_{\text{2}}(\text{g})$

Explanation of Solution

The given equation involves the reaction between solid magnesium and water to form a precipitate of magnesium hydroxide and hydrogen gas. The reactants and products are:

Reactants:

  $\begin{array}{l}\text{Magnesium, solid - Mg (s)}\\ {\text{Water - H}}_{\text{2}}\text{O (l)}\end{array}$

Products:

  $\begin{array}{l}{\text{Hydrogen , gas - H}}_{\text{2}}(\text{g)}\\ {\text{Magnesium hydroxide, solid - Mg(OH)}}_2\text{ (s)}\end{array}$

Unbalanced equation:

  ${\text{Mg (s) + H}}_{\text{2}}\text{O (l)}\to {\text{Mg(OH)}}_2{\text{ (s) + H}}_{\text{2}}(\text{g})$

The above equation is not balanced since the number of H atoms on the reactants side is 2 while that on the products side is 4. In order to balance the above equation, multiply H₂O by 2 to get:

  ${\text{Mg (s) + 2H}}_{\text{2}}\text{O (l)}\to {\text{Mg(OH)}}_2{\text{ (s) + H}}_{\text{2}}(\text{g})$

Conclusion

Therefore, 1 mole of magnesium reacts with 2 moles of water to form 1 mole of magnesium hydroxide and 1 mole of hydrogen gas.