Endothermic Reactions

A chemical reaction that absorbs energy from its surroundings is called an endothermic reaction. In an endothermic reaction, more energy is required to break the bonds in the reactants than is released by the formation of the products.

Figure 18B shows the energy diagram for an endothermic reaction. Notice that the energy of the products is greater than the energy of the reactants. The difference between these amounts of energy equals the amount of heat that must be absorbed from the surroundings.

When mercury(II) oxide is heated to a temperature of about 450°C, it breaks down into mercury and oxygen, as shown in Figure 19. The decomposition of mercury(II) oxide is an endothermic reaction that can be described by the following equation.

$2\mathrm{HgO} + 181.7\mathrm{kJ} \to 2\mathrm{Hg} + {\mathrm{O}}_2$

Because heat is absorbed, the energy term appears on the left side of the equation. For every 2 moles of HgO that decomposes, 181.7 kJ of heat must be absorbed.

Figure 19 The orange-red powder in the bottom of the test tube is mercury(II) oxide. At about 450°C, mercury(II) oxide decomposes into oxygen gas (which escapes from the test tube) and mercury (droplets of which can be seen collecting on the sides of the test tube).