The reaction between zinc and hydrochloric acid is exothermic and releases 126 kJ of energy when 1 mole of zinc reacts with 2 moles of hydrochloric acid to form 1 mole of zinc chloride and 1 mole of hydrogen gas. In contrast, the reaction between nitrogen and oxygen to form nitrogen dioxide is endothermic and absorbs 66 kJ of energy when 1 mole of nitrogen reacts with 2 moles of oxygen to form 2 moles of nitrogen dioxide. As a result of these energy changes, the temperature of the zinc and hydrochloric acid mixture increases while the temperature of the nitrogen and oxygen mixture decreases.
