Consider the reaction: 2 Mg(s) + O2 (g) ? 2MgO(s) ?H = -1204 kJ (a) Is this reac
ID: 604600 • Letter: C
Question
Consider the reaction: 2 Mg(s) + O2 (g) ? 2MgO(s) ?H = -1204 kJ (a) Is this reaction exothermic or endothermic? (b) What is the enthalpy change for the reverse reaction? Is the reverse reaction exothermic or endothermic? (c) Calculate the amount of heat transferred when 7.10 g of Mg(s) reacts at constant pressure with excess oxygen. (d) How many grams of MgO are produced during a run of this reaction which experiences an energy change of - 234 kJ?Explanation / Answer
H = -1204 kJ=>exothermic Enthalpy is viewed from the perspective of the system. If the system loses heat, the reaction is exothemic and the enthalpy is a negative number. If the systems gains heat, the reaction is endothermic and the enthalpy is a positive number. So as it is shown in your problems, if heat is a product then the reaction is exothermic and gives off heat. If heat is a reactant then the reaction is endothermic, as the system requires energy to run the reaciton to completion. A practical example: You have a cup of hot tea. As it sits on the counter, the tea gradually cools down and the counter warms up a little. The system (the cup of tea) has lost heat and the surroundings (the air, the counter, etc) have gained it, so this reaction is exothermic. Another example: A cup of ice sits on the counter. Over time it melts, turning to water which eventually warms to room temperature. The counter (and air) get cold and the water warms up. The system has gained heat and the surroundings have lost it (the heat came from the air and the counter mostly) so the reaction is endothermic. Change in enthalpy is calculated as the heat of the products minus the heat of reactants, or: DeltaH = Hproducts - Hreactants. Heats of formation of a compound or element are usually listed in the appendix of your book.