Energy is a very important quantity in science and the world around us. Although most of our energy ultimately comes from the sun, much of the energy we use on a daily basis is rooted in chemical reactions. The gasoline in your car, the electricity in your house, the food in your diet-all provide substances for chemical reactions to provide energy (gasoline, food) or are produced from chemical reactions (electricity, about 50% of which is generated by burning coal). As such, it is only natural that the study of chemistry involves energy. 7.1: Introduction 7.2: Energy Energy is the ability to do work and uses the unit joule. The law of conservation of energy states that the total energy of an isolated system does not increase or decrease. 7.3: Work and Heat Work can be defined as a gas changing volume against a constant external pressure. Heat is the transfer of energy due to temperature differences. Heat can be calculated in terms of mass, temperature change, and specific heat. 7.4: Enthalpy and Chemical Reactions Every chemical reaction occurs with a concurrent change in energy. The change in enthalpy, a kind of energy, equals heat at constant pressure. Enthalpy changes can be expressed by using thermochemical equations. Enthalpy changes are measured by using calorimetry. 7.5: Stoichiometry Calculations Using Enthalpy The energy change of a chemical reaction can be used in stoichiometry calculations. 7.6: Hess's Law Hess's law allows us to combine reactions algebraically and then combine their enthalpy changes the same way. 7.7: Formation Reactions A formation reaction is the formation of one mole of a substance from its constituent elements. Enthalpies of formation are used to determine the enthalpy change of any given reaction. 7.E: Energy and Chemistry (Exercises) These are exercises and select solutions to accompany Chapter 7 of the "Beginning Chemistry" Textmap formulated around the Ball et al. textbook.