When a chunk of potassium metal is added to water, a vigorous reaction occurs. Hydrogen gas is produced, along with a large amount of heat. In fact, the released heat is enough to ignite the hydrogen, creating a brilliant flame. Energy is an integral component of chemical reactions. Some reactions require an input of energy, whereas others release energy as they proceed. When we burn natural gas in our furnace at home, we are oxidizing small hydrocarbons by reacting them with oxygen. This reaction produces heat, which can be used to warm the house. In this chapter, we will focus primarily on the transfer of heat and energetic changes that occur during chemical reactions.