TEKS 11B: Understand the law of conservation of energy and the processes of heat transfer. What are the ways in which energy changes can occur? Thermochemistry is the study of energy changes that occur during chemical reactions and changes of state. During a chemical reaction, a substance is transformed into another substance with a different amount of chemical potential energy. Energy changes occur as either heat transfer or work, or a combination of both.

TEKS 11B: Understand the law of conservation of energy and the processes of heat transfer. Heat, represented by q, is energy that transfers from one object to another because of a temperature difference between the objects. One of the effects of adding heat to an object is an increase in its temperature. Heat flows spontaneously from a warmer object to a cooler object. If two objects remain in contact, heat will flow from the warmer object to the cooler object until the temperature of both objects is the same. Heat 1. Describe An ice cube at 0ºC is dropped into a cup of water at 45ºC. Describe what happens to the ice and water in terms of heat flow.

TEKS 11B: Understand the law of conservation of energy and the processes of heat transfer. What happens to the energy of the universe during a chemical or physical process? Chemical reactions and changes in physical state generally involve either the absorption or the release of heat. In studying energy changes, you can define a system as the part of the universe on which you focus your attention. Everything else in the universe makes up the surroundings. In thermochemical experiments, you can consider the region in the immediate vicinity of the system as the surroundings. Together, the system and its surroundings make up the universe.

TEKS 11B: Understand the law of conservation of energy and the processes of heat transfer. Conservation of energy The law of conservation of energy states that in any chemical or physical process, energy is neither created nor destroyed. During any chemical or physical process, the energy of the universe remains unchanged. If the energy of the system increases during a process, the energy of the surroundings must decrease by the same amount. If the energy of the system decreases during a process, the energy of the surroundings must increase by the same amount.

TEKS 11B: Understand the law of conservation of energy and the processes of heat transfer. Endothermic process Heat is absorbed from the surroundings in an endothermic process. In an endothermic process, the system gains heat as the surroundings lose heat. Heat flowing into a system from its surroundings is defined as positive; q has a positive value here.

TEKS 11B: Understand the law of conservation of energy and the processes of heat transfer. Exothermic process An exothermic process is one that releases heat to its surroundings. In an exothermic process, the system loses heat as the surroundings gain heat. Heat flowing out of a system into its surroundings is defined as negative; here, q has a negative value.

TEKS 11B: Understand the law of conservation of energy and the processes of heat transfer. Sample problem: Recognizing endothermic and exothermic processes On a sunny winter day, the snow on a rooftop begins to melt. As the melted water drips from the roof, it refreezes into icicles. Show that you understand the processes of heat transfer by describing the direction of heat flow as the water freezes. Is this process endothermic or exothermic? (contd.)

TEKS 11B: Understand the law of conservation of energy and the processes of heat transfer. Sample problem: Recognizing endothermic and exothermic processes 1. Analyze Identify the relevant concepts. Heat flows from a warmer object to a cooler object. An endothermic process absorbs heat from the surroundings. An exothermic process releases heat to the surroundings. (contd.)

TEKS 11B: Understand the law of conservation of energy and the processes of heat transfer. Sample problem: Recognizing endothermic and exothermic processes 2. Solve Apply the concepts to this problem. First, identify the system and the surroundings. System: water Surroundings: air Determine the direction of heat flow. In order for water to freeze, its temperature must decrease. Heat flows out of the water and into the air. (contd.)

TEKS 11B: Understand the law of conservation of energy and the processes of heat transfer. Sample problem: Recognizing endothermic and exothermic processes Determine if the process is endothermic or exothermic. Heat is released from the system to the surroundings. The process is exothermic. 2. Understand the Processes of Heat Transfer On a cold night you use an electric blanket to warm your body. Show that you understand the processes of heat transfer by describing the direction of heat flow. Is this process endothermic or exothermic?