Energy Changes.

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Presentation transcript:

Energy Changes

Energy Changes energy: the ability to do work   thermochemistry: the study of energy changes calorimetry: an experimental technique of measuring energy changes

Energy Changes Energy can take a wide variety of forms. Although there are many forms of energy, they can all be put into two categories: kinetic and potential.

Kinetic and Potential Energy Kinetic Energy Potential Energy Energy due to motion.   q       electrical q       light q       thermal q       sound Stored energy and the energy of position. q       chemical q       nuclear q       mechanical

Energy Changes When studying transfers of energy, it is important to distinguish between the substances undergoing the change, called the chemical system and the system’s environment, called the surroundings. In an open system, both energy and matter can move in/out. In a closed system, only energy can move in/out. In an isolated system, neither matter nor energy can move in/out.

Absorbing or Releasing Heat Energy All chemical reactions occur with either an absorption or release of heat (energy). Reactants Products (bonds broken) (bonds made)   energy absorbed > energy released system absorbs energy (endothermic) energy absorbed < energy released system releases energy (exothermic)

Endothermic vs. Exothermic Reaction Endothermic Reaction q  Energy is absorbed by the system. q  More energy is needed to break bonds than is released by the formation of new bonds. q  Temperature of surroundings decreases. Exothermic Reaction q  Energy is released by the system. q  More energy is released by the formation of new bonds than is needed to break bonds. q  Temperature of the surroundings increases.

Heat Change The following equation can calculate the heat change of a substance.   q = mcT

Heat q = mcT Term definition symbol units   heat amount of energy transferred q J (joules) mass amount of matter m g (grams) specific quantity of heat required to c J heat raise the temperature of a unit gC capacity mass of a substance 1C or 1K temp average kinetic energy of the T C particles in a sample of matter

Practice What would be the final temperature if 163 kJ of energy is transferred to 625 mL of water in an electrical kettle at an initial temperature of 20.0C?

Law of Conservation of Energy According to the Law of Conservation of Energy, energy can be converted from one form to another, or transferred from one set of molecules to another, but the total energy of the system and its surroundings remains the same.

H system = + l q surroundingsl Enthalpy Change An equation showing this relationship is: H system = + l q surroundingsl   where H is “enthalpy change” or “heat of reaction” which is the difference in energy of a substance during a change.

Types of Enthalpy Changes Three types of enthalpy changes observed are during physical changes, chemical reactions, and nuclear reactions. Notice how much more energy is produced in a nuclear change than in a chemical change, and in a chemical change than a physical change.

Practice Handout: # 1-5, 9-11, 14-15 Together #1-3