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Chemistry LibreTexts

10: Introduction to Energy

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  • In previous science classes you may have learned that one way to distinguish chemical changes from physical changes is that physical changes - such as the melting and freezing of water - are reversible but that chemical changes are not. In this unit we will see that this simple answer is not necessarily what it seems.

    • 10.1: Conservation of Mass - There is No New Matter
      The law of conservation of mass states that matter can not be created or destroyed in a chemical reaction. So the mass of the product equals the mass of the reactant. A reactant is when two or more elements chemically interact to make a new substance and a product is the substance that is formed as the result of a chemical reaction. Mass and matter may not be able to be created or destroyed, but it can change forms to other substances like liquids, gasses, solids, etc.
    • 10.2: Energy
      When we speak of using energy, we are really referring to transferring energy from one place to another. Although energy is used in many kinds of different situations, all of these uses rely on energy being transferred in one of two ways. Energy can be transferred as heat or as work.
    • 10.3: Energy and Chemical and Physical Change
      Phase changes involve changes in energy. All chemical reactions involve changes in energy. This may be a change in heat, electricity, light, or other forms of energy.     Reactions that absorb energy are endothermic. Reactions that release energy are exothermic.
    • 10.4: Internal Energy
    • 10.5: 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.
    • 10.6: Enthalpy and Chemical Reactions
      Every chemical reaction occurs with a concurrent change in energy. The change in enthalpy equals heat at constant pressure. Enthalpy changes can be expressed by using thermochemical equations. Enthalpy changes are measured by using calorimetry.

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