ENERGY CHANGES

• All changes that matter undergoes, whether physical or chemical, occur with lost or gain of energy.
• The difference in energy is released as heat and light.
• Heat

  • A thermal energy
  • Energy transferred between two bodies due to difference in their temperature.
• When a chemical reaction occurs, chemical bonds in the reactants break and new chemical bonds form.
• When a bond breaks, it absorbs energy from the surroundings.
• When a bond forms, it releases energy to the surroundings.
• Some chemical reactions release more energy than they absorb, and vice versa.
• For an endothermic reaction to continue, energy must be constantly added.
• In endothermic reactions, more energy is required to break the bonds of the reactants, than is released when the products formed.
• In exothermic reactions, more energy is released when the products formed, than is required to break the bonds in the reactants.
• Most chemical reactions release energy as opposed absorbing it.
• All reactions require energy to start the breaking of bonds.
• The minimum amount of energy needed to start a chemical reaction.

  Activation Energy
• Different reactions have different activation energies.
• In rusting of iron, have low activation energy requirement coming from its surrounding.
• Wood requires the thermal energy of a flame to start burning. Once the reaction started, it releases enough energy to keep the reaction going.
• The study of energy changes that accompany all processes, whether physical or chemical

  Thermodynamics
• The study of energy changes that accompany chemical transformations in matter.

  Thermochemistry
• The formula for change in internal energy is ∆U=Q-W, and ∆U=Q+W
• Q is positive if heat is absorbed.
• Q is negative if heat is released.
• W is positive if work is done on the system.
• W is negative if work is done by the system.
• The total heat absorbed in a process at constant pressure if the only work done is pressure-volume work.

  Enthalpy
• Most reactions take place under constant atmospheric pressure.
• The standard heat of reactions can be evaluated from the heats of formation of the reactants and products in the given chemical reaction.
• The standard heats of formation of pure elements at their most stable form are equal to zero.