Unit 8 – Notes Page 7 • Endothermic and Exothermic Reactions Energy: • The ability to do work • Chemical bonds store energy • Breaking bonds takes energy • Forming bonds releases energy Activation Energy • The minimum amount of energy required for a reaction to take place Heat of Reaction, ΔH • A change in the internal energy of the reaction • + ΔH: Reactants lower energy than products • - ΔH: Reactants higher energy than products Enthalpy • Amount of energy a system or substance contains • Another way to say ΔH Surroundings • Where the reaction is taking place: the beaker, the test tube, the classroom, the universe, etc. System • Substance(s) / chemicals involved in a reaction or phase change. Open System Closed System • Isolated System Exothermic Reactions • Exo = out • Thermic = heat • Energy (heat, light) is given off from a system when a reaction takes place Endothermic Reactions • Endo = into, inside • Thermic = heat • Energy (heat, light) is taken into a system when a reaction takes place Exothermic Reactions • GENERAL EQUATION: REACTANTS PRODUCTS + ENERGY Takes less energy to break bonds in reactants than is released when new bonds form in products Negative ΔH: Exothermic Example: Combustion • Always exothermic • Burns and combines with oxygen • Give off heat as energy and light Example: Hot Hands How does an instant hot pack work? Exothermic reaction of iron in oxygen: 4Fe(s) + 3O2(g) 2Fe2O3(s) Endothermic Reactions • GENERAL EQUATION: REACTANTS + ENERGY PRODUCTS Needs a constant input of energy to continue. Takes more energy to break bonds in reactants than is released when new bonds form. Positive ΔH: Endothermic Example: Photosynthesis • 6 CO2 + 6 H2O → C6H12O6 + 6 O2 • Plants need to aborb energy from the sun (or other source of light) • Absorbs energy endothermic Example: Cold Pack • How does an instant cold pack work? (NO3)NH4 (s) + H2O(l) NH4(OH) (aq) + HNO3 (aq) • Squeezing the cold pack breaks an inner bag of water, and the water mixes with a chemical inside the pack. The chemical and water combine in an endothermic reaction. The energy needed for the reaction to take place comes from the water, which gets colder as the reaction proceeds. Phase Change –Ice Water –Solid gold molten gold –Heat moves from the surroundings to the system • Solid Liquid Gas (Melt, evaporate) • Requires heat (energy). It takes energy from the environment. This is ENDOTHERMIC. • Gas Liquid Solid (Condense, freeze) • Requires absence of heat (energy). It releases energy into its surroundings. • Making ice is EXOTHERMIC!