11 Exothermic Reactions Exo = out Thermic = heat Energy (heat, light) is given off from a system when a reaction takes place
12 Endothermic Reactions Endo = into, insideThermic = heatEnergy (heat, light) is taken into a system when a reaction takes place
13 Exothermic Reactions General equation: Reactants Products + Energy Takes less energy to break bonds in reactants than is released when new bonds form in productsNegative ΔH: Exothermic
14 Example: Combustion Always exothermic Burns and combines with oxygen Give off heat as energy and light
15 Example: Hot Hands How does an instant hot pack work? Exothermic reaction of iron in oxygen:4Fe(s) + 3O2(g) 2Fe2O3(s)
16 Endothermic Reactions General equation:Reactants + Energy ProductsNeeds 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
17 Example: Photosynthesis 6 CO2 + 6 H2O → C6H12O6 + 6 O2Plants need to aborb energy from the sun (or other source of light)Absorbs energy endothermic
18 (NO3)NH4 (s) + H2O(l) NH4(OH) (aq) + HNO3 (aq) 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.
19 Phase Change Ice Water Solid gold molten gold Heat moves from the surroundings to the system
20 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!