KI + Pb(NO3)2 Sugar + H2SO4(aq) Chemical Reactions Section 2.2-2.4

Chemical Reactions occur all around you In a chemical reaction, the substances you start with are not the same as the substances you end up with. Substances with new properties are formed. This type of change cannot be reversed REACTANTS  PRODUCTS The substances that are The substances that there before a chemical form afterwards reaction

Evidence of Chemical Change There are several indicators of chemical change. In some chemical reactions: A color change may occur A new odour is detected A gas is given off A solid (precipitate) is formed Energy is given off – such as heat (temperature change), light or sound

Consider what happens when you light a candle: When you ignite the wick you observe the wick burning, and feel heat and see light being given off This is an example of a COMBUSTION reaction Candle wax + oxygen  carbon dioxide + water (fuel)

Another group of chemical reactions are neutralization reactions. During these reactions, an acid is added to a base to produce a compound and water. The pH of the product is more neutral (closer to 7) than the pH of either reactant Common example: baking soda + vinegar

Chemical changes always involve energy changes. Some are obvious (fireworks) Some are unnoticeable (heat is given off when metal rusts)

FIREWORKS = OBVIOUS ENERGY CHANGE

Chemical reactions can be useful because of: 2) How they occur: Endothermic reactions ABSORB HEAT Exothermic reactions RELEASE HEAT

Energy Changes in Reactions: Exothermic reactions – reactions that release energy in the form of heat, light, electricity I.e. COMBUSTION: a substance reacts rapidly with oxygen to produce new products and heat energy C(s) + O2(g)  CO2(g) + energy i.e. BURNING NATURAL GAS

Energy Changes in Reactions Endothermic reactions – reactions that absorb energy in the form of heat, light, electricity I.e. COLD PACK – used for athletes when they hurt themselves

Practice “Endothermic and Exothermic Reactions” Worksheet Use the following worksheet to test your understanding of Section 2 Decide whether the reaction produces or requires energy. Classify the reaction as exothermic or endothermic.

Design an Investigation: Cold Packs and Hot Packs You are now going to use your knowledge of chemical reactions, specifically exothermic and endothermic reactions, to determine which chemicals make the best hot packs and cold packs. Reference pg. 36 of text You must complete the Pre-Investigation Questions and have your procedure approved by your teacher before you can perform the lab tomorrow