Physical and Chemical Changes and Exothermic and Endothermic Reactions

Physical Change Changes in a substance that do not alter the chemical properties of the substance Things about the substance change, but the substance remains the same For instance, an ice cube melts into water and you can boil that into a gas, but it remains H2O the whole time One or more of its physical properties may be changed Ex: change in size, shape, dissolving. possibly color change (as long as a new substance is not created)

Chemical Change Changes that alter the chemical properties of a substance A change that creates a different substance Clues: bubbles (indicates the production of a gas), getting hot or cold, color change Ex: iron rusting, souring of milk, burning paper

Endothermic and exothermic reactions Step 1: Energy must be SUPPLIED to break chemical bonds: Step 2: Energy is RELEASED when new chemical bonds are made: A reaction is EXOTHERMIC if more energy is RELEASED than SUPPLIED. If more energy is SUPPLIED than is RELEASED then the reaction is ENDOTHERMIC

Endothermic Reactions: a change (e.g. a chemical reaction) that requires (or absorbs) heat. Forming Na+ and Cl- ions from NaCl is an endothermic process Photosynthesis is an endothermic reaction (requires energy input from sun)

Measuring Heat reaction Endothermic reaction = heat taken in & temperature of the substance drops

Ammonium nitrate + water

Starting temperature 20⁰ C

Add ammonium nitrate to water

ending temperature of 12⁰ C a temperature decrease of 8⁰ C

Energy given out by reaction Activation energy Energy given out by reaction

a change (e.g. a chemical reaction) that releases heat. Exothermic Reaction: a change (e.g. a chemical reaction) that releases heat. Burning fossil fuels is an exothermic reaction

Exothermic reaction = heat given off & temperature of substance rises. Heat Released reaction Exothermic reaction = heat given off & temperature of substance rises.

Exothermic vs endothermic: EXOTHERMIC – more energy is given out than is taken in (e.g. burning, respiration) ENDOTHERMIC – energy is taken in but not necessarily given out (e.g. photosynthesis) © Teachable . Some rights reserved. http://teachable.net/res.asp?r=1910

Examples Exothermic Combustion of fuels Evaporation of Water Yeast & Hydrogen Peroxide Epsom salts & water Endothermic Photosynthesis Acetic Acid & Sodium Bicarbonate Cold Packs Melting Ice © Teachable . Some rights reserved. http://teachable.net/res.asp?r=1910

1. Zinc turnings are placed in a hydrochloric acid solution Zinc - Element HCl - compound Bubbles indicate a gas is being produced The Zinc seems to disappear which means its being changed into a different substance Chemical Change What gas do you think is being produced?

2. Ripping Magnesium ribbon into pieces then ignited in a flame

3. A penny is coated with zinc then heated Copper - Element Zinc - Element Coating the Copper with Zinc is a physical change. Nothing new is created Heating dissolves the Zinc and Copper to make brass. A solution of metals is called an alloy. Alloys are considered a mixture of metals and can be separated back, therefore its not a chemical change. Normal Penny Coated with Zinc After being heated

4. Polymers A and B are mixed Polymer A and B are both compounds The reaction gives off heat (exothermic), a new substance is created, therefore its a chemical reaction

5. Hexane is poured down ramp and ignited Hexane is a flammable liquid similar to methane, propane, butane, octane (chemical property) Hexane is a compound with a low boiling point, so just exposing it to room temperature turns it into a gas (similar to nail polish remover) The hexane vapor is denser than air so it sinks (physical property) When its ignited, it turns into a new substance so that is a chemical change

6. A styrofoam cup in acetone Acetone - compound Styrofoam - compound Bonds between the polymers of the styrofoam are broken apart by the acetone The acetone is dissolving the styrofoam Its a physical change

7. Gas bubbles are made, then ignited The gas is methane, a compound Gas causes the bubble to expand - physical change Gas bubble being ignited is a chemical change