Chemical Reactions

• In order for a chemical change to occur, there has to be a reaction first that will produce that change.

• The law of conservation of mass states: • Atoms are neither created, nor destroyed,

during any chemical reaction, just rearranged.

• Every reaction involves energy.• A reaction either absorbs energy or releases

energy.

Endothermic Reaction

• An endothermic reaction is a chemical reaction in which energy is absorbed.

• Cold to the touch• “Endo” means “go in”• “Thermic” means “heat” or “energy”.

• Ammonium thiocyanate is mixed with barium hydroxide. The mixture is stirred and the two solids react. A slush is formed because water is one of the products of the reaction. A drop of water is placed on a block of wood and the beaker is placed on the water drop. The reaction is endothermic, that is, it takes in heat. As a result, the bottom of the beaker becomes cold enough to freeze the water and stick to the wood.

Exothermic Reaction

• An exothermic reaction is a chemical reaction in which energy is released.

• Feels hot to the touch.• “Exo” means “go out” or “exit”• “Thermic” means “heat” or “energy”

Exothermic Reactions

Reactants and Products

• All reactions involve reactants and products.

• Reactants are the substances you start with.• Products are what is produced.

2Na + Cl 2NaCl

• In the equation above, you start with reactants, Na and Cl (on the left side of the equation). NaCl and heat are the products (on the right side of the equation).

• Because of a reaction, you NOW have a change!!!

Rates of Reactions

• The rate of a reaction is a measure of how quickly the reaction takes place.

Four factors that affect the rate of a reaction….

• Temperature• Concentration• Surface area of reactants• Catalyst or inhibitor

Temperature

• A higher temperature means a faster rate of reaction.

• Example: fireworks

Concentration

• Generally, a higher concentration of reactants means a faster rate of reaction.

• Example: Crowded shopping mall when you are bumping into people.

Surface Area

• When the surface area, or the amount of exposed surface, of solid reactants is greater, the rate of reaction is faster.

• Example: Cornstarch Cannon

Catalyst

• A catalyst is a substance that speeds up a reaction without being permanently changed.

• Example: Enzymes in the body that breakdown food for digestion.

Inhibitor

• An inhibitor is a substance that slows down or stops a chemical reaction.

• Example: Preservatives slow down reactions, so food will not spoil.