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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.