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: Twigs vs. logs
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.