Chemical Reactions
Types of Chemical Reactions
• Learning Target: I can balance chemical
reactions.
• Why learn this?
– We are still working with the concept of the
Law of Conservation of Mass.
– You will see chemical reactions again in
biology and chemistry.
– Chemical reactions happen around you and in
you all the time!
Chemical Reactions Review
• A chemical reaction occurs when the
atoms of the starting materials
rearrange to form new substances.
• A chemical reaction involves the valence
electrons only.
Chemical Reactions Review
• All chemical reactions have two parts:
1. Reactants - the substances you start with
(the ones reacting)
AND
2. Products- the substances you end up with
(the ones produced)
Chemical Reactions Review
Law of Conservation of Mass
• Matter is not created or destroyed during a
chemical reaction
• The mass of the reactants is equal to the
mass of the products.
Chemical Reactions
• Chemical reactions cause changes in both
physical and chemical properties.
• A physical change could be a substance
going from a liquid to a gas.
• Water to steam – easily reversed
• A chemical change results in the formation
of a new substance or substances.
• These are difficult to reverse
Chemical Reactions
• Something to notice in this equation:
AgNO3(aq) + NaCl(aq)
AgCl(s) + NaNO3(aq)
• The (aq) and (s) next to the subscripts.
– These indicate the state of each compound shown in the
molecular formula.
• (aq) = the substances are dissolved in water
(aqueous)
• (s) = the substance is a solid
• (l) = the substance is a liquid
• (g) = the substance is a gas
Chemical Reactions
• Chemical reactions may occur instantly or
need help getting started.
• A catalyst is a substance that will increase
how fast a reaction happens.
• An inhibitor is a substance that will slow
the rate of reaction or prevent a reaction.
• Enzymes are biological substances that
can be catalysts or inhibitors.
The heat of Reactions
All chemical reactions result in a
change in energy.
– This energy can be measured in heat
• Some chemical reactions cannot
occur unless heat is added.
• Some chemical reactions occur
spontaneously.
The heat of Reactions
• Exothermic Reactions release
energy to the surroundings.
– Primarily heat
– Also light and sound
• Exothermic reactions occur
spontaneously.
• The containers the reaction occurs
in will feel warm.
– Hand warmers
The heat of Reactions
• Endothermic Reactions must absorb
energy to occur.
– Heat
• They can’t happen spontaneously.
• The containers the reaction occurs
in will feel cold.
– Chemical cold packs