106 - 118 Brief Resume Chemistry

• There are physical and chemical changes.
• In a physical change there is no change of substance.

• In a chemical change new chemical substances are made.
• Chemical and physical changes can be exothermic or endothermic.
• Exothermic changes involve energy, usually heat, being given out; endothermic changes take energy in.
The law of the conservation of mass:

The total mass of all the products of a chemical reaction is equal to the total mass of all the reactants.
A Balanced equation occurs when the numbers of each type of atom are the same on both the reactant side and the
product side of the equation.
In a chemical reaction the reactants are changed into the products.

It is possible to write a word equation for a chemical reaction indicating the new substances formed from the reactants.
In a chemical reaction there is conservation of mass. Matter cannot be created or destroyed in a chemical reaction.
It is possible to write a chemical equation using formulae for a reaction that has taken place
A chemical equation must be balanced so that there are the same number of atoms of each element involved on both
sides of the equation
TYPES OF CHEMICAL REACTIONS

Synthesis (or direct combination) reactions occur where two or more substances react together to form just one product.
There is one very important synthesis reaction which is endothermic: namely photosynthesis. This reaction is essential for
life on Earth. It takes place in the green leaves of plants and requires energy from sunlight. It is a photochemical reaction.
Neutralisation and precipitation.- Neutralisation reactions involve acids. When acids react with bases or alkalis, their
acidity is destroyed. Th ey are neutralised and a salt is produced. Precipitation reactions involve the formation of an
insoluble product.
Precipitation is the sudden formation of a solid, either: • when two solutions are mixed, or • when a gas is bubbled into a
solution.
A displacement reaction occurs because a more reactive element will displace a less reactive one from a solution of one
of its compounds.
Combustion reactions are of great importance and can be very useful or destructive. Combustion of a substance involves
its reaction with oxygen and the release of energy. • These reactions are exothermic and oft en involve a flame.
•Combustion in which a flame is produced is described as burning.
• If a substance gains oxygen during a reaction, it is oxidised. • If a substance loses oxygen during a reaction, it is reduced.
A reducing agent is an element or compound that will remove oxygen from other substances. • Th e commonest reducing
agents are hydrogen, carbon and carbon monoxide
An oxidising agent is a substance that will add oxygen to another substance. • Th e commonest oxidising agents are
oxygen (or air), hydrogen peroxide, potassium manganate(VII) and potassium dichromate(VI).
Remember that, in the process of acting as a reducing agent, that substance will itself be oxidised. The reducing agent
will gain the oxygen it is removing from the other compound. The reverse is true for an oxidising agent.
Th ere are two common examples of oxidation reactions that we might meet in our everyday lives. • Corrosion. If a metal
is reactive, its surface may be attacked by air, water, or other substances around it. The eff ect is called corrosion.
Rancidity. Oxidation also has damaging effects on food. When the fats and oils in butter and margarine are oxidised, they
become rancid.
There is a range of diff erent types of chemical reaction.

Synthesis reactions involve the making of a more complex substance from simpler ones.
Decomposition is the reverse of synthesis: a substance is split into simpler ones. Th is can be brought about by heat in
thermal decomposition
Acids and alkalis take part in neutralisation reactions
Precipitation is the sudden formation of a solid during a reaction.
Combustion reactions involve an exothermic reaction with oxygen
Oxidation is the gain of oxygen during a reaction
Reduction is the opposite of oxidation; a substance loses oxygen during reduction.
Chemical equations can be made more useful by including symbols that give us this information. Th ese are called state
symbols.
They can show clearly when a gas or a precipitate is produced in a reaction (the points of particular interest are shown in
bold type)
Oxidation is the loss of electrons.
Reduction is the gain of electrons
State symbols can be used in chemical reactions to give further information.
Ionic equations can be written for many reactions. These equations help clarify which species (ions) are actually reacting;
the ‘spectator’ ions are omitted from the equation.
Oxidation and reduction can be defined in terms of the loss or gain of electrons during a reaction
Oxidation is the loss of electrons. Reduction is the gain of electrons.

An increase in oxidation state also means that oxidation has taken place. A decrease in oxidation state means that
reduction has happened.

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106 - 118 Brief Resume Chemistry

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• There are physical and chemical changes.

• In a physical change there is no change of substance.

The law of the conservation of mass:

In a chemical reaction the reactants are changed into the products.

TYPES OF CHEMICAL REACTIONS

There is a range of diff erent types of chemical reaction.

Oxidation is the loss of electrons. Reduction is the gain of electrons.

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