Unit 6: Chemical equations
Physical and chemical change
A substance can be changed by heating it, adding water to it, mixing another
substance with it, and so on. The change that takes place will be either chemical
change or a physical change.
Chemical change
In a chemical change, a new chemical substance is produced.
The difference between a mixture and a compound
Mixture: 2 substances are mixed together but not chemically bonded.
Compound: 2 substances are chemically bonded together
The signs of a chemical change
A chemical change is usually called a chemical reaction. You can tell when a chemical
reaction has taken place by these signs:
1. Once or more new chemical substances are formed
The new substance usually looks different from the starting substances.
2. Energy is taken in or given out during the reaction.
A change that gives out heat energy is called exothermic
A change that takes in heat energy is called endothermic
3. The change is usually difficult to reverse.
This means it will be hard to get back the raw materials of the reaction.
Physical change
If no new chemical substance is formed, a change is a physical change.
Equations for chemical reactions
The reaction between carbon and oxygen. When they react together, they form carbon
dioxide. Carbon and oxygen are the reactants. Carbon dioxide is the product of the
reaction.
C
C + O O O O
1 atom 1 molecule of 1 molecule of
of carbon oxygen carbon dioxide
Or in a shorter way, using symbols and numbers like this:
C + O² CO²
This short way to describe the reaction is called a chemical equation.
The reaction between hydrogen and oxygen:
O H O H
H H H H + O O H H
2 molecules 1 molecules 2 molecules
of hydrogen of oxygen of water
And the equation is:
2H² + O² 2H²O
On the left same as on the right, so:
On the left: On the right:
4 hydrogen atoms 4 hydrogen atoms
Adding state
2 Oxygen symbols
atoms 2 Oxygen atoms
Adding state symbols
You can show the state of the reactants and products by adding state symbols to the
equation:
- (s) for solid
- (l) for liquid
- (g) for gas
- (aq) for aqueous solution (solution in water)
The law of conservation of mass and balancing
chemical equations
First, let’s briefly discuss the theory.
The law of conservation of mass says that in a chemical reaction:
the mass of the products
must equal
the mass of the reactants
So what does this mean, exactly? In practical terms it means that:
the number of atoms of each element on the left-hand side (LHS) of a chemical
equation
must equal
the number of atoms of each element on the right-hand side (RHS) of a
chemical equation
Often, this is easy enough to check. For example, take the simple equation
below:
C + O2 ⟶ CO2
This is clearly a balanced equation: there’s one carbon and two oxygen atoms
on the LHS and the same on the RHS. So the equation is balanced.
But sometimes, the answer isn’t obvious. There are several ways to approach a
difficult equation. Our favourite method is to use a table to tally the number of
atoms of each element on the LHS of the equation and the RHS of the
equation.
How to balance equations
The steps are:
If the number of each atom on either side is the same, you’re all good, and the
equation is balanced.
If the number of each atom on either side is NOT the same, you need to use big
numbers in front of each element or compound to even them out.
Every time you add a big number, you multiply the whole compound by that number,
and so you adjust the numbers in the table accordingly.
Follow this balancing order:
1) Metals
2) Non metals
3) Oxygen
4) Hydrogen
