The state (phase) of the compounds can be expressed in the chemical equation. This is done by placing the correct label on the right hand side of the formula. There are only four labels that can be used:
(g) for gaseous compounds
(l) for liquids
(s) for solid compounds
(aq) for an aqueous (water) solution
To show that heat is needed for a reaction, a Greek delta (
) is placed above the arrow.
You may remember from
Physical and chemical change that energy cannot be created or destroyed during a chemical reaction but it may change form. In an exothermic reaction,
H is less than zero and in an endothermic reaction,
H is greater than zero. This value is often written at the end of a chemical equation.
Solid zinc metal reacts with aqueous hydrochloric acid to form an aqueous solution of zinc chloride (
)and hydrogen gas. Write a balanced equation for this reaction.
The reactants are zinc (
) and hydrochloric acid (
). The products are zinc chloride (
) and hydrogen (
).
You will notice that the zinc atoms balance but the chlorine and hydrogen atoms don't. Since there are two chlorine atoms on the right and only one on the left, we will give
a coefficient of 2 so that there will be two chlorine atoms on each side of the equation.
When you look at the equation again, you will see that all the atoms are now balanced.
In the initial description, you were told that zinc was a metal, hydrochloric acid and zinc chloride were in aqueous solutions and hydrogen was a gas.
Balance the following equation:
In this example, the first two steps are not necessary because the reactants and products have already been given.
With a complex equation, it is always best to start with atoms that appear only once on each side i.e.
,
and
atoms. Since the
atoms already balance, we will start with
and
atoms. There are two
atoms on the right and one on the left. We will add a second
atom by giving
a coefficient of two. There are two
atoms on the left and one on the right. To balance the
atoms,
will be given a coefficient of two. The equation now looks as follows:
,
and
atoms balance, but
and
atoms do not. There are six
atoms and ten
atoms on the left, and five
atoms and eight
atoms on the right. We need to add one
atom and two
atoms on the right to balance the equation. This is done by adding another
molecule on the right hand side. We now need to check the equation again:
The equation is now balanced.
The following video explains some of the concepts of balancing chemical equations.
Khan academy video on balancing chemical equations
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