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State symbols and other information

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:

  1. (g) for gaseous compounds
  2. (l) for liquids
  3. (s) for solid compounds
  4. (aq) for an aqueous (water) solution

To show that heat is needed for a reaction, a Greek delta ( Δ ) 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, Δ H is less than zero and in an endothermic reaction, Δ 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 ( ZnCl 2 )and hydrogen gas. Write a balanced equation for this reaction.

  1. The reactants are zinc ( Zn ) and hydrochloric acid ( HCl ). The products are zinc chloride ( ZnCl 2 ) and hydrogen ( H 2 ).

  2. Zn + HCl ZnCl 2 + H 2
  3. 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 HCl a coefficient of 2 so that there will be two chlorine atoms on each side of the equation.

    Zn + 2 HCl ZnCl 2 + H 2

  4. When you look at the equation again, you will see that all the atoms are now balanced.

  5. 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.

    Zn ( s ) + 2 HCl ( aq ) ZnCl 2 ( aq ) + H 2 ( g )

Balance the following equation:

( NH 4 ) 2 SO 4 + NaOH NH 3 + H 2 O + Na 2 SO 4

In this example, the first two steps are not necessary because the reactants and products have already been given.

  1. With a complex equation, it is always best to start with atoms that appear only once on each side i.e. Na , N and S atoms. Since the S atoms already balance, we will start with Na and N atoms. There are two Na atoms on the right and one on the left. We will add a second Na atom by giving NaOH a coefficient of two. There are two N atoms on the left and one on the right. To balance the N atoms, NH 3 will be given a coefficient of two. The equation now looks as follows:

    ( NH 4 ) 2 SO 4 + 2 NaOH 2 NH 3 + H 2 O + Na 2 SO 4

  2. N , Na and S atoms balance, but O and H atoms do not. There are six O atoms and ten H atoms on the left, and five O atoms and eight H atoms on the right. We need to add one O atom and two H atoms on the right to balance the equation. This is done by adding another H 2 O molecule on the right hand side. We now need to check the equation again:

    ( NH 4 ) 2 SO 4 + 2 NaOH 2 NH 3 + 2 H 2 O + Na 2 SO 4

    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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Source:  OpenStax, Siyavula textbooks: grade 10 physical science [caps]. OpenStax CNX. Sep 30, 2011 Download for free at http://cnx.org/content/col11305/1.7
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