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    This module is included inLens: Siyavula: Chemistry (Gr. 10-12)
    By: Siyavula

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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

Occasionally, a catalyst is added to the reaction. A catalyst is a substance that speeds up the reaction without undergoing any change to itself. In a chemical equation, this is shown by using the symbol of the catalyst above the arrow in the equation.

To show that heat was needed for the reaction, a Greek delta (ΔΔ) is placed above the arrow in the same way as the catalyst.

Tip:

You may remember from chapter (Reference) that energy cannot be created or destroyed during a chemical reaction but it may change form. In an exothermic reaction, ΔΔH is less than zero and in an endothermic reaction, ΔΔH is greater than zero. This value is often written at the end of a chemical equation.

Exercise 1: Balancing chemical equations 5

Solid zinc metal reacts with aqueous hydrochloric acid to form an aqueous solution of zinc chloride (ZnCl22)and hydrogen gas. Write a balanced equation for this reaction.

Solution

  1. Step 1. Identify the reactants and products and their chemical formulae :

    The reactants are zinc (Zn) and hydrochloric acid (HCl). The products are zinc chloride (ZnCl22) and hydrogen (H22).

  2. Step 2. Place the reactants on the left of the equation and the products on the right hand side of the arrow :

    Zn + HCl ZnCl2 + H2

  3. Step 3. Balance the equation :

    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 + 2HCl ZnCl2 + H2

  4. Step 4. Check that all the atoms balance :

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

  5. Step 5. Ensure all details (e.g. state symbols) are added :

    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) + 2HCl(aq) ZnCl2(aq) + H2(g)

Exercise 2: Balancing chemical equations 5 (advanced)

Balance the following equation:

(NH4)2SO4 + NaOH NH3 + H2O + Na2SO4

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

Solution

  1. Step 1. Balance the equation :

    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, NH33 will be given a coefficient of two. The equation now looks as follows:

    (NH4)2SO4 + 2NaOH 2NH3 + H2O + Na2SO4

  2. Step 2. Check that all atoms balance :

    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 H22O molecule on the right hand side. We now need to check the equation again:

    (NH4)2SO4 + 2NaOH 2NH3 + 2H2O + Na2SO4

    The equation is now balanced.

The following video explains some of the concepts of balancing chemical equations.

Figure 1
Khan Academy video on balancing chemical equations

Balancing more advanced chemical equations

Write balanced equations for each of the following reactions:

  1. Al2O3(s) + H2SO4(aq) Al2(SO4)3(aq) + 3H2O (l)
  2. Mg(OH)2(aq) + HNO3(aq) Mg(NO3)2(aq) + 2H2O (l)
  3. Lead(ll)nitrate solution reacts with potassium iodide solution.
  4. When heated, aluminium reacts with solid copper oxide to produce copper metal and aluminium oxide (Al2O3).
  5. When calcium chloride solution is mixed with silver nitrate solution, a white precipitate (solid) of silver chloride appears. Calcium nitrate (Ca(NO3)2) is also produced in the solution. Click here for the solution

Figure 2

Summary

  • A chemical equation uses symbols to describe a chemical reaction.
  • In a chemical equation, reactants are written on the left hand side of the equation and the products on the right. The arrow is used to show the direction of the reaction.
  • When representing chemical change, it is important to be able to write the chemical formula of a compound.
  • In any chemical reaction, the law of conservation of mass applies. This means that the total atomic mass of the reactants must be the same as the total atomic mass of the products. This also means that the number of atoms of each element in the reactants must be the same as the number of atoms of each element in the product.
  • If the number of atoms of each element in the reactants is the same as the number of atoms of each element in the product, then the equation is balanced.
  • If the number of atoms of each element in the reactants is not the same as the number of atoms of each element in the product, then the equation is not balanced.
  • In order to balance an equation, coefficients can be placed in front of the reactants and products until the number of atoms of each element is the same on both sides of the equation.

Summary exercise

Balance each of the following chemical equations:

  1. NH4 + H2O NH4OH
  2. Sodium chloride and water react to form sodium hydroxide, chlorine and hydrogen.
  3. Propane is a fuel that is commonly used as a heat source for engines and homes. Balance the following equation for the combustion of propane: C3H8(l) + O2(g)CO2(g) + H2O(l)
     
    Click here for the solution
  4. Aspartame, an artificial sweetener, has the formula C14H18N2O5. Write the balanced equation for its combustion (reaction with O2) to form CO2 gas, liquid H2O, and N2 gas. Click here for the solution
  5. Fe2(SO4)3 + K(SCN) K3Fe(SCN)6 + K2SO4 Click here for the solution
  6. Chemical weapons were banned by the Geneva Protocol in 1925. According to this protocol, all chemicals that release suffocating and poisonous gases are not to be used as weapons. White phosphorus, a very reactive allotrope of phosphorus, was recently used during a military attack. Phosphorus burns vigorously in oxygen. Many people got severe burns and some died as a result. The equation for this spontaneous reaction is: P4(s) + O2(g) P2O5(s)
    1. Balance the chemical equation.
    2. Prove that the law of conservation of mass is obeyed during this chemical reaction.
    3. Name the product formed during this reaction.
    4. Classify the reaction as endothermic or exothermic. Give a reason for your answer.
    5. Classify the reaction as a synthesis or decomposition reaction. Give a reason for your answer. Click here for the solution
    (DoE Exemplar Paper 2 2007)

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