CHAPTER 8 Writing Chemical Equations © 2013 Marshall Cavendish International (Singapore) Private Limited.

CHAPTER 8Writing Chemical Equations

© 2013 Marshall Cavendish International (Singapore) Private Limited

8.1 Chemical Equations

8.2 Ionic Equations

Writing Chemical EquationsChapter 8

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Learning Outcomes

• interpret chemical equations with state symbols;

• write balanced chemical equations with state symbols.

At the end of this section, you should be able to:

Chemical Equations8.1

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What is a Chemical Equation?

A chemical equation shows what happens in a chemical reaction. It tells us

• which reactants and products are involved in the reaction;

• the relative amounts of reactants and products;

• the physical states of the reactants and products of the reaction.

sodium + chlorine → sodium chloride (word equation)

2 Na + Cl2 → 2 NaCl (chemical equation)

Example:

Chemical Equations8.1

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Balancing a Chemical Equation

A balanced chemical equation must contain equal numbers of atoms of each element on both sides of the equation.

The reactant(s) are written on the left-hand side of the equation.

The product(s) are written on the right-hand side of the equation.

The state symbols(s) – solid(g) – gas(l) – liquid(aq) – aqueous solution

2H2(g) + O2(g) → 2H2O(l)

Chemical Equations8.1

The chemical equation is balanced by adding a number in front of the chemical formula. This is the same as multiplying the formula by that number. 5

Writing a Balanced Chemical Equation

Step 1: Write down the chemical formulae of the reactants and products to get the chemical equation.

Example: Reacting hydrogen and oxygen to get water

H2 + O2 H2O

Chemical Equations8.1

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Step 2: Check the number of atoms of each element in the formulae on both sides of the equation. If the equation is not balanced, proceed to step 3.

Writing a Balanced Chemical Equation

H2 + O2 H2O

Chemical Equations8.1

Left-hand side Right-hand side

2 H atoms 2 H atoms

2 O atoms 1 O atom

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Step 3a: To balance the number of oxygen atoms, put a ‘2’ in front of H2O. This means that two molecules of water are formed.

The equation is still not balanced because there are four hydrogen atoms on the right-hand side and only two on the left-hand side.

Writing a Balanced Chemical Equation

H2 + O2 2H2O

Chemical Equations8.1

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Step 3b: To balance the equation, we need to put a ‘2’ in front of H2.

Writing a Balanced Chemical Equation

2H2 + O2 2H2O

Chemical Equations8.1

Left-hand side Right-hand side

4 H atoms 4 H atoms

2 O atoms 2 O atoms

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Step 4: Add the state symbols to indicate the physical state of each reactant and product.

Writing a Balanced Chemical Equation

2H2(g) + O2(g) 2H2O(l)

Chemical Equations8.1

State Symbol

Solid (s)

Liquid (l)

Gas (g)

Aqueous (aq)

Aqueous means dissolved in water

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8.1 Chemical Equations

8.2 Ionic Equations

Writing Chemical EquationsChapter 8

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Learning Outcome

• write ionic equations with state symbols.

At the end of this section, you should be able to:

Ionic Equations8.2

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What is an Ionic Equation?

An ionic equation is a simplified chemical equation that shows the reactions involving ions in aqueous solution.

In chemical reactions, there are ions that do not take part in the chemical reaction.

Such ions are called spectator ions.

Spectator Ions

Ionic Equations8.2

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Writing an Ionic Equation

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Let’s look at the reaction between hydrochloric acid and sodium hydroxide.

hydrochloric acid + sodium hydroxide sodium chloride + water

H+(aq) + Cl–(aq) Na+(aq) + OH–(aq) Na+(aq) + Cl–(aq)

Na+(aq) and Cl–(aq) are spectator ions. Removing them from the equation, we are left with:

H+(aq) + OH–(aq) → H2O(l)

Ionic Equations8.2

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How to Write an Ionic Equation

Step 1:Write a balanced chemical equation including state symbols.

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Ionic Equations8.2

Example: Reaction of hydrochloric acid with sodium hydroxide

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+ H2O(l)

Step 2:Rewrite the chemical equation in terms of ions.

HCl

H+(aq) + Cl–(aq)

NaOH

Na+(aq) + OH–(aq)+

Ionic Equations8.2

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How to Write an Ionic Equation

H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq)

→ Na+(aq) + Cl–(aq) + H2O(l)

Step 3:Cancel out the spectator ions.

Step 4:Write the ionic equation.

H+(aq) + OH–(aq) H2O(l)

Ionic Equations8.2

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• Insoluble solids (e.g metals, insoluble salts)

• Covalent liquids (such as water)

• Gases

Do not break them into ions when writing an ionic equation. These substances should be written in full, for example, H2O(l) and AgCl (s).

Substances that do not ionise in solution:

Ionic Equations8.2

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Example:Reaction of sodium chloride with silver nitrate solution

Step 1:Write a balanced chemical equation including state symbols.

NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)

Ionic Equations8.2

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Step 2:Rewrite the chemical equation in terms of ions.

Na+(aq) + Cl–(aq) +

→ AgCl(s) + Na+(aq) + NO3–(aq)

Ag+(aq) + NO3–(aq)

Ionic Equations8.2

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Step 3:Cancel out the spectator ions.

Step 4:Write the ionic equation.

Na+(aq) + Cl–(aq) + Ag+(aq) + NO3–(aq)

→ AgCl(s) + Na+(aq) + NO3– (aq)

Cl–(aq) + Ag+(aq) → AgCl(s)

Can you give another example of ionic precipitation?

Ionic Equations8.2

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Aqueous silver nitrate consists of Ag+ and NO3

– ions.

Silver chloride precipitates as a white solid.

Aqueous sodium chloride consists of Na+ and Cl–

ions.

Aqueous sodium nitrate consists of Na+(aq) + NO3

– (aq) which remained unchanged throughout the reaction.

URL

Ionic Equations8.2

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Writing Chemical EquationsChapter 8

Concept Map

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Writing Chemical EquationsChapter 8

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Acknowledgements

(slide 1) chemical reactions © Aushulz | Wikimedia Commons | CC BY 3.0 (http://creativecommons.org/licenses/by/3.0/deed.en)

(slide 22) © Marshall Cavendish International (Singapore)

The URLs are valid as at 15 October 2012.