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## Wednesday, 15 July 2015

### Writing chemical and ionic equations

1. Writing chemical equation
There are generally three steps to writing a chemical equation:
• Determine the reactants and products.
• Write out the formula of the reactants and products.
• Balance the equation.
Note that the first two steps should come naturally, and you should be able to write the equation almost immediately after some practice!

Example. Write a balanced equation for the reaction between calcium carbonate and hydrochloric acid.
Step 1: An acid and carbonate reaction will produce carbon dioxide, water, and salt. In this case, the salt is calcium chloride.

Step 2:
Reactants are calcium carbonate, hydrochloric acid, and their formulae are determined as follows.
 How to find formula - calcium carbonate
 How to find formula  - hydrochloric acid

Products are carbon dioxide, water and calcium chloride.
Formula of carbon dioxide is CO2, and water is H2O.
Formula of calcium chloride is determined as follows:

 How to find formula - calcium chloride

Step 3: Balance the equation.
calcium carbonate + hydrochloric acid ---> calcium chloride + water + carbon dioxide
CaCO3 + HCl ---> CaCl2 + H2O + CO2
Balancing both sides, you get the final answer:
CaCO3 + 2HCl ---> CaCl2 + H2O + CO2

2. State symbols
To represent the state in a chemical equation, we add in state symbols.
(aq) represents aqueous, which means dissolved in water.;
(l) represents liquid state;
(g) represents gaseous state;
(s) represents solid state.

Let's use the above equation, and add in the state symbols. The following is the balanced chemical equation, which includes state symbol.
CaCO3 (s) + 2HCl (aq) ---> CaCl2 (aq)+ H2O (l)+ CO2(g)

3. Ionic equations
As the name suggest, it is related to ionic compounds. Remember that you only split ionic compounds in the liquid, aqueous or gaseous state into their constituent ions. This is because in the solid states, the ions are held together tightly.

Example. Write the ionic equation for CaCO3 (s) + 2HCl (aq) ---> CaCl2 (aq)+ H2O (l)+ CO2(g).
Step 1: Identify the ionic compounds. They are CaCO3, HCl and CaCl2
Step 2: Identify the ionic compounds that are in liquid, aqueous or gaseous state. They would need to be broken down into the constituent ions.
Only HCl and CaClneeds to be broken down into their constituent ions.
Step 3: Split the liquid, aqueous or gaseous ionic compounds into their constituent ions.
CaCO3 (s) + 2H+ + 2Cl- (aq) ---> Ca2+ (aq) + 2Cl- (aq)+ H2O (l)+ CO2(g)
Step 4: Cancel the same ions that are found on both the reactant and product side.
CaCO3 (s) + 2H+ + 2Cl- (aq) ---> Ca2+ (aq) + 2Cl- (aq)+ H2O (l)+ CO2(g)
Hence, the ionic equation is:

CaCO3 (s) + 2H+ ---> Ca2+ (aq) + H2O (l)+ CO2(g)

Questions
- Quizzes and Test on Writing Chemical and Ionic Equations