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A chemical equation is a representation of a chemical reaction with the help of symbols and formulae of the substances involved in the reaction. It is a chemical shorthand for representing the reacting substance or substances combining (the reactants) and the substance or substances formed as a result of the reaction (the products).
A Molecular equation is the one which shows the reactants combining and the products formed, in their elemental or molecular forms in a chemical reaction. An example of a molecular equation is the reaction between sodium and water to produce sodium hydroxide solution and hydrogen gas:
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
In this context, sodium (in elemental form) reacts with water (in molecular form) to produce sodium hydroxide (in molecular form) and hydrogen gas (in molecular form).
Word Equations for given Chemical Reactions
Write word equations for given chemical reactions
A word equation is a short form of expressing a chemical reaction by word. Chemical reactions can be summarized by word equations that show all the reactants and the products. This type of equation links together the names of the reactants and the products. For examples, the burning of magnesium in air to produce magnesium oxide can be represented by the following word equation:
Magnesium + Oxygen → Magnesium oxide
Another example is the reaction between sodium and chlorine to give sodium chloride (common salt)
Sodium + Chlorine → Sodium chloride
Equations like these sometimes give us some information about the products formed when different substances are reacted together. But equations can be made even more useful by writing them using chemical symbols and formulae.
Any method for representing a chemical reaction must meet basic certain requirements. These are:
- the chemical nature of the reactants as well as those of the products must be clear. The reactants can be in solid, gaseous, liquid or aqueous forms.
- the mole ratios in which the products are combined and the products are formed must be deducible. This means that atoms of the reactants and the products must be balanced.
- the direction of the reaction must be established. This means that it should be clearly shown which substances are the reactants and which ones are the products. This is normally done by separating the reactants from the products by an arrow. The arrow normally points from the reactants to the products.
Consider the reaction between potassium and water:
2K(s) + 2H2O (l) → 2KOH (aq) + H2 (g).
In this reaction, the three requirements have been met:
- The chemical nature of the reactants [potassium (solid); water (liquid)] and the products [potassium hydroxide (aqueous); hydrogen (gas)] has been shown.
- The mole ratios of the reactants and products are clearly shown: 2 moles of potassium combines with 2 moles of2water to produce 2 moles of potassium hydroxide and one mole of hydrogen gas.
- The reactants (potassium and water) and the products (potassium hydroxide and hydrogen) are separated by an arrow (→) which also indicates the direction of the reaction.
Formula Equations Using Chemical Symbols
Write formula equations using chemical symbols
Essentially, chemical reactions can be expressed in two forms. The chemical reaction can be expressed either as a word equation or as a formula (or symbolic) equation. We have already seen how chemical equations can be represented by words (word equation). The formula equation makes use of chemical symbols and formulae to represent a chemical reaction. An example is the reaction between iron and sulphur to form iron (II) sulphide: Fe + S → FeS
Balancing Chemical Equations
Balance chemical equations
A balanced chemical equation has an equal number of atoms of different elements of the reactants and the products on both sides of the equation. A balanced equation gives us more information about a reaction than we get from a simple word equation.
Below is a step-by-step approach to working out the balanced equation for the reaction:
- Write the chemical equation for the reaction with the correct symbols and formulae of the reactant(s) and the product(s).
- Identify different atoms of the different elements of the reactant(s) and the product(s).
- Check whether these different atoms are equal on both sides of the equation. Some atoms may balance each other directly.
- Balance the atoms on each sides of the equation by Hit and Trial Method.
- Add state symbols.
The reaction between hydrogen and oxygen to produce water:
Hydrogen + Oxygen → Water
H2 + O2 → H2O (not balanced)
The atoms involved in the reaction are hydrogen and oxygen. It is these atoms that we are going to balance. The atoms must be equal on both sides of the reaction equation. There are two hydrogen atoms on each side of the equation. But, as you can see there are two oxygen atoms on the left-hand side (LHS) of the equation and only one oxygen atom on the right-hand side (RHS). To balance oxygen atoms, we write 2 before water.
H2 + O2 → 2H2O (not balanced yet)
By introducing 2 before water, another problem has been created. Now we have 4 hydrogen atoms on the RHS but only 2 hydrogen atoms on the LHS. To equalize the number of hydrogen atoms we write 2 before hydrogen on the LHS.
2H2 + O2 → 2H2O (balanced).
You can still check to find out whether the atoms are balanced or not. Now look at the number of atoms on each side of the equation:
Now, the number of hydrogen and oxygen atoms is the same on both sides of the equation. This is because the atoms do not disappear during a reaction. They are neither created nor destroyed. They obey the Law of Conservation of Mass. When the numbers of different atoms are the same on the both sides, an equation is said to be balanced. Once the equation is balanced you can now add the state symbols.
This gives a standard and an acceptable chemical equation.
An equation which is not balanced is not correct. An unbalanced equation implies that the atoms have been created or destroyed. It is therefore, wrong and calculations based on it are certainly unreliable.
Remember that we cannot change the formulae of the substances involved in the reaction. These are fixed by the bonding in the substance itself. For instance, in attempt to balance the number of oxygen in water, H2O, we cannot write H2O2. We can only put a multiplying numbers before symbols and formulae, e.g. 2H2O.
Hydrogen burns in oxygen to form water. The equation for the reaction is:
2H2(g) + O2(g) →2H2O(l)
- How much oxygen is needed to burn 1g of hydrogen?
- How much water is formed when 5g of hydrogen is completely burned in oxygen? (Atomic weights: H = 1, O = 16)
a. Reaction equation:2H2(g)+ O2(g)→2H2O(l)
Molecular weights: 4 : 32
Reacting weights: 1g : Xg
The weight, X, of oxygen = 1×32⁄4= 8g
So, 1g of hydrogen needs 8g of oxygen
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