Chemical Equations
Chemical reactions are represented in a simplified form using chemical equations.
Key Terms
- Reactants: Substances that participate in the reaction, placed on the left side of the arrow.
- Products: Substances formed during the reaction, shown on the right side of the arrow.
- Coefficients: Numbers placed before chemical formulas to indicate the quantity of molecules or ions. If the number is 1, it is usually omitted.
- Subscripts: Small numbers written at the lower right of chemical symbols, indicating the number of atoms of an element. A subscript of 1 is not written.
In chemical equations:
- The "+" sign means "reacts with."
- The arrow "→" means "produces" or "yields."
Chemical Equations
A chemical equation is a short way of representing the relative amounts of reactants and products involved in a chemical reaction.
Steps in Writing a Chemical Equation
- Word Equation: Names of the reactants and products are written out.
- Skeleton Equation: Chemical formulas and symbols are used to represent the reactants and products.
- Balanced Equation: Coefficients are added to the skeleton equation to balance the number of atoms on both sides.
Example
Reaction: Hydrogen burns in oxygen to form water.
Word Equation:
Hydrogen + Oxygen → Water
This reads as "Hydrogen reacts with oxygen to produce water."
Skeleton Equation:
H2 + O2 → H2O
Balanced Equation:
2H2 + O2 → 2H2O
Balanced Equation
For a chemical equation to be balanced, there must be an equal number of atoms of each element on both sides of the arrow. Once this is achieved, the equation is said to be balanced.
Example: Formation of Water
In the skeleton equation for water, two hydrogen atoms react with two oxygen atoms on the reactant side. However, on the product side, water (H2O) only contains two hydrogen atoms and one oxygen atom.
This means there is one fewer oxygen atom on the product side. To fix this, we add coefficients (whole numbers) in front of the reactants or products. Note: Subscripts should never be changed, as this would alter the identity of the substance.
Steps to Balance:
- Place a "2" in front of H2O to balance the number of oxygen atoms:
H2 + O2 → 2H2O
- Now, the hydrogen atoms are unbalanced. Place a "2" in front of H2:
2H2 + O2 → 2H2O
The equation is now balanced with four hydrogen atoms and two oxygen atoms on both sides.
Counting Atoms of Elements
To count atoms in a chemical formula, multiply the coefficient by the subscript of each element.
Example: 2Al2(SO4)3
- Aluminium (Al):
Coefficient = 2, Subscript = 2.
Number of aluminium atoms = 2 × 2 = 4 atoms. - Sulphur (S):
Coefficient = 2, Subscript inside parentheses = 1, Subscript outside = 3.
Number of sulphur atoms = 2 × (1 × 3) = 6 atoms. - Oxygen (O):
Coefficient = 2, Subscript inside parentheses = 4, Subscript outside = 3.
Number of oxygen atoms = 2 × (4 × 3) = 24 atoms.
Important: Follow the steps carefully when counting atoms to avoid mistakes. Although shortcuts exist, using a systematic approach ensures accuracy.
Physical States of Elements
The physical state of substances in a chemical reaction is shown by placing symbols to the right of the chemical formula:
- (g) — gas
- (l) — liquid
- (s) — solid
- (aq) — aqueous (dissolved in water)
Example:
2H2(g) + O2(g) → 2H2O(l)
Steps in Writing a Balanced Equation
- Step 1: Write down the reactants and products. If the products are unknown, they can often be predicted based on the reactants.
- Step 2: Ensure that the chemical formulas of all substances are correct. Remember, diatomic gases like Cl2, N2, I2, O2, Br2, and H2 must be written with a subscript of 2.
- Step 3: Balance the equation by placing coefficients in front of compounds or elements so that the number of atoms of each element is the same on both sides of the equation.
Limiting Reactant
The limiting reactant is the substance that is completely used up during a chemical reaction. It is not available in enough quantity to react with all other reactants.
Once the limiting reactant is consumed, the reaction stops. Any reactants left over are called excess reactants.
The amount of product formed depends on the amount of the limiting reactant.
Steps in Solving a Limiting Reactant Problem
- Step 1: Write a balanced chemical equation for the reaction.
- Step 2: Convert the given quantities of both reactants to moles.
- Step 3: Calculate the moles of product that can be formed from each reactant.
- Step 4: Identify the limiting reactant as the one that produces the least amount of product.
- Step 5: Use the moles of product from the limiting reactant to determine the mass of the product formed.