Stoichiometry and Chemical Calculations
1. Stoichiometry
Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. It allows chemists to predict the amounts of substances involved in a reaction based on balanced chemical equations.
2. Balanced Chemical Equations
A balanced chemical equation shows the correct ratios of reactants and products. The law of conservation of mass states that the total mass of the reactants must equal the total mass of the products. Balancing an equation ensures that the number of atoms of each element is the same on both sides of the equation.
Example: The combustion of methane (CH₄) can be represented by the balanced equation:
CH₄ + 2O₂ → CO₂ + 2H₂O
This equation shows that 1 mole of methane reacts with 2 moles of oxygen to produce 1 mole of carbon dioxide and 2 moles of water.
3. Moles and Avogadro's Number
The mole is a unit used to express the amount of a substance. One mole of a substance contains 6.022 x 10²³ particles (atoms, molecules, ions, etc.). This number is known as Avogadro's number. The molar mass of a substance is the mass of one mole of that substance, typically expressed in grams per mole (g/mol).
Example: The molar mass of water (H₂O) is approximately 18 g/mol. This means that 1 mole of water weighs 18 grams and contains 6.022 x 10²³ water molecules.
4. Limiting Reactant
The limiting reactant is the reactant that is completely consumed in a chemical reaction and limits the amount of product that can be formed. Once the limiting reactant is used up, the reaction stops.
Example: In the reaction 2H₂ + O₂ → 2H₂O, if you have 2 moles of hydrogen and 1 mole of oxygen, hydrogen is the limiting reactant because it will be completely consumed before all the oxygen is used up.
5. Percent Yield
Percent yield is a measure of the efficiency of a chemical reaction. It is calculated by dividing the actual yield (the amount of product obtained) by the theoretical yield (the amount of product expected based on the stoichiometry) and multiplying by 100.
Example: If the theoretical yield of a reaction is 100 grams and the actual yield is 85 grams, the percent yield is:
(85 g / 100 g) × 100 = 85%
6. Stoichiometric Calculations
Stoichiometric calculations involve using the balanced chemical equation to determine the amounts of reactants and products. These calculations can be used to find the moles, mass, or volume of substances involved in a reaction.
Example: Given the balanced equation 2H₂ + O₂ → 2H₂O, if you start with 4 grams of hydrogen, how many grams of water will be produced?
First, convert grams of hydrogen to moles: (4 g H₂) / (2 g/mol) = 2 mol H₂
Using the stoichiometry, 2 moles of hydrogen produce 2 moles of water.
Convert moles of water to grams: (2 mol H₂O) × (18 g/mol) = 36 g H₂O
7. Reaction Stoichiometry in Solutions
Reaction stoichiometry can also be applied to reactions in solution. The concentration of a solution is often expressed in molarity (M), which is the number of moles of solute per liter of solution. Stoichiometric calculations can be used to determine the volumes and concentrations of reactants and products in solution.
Example: If you have a 0.5 M solution of HCl and you need to react it with a 0.2 M solution of NaOH, how many liters of each solution are needed to produce 0.1 moles of NaCl?
The balanced equation is HCl + NaOH → NaCl + H₂O.
Since 1 mole of HCl reacts with 1 mole of NaOH to produce 1 mole of NaCl, you need 0.1 moles of each reactant.
Volume of HCl solution needed: (0.1 mol) / (0.5 M) = 0.2 L
Volume of NaOH solution needed: (0.1 mol) / (0.2 M) = 0.5 L