Chemical Reactions and Stoichiometry
1. Chemical Reactions
Chemical reactions are processes that transform one set of substances (reactants) into another set of substances (products). These reactions involve the breaking and forming of chemical bonds, resulting in changes in the chemical composition of substances.
Example: The combustion of methane (CH₄) in the presence of oxygen (O₂) produces carbon dioxide (CO₂) and water (H₂O). The reaction can be represented as: CH₄ + 2O₂ → CO₂ + 2H₂O.
2. Stoichiometry
Stoichiometry is the quantitative study of reactants and products in a chemical reaction. It involves the use of balanced chemical equations to determine the amounts of substances involved in a reaction.
Example: In the reaction 2H₂ + O₂ → 2H₂O, stoichiometry tells us that 2 moles of hydrogen (H₂) react with 1 mole of oxygen (O₂) to produce 2 moles of water (H₂O).
3. Balanced Chemical Equations
A balanced chemical equation shows the correct ratios of reactants and products, ensuring that the law of conservation of mass is obeyed. This means that the number of atoms of each element must be the same on both sides of the equation.
Example: The reaction between sodium (Na) and chlorine (Cl₂) to form sodium chloride (NaCl) is balanced as: 2Na + Cl₂ → 2NaCl.
4. Moles and Molecular Weight
The mole is a unit used to express the amount of substance in chemistry. One mole of a substance contains 6.022 x 10²³ particles (Avogadro's number). The molecular weight of a substance is the mass of one mole of that substance, typically expressed in grams per mole (g/mol).
Example: The molecular weight of water (H₂O) is approximately 18 g/mol. This means that one mole of water has a mass of 18 grams.
5. 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. The other reactants are in excess and are not completely used up.
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, while some oxygen will remain unreacted.
6. 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 maximum amount of product that could be obtained) 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 (85/100) x 100 = 85%.
7. Reaction Types
Chemical reactions can be classified into several types, including synthesis (combination), decomposition, single replacement, double replacement, and combustion. Each type has a specific pattern of reactants and products.
Example: A synthesis reaction involves the combination of two or more substances to form a single compound, such as 2H₂ + O₂ → 2H₂O. A decomposition reaction involves a single compound breaking down into two or more simpler substances, such as 2H₂O₂ → 2H₂O + O₂.