Chemical Reactions and Chemical Quantities: Study Notes

Chemical Reactions and Chemical Quantities

Chemical Equations

Chemical equations are symbolic representations of chemical reactions, showing the identities and quantities of substances involved. They are fundamental tools for describing chemical changes and for performing quantitative calculations in chemistry.

• Chemical equation: Shows the identities and quantities of substances involved in a reaction.

• Example: $2H_2O \rightarrow 2H_2 + O_2$

Components of Chemical Equations

• Reactants: Substances initially consumed in the reaction.

• Products: Substances formed as a result of the reaction.

• Example: $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$

Symbols in Chemical Equations

• States of Matter:

  • Gas (g)

  • Liquid (l)

  • Solid (s)

  • Aqueous (aq): dissolved in water

• Reaction Conditions:

  • High temperature (Δ)

  • Pressure, catalysts, etc., are written above or below the arrow.

Balanced Chemical Equations

Balanced chemical equations are required to follow the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction. Therefore, the total number of each type of atom on the reactant side must equal the total number on the product side.

• Never introduce extraneous chemical species to balance.

• Never change a formula for the purpose of balancing an equation.

• Cannot change subscripts in chemical formulas.

Balancing Equation Strategy

• Balance atoms that occur in more complex substances first.

• Balance free elements last.

• Balance unchanged polyatomic ions or other groups of atoms as units.

• Fractional coefficients are acceptable and can be cleared at the end by multiplication.

Combustion Reactions

Combustion reactions are a class of chemical reactions that occur when a substance reacts with oxygen, often producing heat and light.

• Combustion reaction: A reaction that occurs when a substance reacts with oxygen and another substance.

• Hydrocarbons: Molecular compounds composed of only hydrogen and carbon.

• General chemical formula: $C_xH_y$

• The products of complete combustion are carbon dioxide and water.

Stoichiometry and Chemical Calculations

Stoichiometry involves quantitative relationships between reactants and products in a chemical reaction, allowing chemists to predict the amounts of substances consumed and produced.

• Formula equations: Used to solve questions of how much or how many.

• Mole-to-mole ratio: Key conversion factor known as the stoichiometric factor.

Steps for Stoichiometric Calculations

1. Convert mass of chemical species to moles using its molar mass.

2. Convert moles of one chemical species to moles of another using a mole ratio from the balanced equation.

3. Convert moles of chemical species back to mass using its molar mass if required.

The Limiting Reactant Concept

In many reactions, one reactant is completely consumed before the others, limiting the amount of product that can be formed. This reactant is called the limiting reactant.

• Limiting reactant: The reactant that is entirely consumed when a reaction goes to completion.

• Excess reactant: Any other reactant that is not completely consumed.

• The maximum amount of product that can be formed is determined by the amount of limiting reactant.

Percent Yield

Percent yield is a measure of the efficiency of a chemical reaction, comparing the actual amount of product obtained to the theoretical maximum predicted by stoichiometry.

• Theoretical yield: The stoichiometric amount of product expected.

• Actual yield: The experimentally determined amount of product formed.

• Percent yield: $\text{Percent yield} = \frac{\text{Actual yield}}{\text{Theoretical yield}} \times 100\%$

Why Actual Yield Differs from Theoretical Yield

• Side reactions may occur, producing undesired products.

• Losses during product recovery and purification.

• Incomplete reactions or measurement errors.