Chemical Equations and Chemical Reactions

Introduction to Chemical Equations

Chemical changes are represented by chemical equations, which summarize the transformation of reactants into products. These equations are fundamental tools in chemistry for describing and analyzing chemical reactions.

• Reactants: Substances present on the left-hand side of the arrow; they undergo change during the reaction.

• Products: Substances present on the right-hand side of the arrow; they are formed as a result of the reaction.

Example chemical equations:

Overall, a chemical change expressed as a chemical equation is called a chemical reaction. In chemical reactions, bonds are broken and new bonds are formed.

• Law of Conservation of Mass: Atoms are neither created nor destroyed in a chemical reaction. The number of each type of atom must be the same on both sides of the equation.

Physical States in Chemical Equations

The physical state of each substance in a chemical equation is indicated by a symbol to the lower right of the formula:

• (s): Solid

• (l): Liquid

• (g): Gas

• (aq): Aqueous (dissolved in water)

Example:

Writing and Balancing Chemical Equations

Steps for Writing and Balancing

Balancing chemical equations ensures that the law of conservation of mass is obeyed. The process involves the following steps:

1. Identify the reaction: Determine the reactants, products, and their physical states.

2. Write the unbalanced equation: Summarize the reaction with correct formulas.

3. Balance the equation by inspection: Adjust coefficients to ensure the same number of each type of atom appears on both sides. Start with the most complex molecule and proceed to the simplest. Do not change the chemical formulas of reactants or products.

Note: Only coefficients (whole numbers in front of formulas) are changed to balance equations, not subscripts within formulas.

Example: Balancing a Chemical Equation

Consider the reaction of methane with oxygen:

• Unbalanced:

• Balanced:

Here, the number of each atom is the same on both sides of the equation.

Stoichiometry: Quantitative Relationships in Chemical Reactions

Definition and Importance

Stoichiometry is the calculation of the quantities of reactants and products involved in a chemical reaction. It is based on the balanced chemical equation and allows chemists to predict the amounts of substances consumed and produced.

• The coefficients in a balanced equation represent the relative minimum number of molecules or moles needed for the reaction to occur.

• We often use mole ratios derived from these coefficients to perform calculations.

Steps in Stoichiometric Calculations

1. Write and balance the chemical equation.

2. Convert the given mass of a substance to moles using its molar mass.

3. Use the mole ratio from the balanced equation to relate moles of one substance to moles of another.

4. Convert moles of the desired substance to mass, if required.

Example Problem: Combustion of Propane

If propane () reacts with oxygen to produce water and carbon dioxide, how much is needed to react with 96.1 g of propane?

1. Write the balanced equation:

2. Convert 96.1 g to moles: moles $\mathrm{C_3H_8}$

3. Use the mole ratio: mole reacts with moles

4. Calculate moles of needed: moles $\mathrm{O_2}$

5. Convert moles to grams (if needed): g $\mathrm{O_2}$

Sample Problems and Applications

Balancing and Stoichiometry Practice

• Aluminum and Cobalt(II) Nitrate Reaction: Aluminum metal reacts with aqueous cobalt(II) nitrate to form aqueous aluminum nitrate and cobalt metal. Find the stoichiometric coefficient for aluminum in the balanced equation using the lowest whole-number coefficients.

• Radium Phosphate and Sulfuric Acid Reaction: Radium phosphate reacts with sulfuric acid to form radium sulfate and phosphoric acid. Find the coefficient for sulfuric acid in the balanced equation using the lowest whole-number coefficients.

These problems require writing the correct formulas, balancing the equation, and identifying the required coefficients.

Sample Problem 2: Acid Rain Formation

A component of acid rain is nitric acid, which forms when nitrogen dioxide (NO2), a pollutant, reacts with oxygen and water:

If a power plant produces 16 kg of NO2 per year, and there is sufficient O2 and H2O, what mass of HNO3 can form?

1. Convert 16 kg NO2 to moles: moles NO2

2. Use the mole ratio: moles NO2 produce $4$ moles HNO3

3. Calculate moles of HNO3: moles

4. Convert moles HNO3 to mass: g = 21.9 kg HNO3

Summary Table: Physical State Symbols

Key Terms

• Chemical equation: A symbolic representation of a chemical reaction.

• Reactant: A starting substance in a chemical reaction.

• Product: A substance formed as a result of a chemical reaction.

• Stoichiometry: The calculation of reactants and products in chemical reactions.

• Mole ratio: The ratio of moles of one substance to another as indicated by the coefficients in a balanced equation.

• Coefficient: The number in front of a chemical formula in an equation, indicating the number of units involved.

Additional info: Some example problems and explanations have been expanded for clarity and completeness, including step-by-step stoichiometric calculations and the use of physical state symbols.