Chemical Reactions, Balancing Equations, and Stoichiometry

Chemical Reactions and Chemical Changes

Definition of Chemical Reaction

A chemical reaction is a process in which one or more substances (reactants) are transformed into one or more different substances (products). Chemical reactions involve chemical changes in matter, resulting in the formation of new chemical substances.

• Reactants: The starting materials in a chemical reaction.

• Products: The substances formed as a result of the reaction.

Combustion Reactions

A combustion reaction is a specific type of chemical reaction in which a substance combines with oxygen to form one or more oxygen-containing compounds, usually releasing heat.

• General form: Fuel + O2 → CO2 + H2O (for hydrocarbons)

• Commonly produces carbon dioxide and water as products.

• Exothermic: releases energy in the form of heat.

Writing and Balancing Chemical Equations

Parts of a Chemical Equation

Chemical equations provide information about the reaction, including:

• The formulas of the reactants and products

• The physical states of the reactants and products

• The relative numbers of reactants and products (coefficients)

• Stoichiometric relationships required to determine how much product will be made or how much reactant is needed

Physical States in Chemical Equations

The physical state of each reactant and product is indicated by an abbreviation in parentheses:

Abbreviation	State
(g)	Gas
(l)	Liquid
(s)	Solid
(aq)	Aqueous (water solution)

Example: Combustion of Methane

When methane gas burns, it reacts with oxygen to produce carbon dioxide and water vapor:

• Unbalanced equation: $\mathrm{CH_4(g) + O_2(g) \rightarrow CO_2(g) + H_2O(g)}$

• Check atom balance:

  • Left: C = 1, H = 4, O = 2

  • Right: C = 1, H = 2, O = 3

• Balance hydrogen by changing the coefficient of water: $\mathrm{CH_4(g) + O_2(g) \rightarrow CO_2(g) + 2 H_2O(g)}$

  • Left: C = 1, H = 4, O = 2

  • Right: C = 1, H = 4, O = 4

• Balance oxygen by changing the coefficient of O2: $\mathrm{CH_4(g) + 2 O_2(g) \rightarrow CO_2(g) + 2 H_2O(g)}$

  • Left: C = 1, H = 4, O = 4

  • Right: C = 1, H = 4, O = 4

Important: When balancing equations, change the coefficients, never the subscripts in chemical formulas.

Practice: Writing Balanced Equations

• Write a balanced equation for the reaction of solid copper(I) oxide and solid carbon to produce solid copper and carbon dioxide gas.

• Write a balanced equation for the combustion of ethanol (C2H5OH).

• Write a balanced equation for the reaction between aqueous calcium chloride and aqueous lithium phosphate to form solid calcium phosphate and aqueous lithium chloride.

Additional info: These are typical practice problems to reinforce balancing skills and understanding of reaction types.

Stoichiometry

Definition and Importance

Stoichiometry is the study of the numerical relationships between the amounts of reactants and products in a chemical reaction. It is based on the law of conservation of mass, which states that matter is neither created nor destroyed in a chemical reaction.

• Balanced equations are essential for stoichiometric calculations.

• Coefficients in a balanced equation specify the relative amounts (in moles) of each substance involved.

Stoichiometric Ratios and Conversion Factors

From a balanced equation, you can extract valuable ratios to use as conversion factors in chemical calculations.

• Example: For the combustion of octane: $\mathrm{2 C_8H_{18}(l) + 25 O_2(g) \rightarrow 16 CO_2(g) + 18 H_2O(g)}$

• From this equation, the following ratios can be derived:

  • 2 mol C8H18 : 25 mol O2

  • 2 mol C8H18 : 16 mol CO2

  • 2 mol C8H18 : 18 mol H2O

  • 25 mol O2 : 16 mol CO2

  • ...and so on

Example Stoichiometry Problems

• How many moles of H2O will be produced from 17 mol of C8H18? Use the ratio from the balanced equation: $\frac{18\ \text{mol}\ H_2O}{2\ \text{mol}\ C_8H_{18}}$ $17\ \text{mol}\ C_8H_{18} \times \frac{18\ \text{mol}\ H_2O}{2\ \text{mol}\ C_8H_{18}} = 153\ \text{mol}\ H_2O$

• How many moles of H2O will be produced from 23 L of C8H18 (liquid)? Additional info: To solve this, you would need the density of liquid octane to convert liters to moles before using the stoichiometric ratio.

Summary Table: Physical States in Chemical Equations

Abbreviation	State
(g)	Gas
(l)	Liquid
(s)	Solid
(aq)	Aqueous (water solution)

Additional info: Mastery of balancing equations and stoichiometry is foundational for all further study in chemistry, including quantitative analysis and laboratory work.