Chapter 4: Chemical Reactions – Stoichiometry and Balancing Equations

Chemical Reactions

Introduction to Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of chemical bonds, resulting in new products. Understanding how to represent and balance chemical equations is fundamental in general chemistry.

• Chemical Equation: A symbolic representation of a chemical reaction showing reactants and products.

• Balancing Equations: Ensures the conservation of mass and atoms for each element involved in the reaction.

• Stoichiometry: The quantitative relationship between reactants and products in a chemical reaction.

Combustion Reactions

Combustion of Methane

Methane combusts in the presence of oxygen to produce carbon dioxide and water. The balanced chemical equation is essential for determining the stoichiometric coefficients.

• Unbalanced Equation: $\mathrm{CH_4 + O_2 \rightarrow CO_2 + H_2O}$

• Balanced Equation: $\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$

• Stoichiometric Coefficient for $O_2$: 2 (for the combustion of one mole of methane)

• Example: Burning 1 mole of methane requires 2 moles of oxygen.

Combustion of Butane

Butane, commonly used in lighters, reacts with oxygen to produce carbon dioxide and water. Balancing the equation is crucial for stoichiometric calculations.

• Unbalanced Equation: $\mathrm{C_4H_{10} + O_2 \rightarrow CO_2 + H_2O}$

• Balanced Equation: $\mathrm{2C_4H_{10} + 13O_2 \rightarrow 8CO_2 + 10H_2O}$

• Stoichiometric Coefficient for $O_2$: $\frac{13}{2}$ (for the combustion of one mole of butane)

• Example: Burning 1 mole of butane requires $6.5$ moles of oxygen.

Stoichiometry in Chemical Reactions

Reaction of Aluminum with Hydrochloric Acid

Aluminum reacts with hydrochloric acid to produce aluminum chloride and hydrogen gas. Stoichiometric relationships allow us to determine the amount of reactants consumed and products formed.

• Balanced Equation: $\mathrm{2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)}$

• Key Stoichiometric Relationships:

  • For every mole of Al consumed, 3 moles of HCl are consumed.

  • For every mole of Al consumed, $1.5$ moles of $H_2$ are produced.

  • For every mole of HCl consumed, $\frac{1}{3}$ moles of Al and $\frac{1}{2}$ moles of $H_2$ are produced.

• Incorrect Statement: For every mole of $H_2$ produced, $\frac{3}{2}$ moles of Al are consumed. (This is false; for every mole of $H_2$ produced, $\frac{2}{3}$ moles of Al are consumed.)

• Example: If 2 moles of Al react, 6 moles of HCl are required, and 3 moles of $H_2$ are produced.

Summary Table: Stoichiometric Coefficients in Combustion Reactions

Hydrocarbon	Balanced Equation	Oxygen Coefficient (per mole hydrocarbon)
Methane ($CH_4$)	$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$	2
Butane ($C_4H_{10}$)	$2C_4H_{10} + 13O_2 \rightarrow 8CO_2 + 10H_2O$	$\frac{13}{2}$

Key Concepts

• Balancing Equations: Always ensure the same number of atoms of each element on both sides of the equation.

• Stoichiometric Coefficients: Indicate the relative number of moles of each substance involved in the reaction.

• Applications: Used to calculate reactant requirements and product yields in laboratory and industrial settings.

Additional info: The notes above expand on the brief question format by providing full academic context, definitions, and examples for each reaction and concept.