Chemical Reactions and Chemical Equations: Foundations and Applications

Chapter 7A: Chemical Reactions

Introduction to Chemical Reactions

Chemical reactions are fundamental processes in chemistry where substances (reactants) are transformed into new substances (products) through the rearrangement of atoms. These changes are central to both everyday phenomena and industrial applications, such as rocket propulsion and food chemistry.

• Chemical Change: Involves the formation of new substances with different properties from the original materials.

• Atomic Rearrangement: Atoms are not created or destroyed but are rearranged to form new molecules.

• Law of Conservation of Mass: The total mass of reactants equals the total mass of products in a chemical reaction.

• Example: The combustion of octane in an auto engine produces carbon dioxide and water from octane and oxygen.

Chemical Equations

Describing Chemical Reactions

Chemical equations provide a concise way to represent chemical reactions, showing the reactants, products, and their relative quantities.

• General Format: Reactants → Products

• Example: $(NH_4)_2Cr_2O_7(s) \rightarrow Cr_2O_3(s) + N_2(g) + H_2O(g)$

• Information Provided:

  • Formulas of reactants and products

  • Physical states: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous

  • Relative numbers (coefficients) of molecules or moles

  • Can be used to calculate masses of substances involved

Combustion of Methane: An Example

Combustion reactions are a key class of chemical reactions, especially for fuels. Methane combustion is a classic example:

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

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

• Interpretation: 1 molecule of methane reacts with 2 molecules of oxygen to produce 1 molecule of carbon dioxide and 2 molecules of water.

• Law of Conservation of Mass: The number of each type of atom is the same on both sides of the equation.

Key Points in Writing and Balancing Chemical Equations

• Only change coefficients, not subscripts, to balance equations.

• Balance polyatomic ions as units if they appear unchanged on both sides.

• Track each atom or ion carefully as you adjust coefficients.

• Physical state symbols and energy symbols (e.g., Δ for heat) provide additional information about reaction conditions.

Everyday Chemistry Applications

• Food Chemistry: Capsaicin in hot peppers binds to sensory neurons, causing a sensation of heat.

• Acid Rain: Acid rain reacts with calcium carbonate in statues, causing deterioration: $CaCO_3(s) + H_2SO_4(aq) \rightarrow CaSO_4(aq) + CO_2(g) + H_2O(l)$

• Corrosion: Bridges corrode due to electrochemical reactions involving iron, oxygen, and water.

• Nanotechnology: Gold nanoparticles can be used to detect glucose via specific chemical reactions.

Summary Table: Physical State Symbols in Chemical Equations

Symbol	Meaning
(s)	Solid
(l)	Liquid
(g)	Gas
(aq)	Aqueous (dissolved in water)

Additional info:

• Balancing equations is essential for quantitative chemical calculations (stoichiometry).

• Combustion reactions are a major source of energy in both natural and industrial processes.