Chemical Reactions and Stoichiometry

Law of Conservation of Matter and Balancing Chemical Equations

Chemical reactions involve the transformation of reactants into products, but the total number of atoms of each element is conserved. This is known as the law of conservation of matter. Chemical equations must be balanced by adjusting coefficients, not by changing subscripts, to reflect this conservation.

• Reactants: Substances that undergo change in a reaction.

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

• Coefficients in a balanced equation indicate the relative number of moles of each substance involved.

• Example:

Important: Never balance equations by changing the chemical formulas (subscripts) of the substances involved.

The Mole and Avogadro's Number

The mole (mol) is a counting unit in chemistry, representing particles (Avogadro's number). It allows chemists to relate the mass of substances to the number of particles involved in reactions.

• 1 mole of any element or compound contains atoms, molecules, or formula units.

• The mass of 1 mole of a substance (in grams) is its molar mass.

• Example: 1 mol of NaCl = 58.4 g; 1 mol of Cu = 63.55 g

Stoichiometry and Reaction Ratios

Stoichiometry uses the coefficients of a balanced chemical equation to determine the relative amounts of reactants and products. These coefficients can be used to set up mole ratios for calculations.

• Example:

• Mole ratios: , , etc.

Mass can be related to moles using molar mass, but calculations are not required here.

Redox Reactions

Oxidation and Reduction

Redox reactions involve the transfer of electrons between substances. Oxidation is the loss of electrons, while reduction is the gain of electrons. These processes always occur together.

• Oxidizing agent (oxidant): Causes oxidation, is itself reduced.

• Reducing agent (reductant): Causes reduction, is itself oxidized.

• Mnemonic: "LEO says GER" (Lose Electrons Oxidation, Gain Electrons Reduction) or "OIL RIG" (Oxidation Is Loss, Reduction Is Gain).

Oxidation Numbers

Oxidation numbers are assigned to atoms to track electron transfer in reactions. They help identify which atoms are oxidized or reduced.

• Uncombined elements: 0

• Monatomic ions: Equal to their charge

• Group IA: +1; Group IIA: +2

• Hydrogen: +1; Oxygen: -2 (except peroxides)

• The sum of oxidation numbers in a compound equals its overall charge.

Example: In , C = +4, O = -2; in , C = -4, H = +1.

Combustion and Biological Redox

Combustion is a redox reaction where a substance reacts with oxygen, producing energy, carbon dioxide, and water. In metabolism, organic molecules are oxidized to release energy.

• Example:

• In biological systems, cofactors like NAD+ act as oxidizing agents.

Thermodynamics of Chemical Reactions

Enthalpy (ΔH): Exothermic and Endothermic Reactions

Enthalpy (ΔH) is the heat energy change in a reaction. Reactions can be:

• Exothermic: Release heat (ΔH < 0), energy is a product.

• Endothermic: Absorb heat (ΔH > 0), energy is a reactant.

Example: Combustion of glucose is exothermic.

Energy in Food

The energy content of food is measured in kilocalories (kcal). Carbohydrates and proteins provide about 4 kcal/g, while fats provide about 9 kcal/g.

• Example: A Snickers bar contains carbohydrates, fats, and proteins, each contributing to the total caloric value.

Spontaneity and Thermodynamics

Free Energy (ΔG) and Spontaneity

Free energy (ΔG) determines whether a reaction is spontaneous (can proceed without external input). The sign of ΔG indicates spontaneity:

• ΔG < 0: Spontaneous (exergonic)

• ΔG > 0: Non-spontaneous (endergonic)

ΔG depends on enthalpy (ΔH), entropy (ΔS), and temperature (T):

Entropy (ΔS)

Entropy (ΔS) is a measure of disorder. Processes that increase disorder (e.g., solid to liquid to gas, breaking molecules into smaller pieces) have positive ΔS.

• Example: Dissolving a solid or producing more gas molecules increases entropy.

Coupled Reactions in Metabolism

Endergonic reactions (ΔG > 0) can occur in cells by coupling them to exergonic reactions (ΔG < 0), such that the overall process is exergonic. ATP is a key molecule for energy transfer in cells.

Reaction Kinetics

Activation Energy and Reaction Rate

Activation energy (Eact) is the minimum energy required for a reaction to proceed. The reaction rate is how quickly reactants are converted to products.

• Higher activation energy = slower reaction

• Factors affecting rate: temperature (higher = faster), concentration (higher = faster), and catalysts (lower activation energy, faster rate)

• Catalyst: Substance that increases reaction rate without being consumed

Chemical Equilibrium

Dynamic Equilibrium and the Equilibrium Constant (Keq)

At equilibrium, the rates of the forward and reverse reactions are equal. The equilibrium constant (Keq) expresses the ratio of product to reactant concentrations at equilibrium:

• Keq > 1: Products favored

• Keq < 1: Reactants favored

Le Châtelier’s Principle

If a system at equilibrium is disturbed (by changing concentration, temperature, or pressure), it will shift to counteract the disturbance and re-establish equilibrium.

• Adding reactant: shifts right (toward products)

• Removing product: shifts right

• Adding product or removing reactant: shifts left (toward reactants)

• Increasing temperature: shifts away from the side with heat (depends on exothermic/endothermic nature)

• Catalysts do not affect the position of equilibrium, only the rate at which equilibrium is reached.

Summary Table: Exothermic vs. Endothermic Reactions