Chapter 5: Chemical Reactions – Thermodynamics, Kinetics, and Reaction Types

Chapter 5: Chemical Reactions

Thermodynamics and Kinetics

Chemical reactions are governed by two main fields: thermodynamics and kinetics. Thermodynamics describes the energy changes and heat flow in reactions, while kinetics focuses on the speed at which reactions occur.

• Thermodynamics: Examines heat and energy changes during chemical reactions.

• Kinetics: Studies the rate (speed) of chemical reactions.

• Chemical Reaction Interpretation: Reactants → Products (arrow shows direction of reaction).

Enthalpy (H)

Enthalpy is the heat content of a system, largely determined by bonding interactions (covalent, ionic, hydrogen-bonding, dipole-dipole, London forces).

• Change in Enthalpy (ΔH): $\Delta H = H_{products} - H_{reactants}$

• Exothermic Reaction: ΔH is negative; heat is released; reaction tends toward spontaneity.

• Endothermic Reaction: ΔH is positive; heat is absorbed; reaction resists spontaneity.

Entropy (S)

Entropy measures the disorder or randomness in a system. Increased entropy means more freedom of movement, more particles, or more conformations.

• Change in Entropy (ΔS): $\Delta S = S_{products} - S_{reactants}$

• Negative ΔS: Products are more ordered; reaction resists spontaneity.

• Positive ΔS: Products are more disordered; reaction tends toward spontaneity.

Free Energy (G)

Free energy is the useful work a reaction can do. The Gibbs free energy combines enthalpy and entropy to predict spontaneity.

• Change in Free Energy (ΔG): $\Delta G = G_{products} - G_{reactants}$

• Relationship: $\Delta G = \Delta H - T\Delta S$

• Spontaneous Reaction: ΔG is negative (exergonic).

• Non-spontaneous Reaction: ΔG is positive (endergonic).

• Coupled Reactions: Endergonic reactions can proceed if paired with exergonic reactions so that overall ΔG is negative.

Activation Energy and Reaction Rate

The activation energy (Ea) is the energy barrier that must be overcome for a reaction to proceed. The higher the barrier, the slower the reaction.

• Factors Increasing Reaction Rate:

  • Increased temperature → more frequent and energetic collisions

  • Increased concentration of reactants → more collisions

  • Catalysts → lower activation energy (enzymes are biological catalysts)

Energy Content in Food

Different macronutrients provide varying amounts of energy.

Nutrient	Energy (Cal/g)
Carbohydrates	4
Protein	4
Fat	9

Calorie (Cal): 1 Cal = 1000 cal; enough to raise 1 kg of water by 1°C.

Fat provides more than twice the energy of carbohydrates or protein due to:

• Higher proportion of reduced carbon atoms (more C-H bonds)

• Lower degree of oxidation compared to carbohydrates/proteins

Types of Chemical Reactions

Reactions are classified by how atoms and molecules are rearranged.

• Synthesis: Two or more substances combine to form one product.

• Decomposition: One substance breaks down into two or more products.

• Exchange: Parts of two substances swap to form new products.

Reversible and Irreversible Reactions

Reactions can be reversible (⇌) or irreversible (→).

• Irreversible Reaction: Large negative ΔG; reaction goes to completion.

• Reversible Reaction: Both forward and backward reactions occur; equilibrium is possible.

Combustion Reactions

Combustion is the rapid reaction of a substance with oxygen, producing heat and light.

• Requirements: Oxygen (O2) and heat to initiate.

• Products: Carbon dioxide (CO2), water (H2O), and heat.

• Incomplete Combustion: Produces carbon monoxide (CO), which is toxic.

• Alkanes: Good fuels due to high energy content.

Example: Balancing Alkane Combustion

• General equation: $C_nH_{2n+2} + O_2 \rightarrow CO_2 + H_2O$

Organic Reaction Examples

Organic reactions often require catalysts and energy sources.

• Hydrogenation: Addition of hydrogen to an alkene to form an alkane; requires a catalyst.

• Glucose Phosphorylation: First step in glycolysis; requires a catalyst (enzyme) and energy (ATP).

Redox Reactions

Redox reactions involve the transfer of electrons. Oxidation is loss of electrons; reduction is gain of electrons.

• OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

• Oxidizing Agent: Accepts electrons.

• Reducing Agent: Donates electrons.

• Metals: Oxidize to form positive ions.

• Organic Compounds:

  • Oxidation: Loss of C-H bonds, gain of C-O bonds.

  • Reduction: Gain of C-H bonds, loss of C-O bonds.

• Oxidation States: $R-COOH$ (carboxylic acid) > $R-COH$ (aldehyde) > $R-CH_2OH$ (alcohol) in terms of oxidation.

• Redox Cofactors: NAD+ and FAD are oxidizing agents; become NADH and FADH2 when reduced.

Hydrolysis and Condensation Reactions

Hydrolysis: Water breaks a molecule into two parts.

Condensation: Two molecules join, releasing water (reverse of hydrolysis).

• Example: ATP hydrolysis to ADP and inorganic phosphate (Pi) is spontaneous and releases energy.

• ATP Formation: Condensation of ADP and Pi is not spontaneous; requires energy input.

Carboxylation and Phosphatase Reactions

• Carboxylation: Addition of a carboxyl group; requires energy and is coupled to an exergonic reaction.

• Phosphatase: Enzyme catalyzing removal of phosphate group; reaction is spontaneous.

Triacylglyceride Structure and Ester Formation

Triacylglycerides are formed by attaching fatty-acyl groups to glycerol via ester bonds.

• Functional Groups: Ester bonds link fatty acids to glycerol.

• Hydrolysis: Ester bonds can be hydrolyzed spontaneously.

• Formation: Attaching fatty acids requires energy (2 ATP equivalents per bond).

• Esters: Formed by condensation of carboxylic acids and alcohols.

Addition Reactions to Alkenes

Alkenes can undergo addition reactions, important in metabolism.

• Hydrogenation: Addition of hydrogen; requires a catalyst (often metal or enzyme).

• Trans Fats: Formed during industrial partial hydrogenation; associated with health risks.

• Hydration: Addition of water; requires a catalyst (acid or enzyme).

• Enzyme vs. Acid Catalyst: Enzymes produce one product; acid catalysts can yield multiple products.

Markovnikov's Rule

Predicts the outcome of addition reactions to alkenes.

• Rule: The -OH group attaches to the second carbon (not the end carbon) of a 1-alkene, or to the same carbon as a branch if present.

Enzyme Deficiencies and Disease

Enzyme	Function	Deficiency Effect
Lactase	Hydrolyzes lactose	Lactose intolerance
Lysosomal hydrolytic enzyme	Recycles sugar linkages	Tay-Sachs disease

Enzymes: Biological catalysts essential for metabolic reactions.

Example: Fumarase catalyzes hydration in the citric acid cycle.

Additional info: Enzyme-catalyzed reactions are highly specific and efficient, often producing only one product.