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

Thermodynamics

Introduction to Thermodynamics

Thermodynamics is the study of energy and heat exchange in chemical reactions. It helps us understand how energy is transferred and transformed during chemical processes, such as those occurring in living organisms and in food metabolism.

• Energy is defined as the capacity to do work.

• In chemical reactions, energy can be dissipated as heat or transformed between potential and kinetic forms.

• Reactants are written on the left, products on the right, and the arrow indicates the direction of the reaction: Reactants → Products

Types of Reaction: Exothermic and Endothermic

• Exothermic reactions release heat to the surroundings. The products have lower energy than the reactants. Example: $\mathrm{NaC_2H_3O_2(aq) \rightarrow NaC_2H_3O_2(s) + Heat}$

• Endothermic reactions absorb heat from the surroundings. The products have higher energy than the reactants. Example: $\mathrm{Heat + NH_4NO_3(s) \rightarrow NH_4NO_3(aq)}$

Change in Enthalpy ($\Delta H$)

The change in enthalpy ($\Delta H$) measures the heat exchanged in a reaction at constant pressure.

• For exothermic reactions, $\Delta H$ is negative.

• For endothermic reactions, $\Delta H$ is positive.

Equation:

$\Delta H_{\text{reaction}} = H_{\text{products}} - H_{\text{reactants}}$

Entropy ($\Delta S$)

Entropy is a measure of randomness or disorder in a system.

• Change in entropy ($\Delta S$) is calculated as the entropy of products minus the entropy of reactants.

• If $\Delta S$ is positive, randomness increases; if negative, randomness decreases.

Gibbs Free Energy ($\Delta G$)

Gibbs free energy determines whether a reaction is spontaneous (can occur without outside energy input).

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

• If $\Delta G$ is negative, the reaction is exergonic (spontaneous).

• If $\Delta G$ is positive, the reaction is endergonic (nonspontaneous).

Energy in Biological Systems

• ATP (adenosine triphosphate) is the main energy carrier in cells.

• Free energy released from one reaction can drive another reaction (energy coupling).

Chemical Reactions: Kinetics

Reaction Rate and Activation Energy

Kinetics is the study of the speed (rate) of chemical reactions and the factors that affect it.

• The rate of reaction is measured by the amount of product formed or reactant used per unit time.

• Activation energy is the minimum energy required for reactants to collide and react.

Factors Affecting Reaction Rate

• Temperature: Higher temperature increases molecular motion, leading to more frequent and energetic collisions.

• Concentration of Reactants: More reactant molecules increase the likelihood of collisions.

• Catalysts: Catalysts lower activation energy, speeding up reactions without being consumed.

Enzymes as Biological Catalysts

• Enzymes are proteins that act as highly specific biological catalysts.

• They increase reaction rates by immobilizing reactants at the active site and orienting them for reaction.

• Defective enzymes can lead to diseases (e.g., albinism due to tyrosinase deficiency).

Overview of Chemical Reactions

Types of Chemical Reactions

• Synthesis (Combination): Two or more small molecules combine to form a larger molecule. General form: $A + B \rightarrow AB$

• Decomposition: A large molecule breaks down into smaller molecules. General form: $AB \rightarrow A + B$

• Exchange (Displacement): Atoms or groups are exchanged between molecules. General forms: $AB + C \rightarrow AC + B$ (single), $AB + CD \rightarrow AD + CB$ (double)

Reaction Type	General Scheme
Synthesis	A + B → AB
Decomposition	AB → A + B
Exchange (Single)	AB + C → AC + B
Exchange (Double)	AB + CD → AD + CB

Reversible and Irreversible Reactions

• Reversible reactions can proceed in both directions, reaching equilibrium when the rates of forward and reverse reactions are equal.

• Irreversible reactions proceed in one direction, often because they are highly exothermic (e.g., combustion).

Combustion Reactions

• Combustion involves an organic molecule reacting with oxygen to produce carbon dioxide and water.

• These reactions are highly exothermic and considered irreversible.

• Example: $\mathrm{C_xH_y + O_2 \rightarrow CO_2 + H_2O}$

Oxidation and Reduction (Redox Reactions)

Definitions and Identification

• Oxidation is the loss of electrons (or gain of oxygen/loss of hydrogen in organic molecules).

• Reduction is the gain of electrons (or gain of hydrogen/loss of oxygen in organic molecules).

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

Redox in Inorganic and Organic Chemistry

• Inorganic: Metal atoms lose electrons to form cations (oxidized); nonmetals gain electrons to form anions (reduced).

• Organic: Oxidation often involves adding oxygen or removing hydrogen; reduction involves adding hydrogen or removing oxygen.

• Example: Aldehyde can be oxidized to carboxylic acid or reduced to alcohol.

Process	Example
Oxidation	R-CH2OH → R-CHO → R-COOH
Reduction	R-COOH → R-CHO → R-CH2OH

Biological Redox Reactions

• Redox reactions are essential for energy production in cells (e.g., cellular respiration).

• NAD+ and FAD are important coenzymes in metabolic redox reactions.

• Example: Ethanol is oxidized to acetaldehyde by alcohol dehydrogenase, with NAD+ reduced to NADH.

Organic Reactions: Condensation and Hydrolysis

Condensation Reactions

• Two molecules combine to form a larger molecule, releasing water as a byproduct.

• Common in the formation of biological macromolecules (e.g., peptide bond formation).

Hydrolysis Reactions

• A molecule is split into two smaller molecules by the addition of water.

• Important in digestion and breakdown of biomolecules (e.g., ATP hydrolysis).

Phosphorylation and Dephosphorylation

• Phosphorylation is the addition of a phosphate group (condensation reaction).

• Dephosphorylation is the removal of a phosphate group (hydrolysis reaction).

• Enzymes: phosphorylases (add phosphate), phosphatases (remove phosphate).

Hydrolyzable and Nonhydrolyzable Lipids

• Hydrolyzable lipids (e.g., fats, oils, waxes) can be broken down by hydrolysis to form smaller molecules (e.g., fatty acids, alcohols).

• Nonhydrolyzable lipids cannot be hydrolyzed.

Organic Addition Reactions to Alkenes

Addition Reactions

• Atoms or groups are added to the carbons of a double bond, converting it to a single bond.

• Common addition reactions include hydrogenation and hydration.

Hydrogenation

• Addition of hydrogen (H2) across a double bond, converting an alkene to an alkane.

• Requires a metal catalyst (e.g., Pt, Ni, Pd).

• Partial hydrogenation can produce trans fats.

Hydration

• Addition of water (H and OH) across a double bond, forming an alcohol.

• Requires an acid or enzyme catalyst.

• Follows Markovnikov's rule: the H adds to the carbon with more hydrogens, the OH to the carbon with more carbon groups.

Summary Table: Key Reaction Types

Reaction Type	Description	General Equation
Synthesis	Combining smaller molecules	A + B → AB
Decomposition	Breaking down larger molecules	AB → A + B
Exchange	Swapping components	AB + CD → AD + CB
Combustion	Reaction with O2 to form CO2 and H2O	CxHy + O2 → CO2 + H2O
Redox	Electron transfer	Varies
Condensation	Joining with loss of H2O	R-OH + R'-H → R-R' + H2O
Hydrolysis	Splitting with addition of H2O	R-R' + H2O → R-OH + R'-H
Addition to Alkene	Adding atoms/groups to double bond	RCH=CH2 + X-Y → RCHX-CH2Y

Additional info: These notes are based on textbook slides and are suitable for GOB Chemistry students preparing for exams on chemical reactions, thermodynamics, kinetics, and basic organic reaction types.