Chapter 5: Chemical Reactions

5.1 Thermodynamics

Thermodynamics is the study of energy and heat exchange in chemical reactions. It helps explain how energy is transferred and transformed during chemical processes, including those in living systems.

• Energy: Defined as the capacity to do work. Chemical reactions involve exchanges of energy, often transforming potential energy into kinetic energy or vice versa.

• Heat of Reaction:

  • Exothermic reactions: Release heat; products have lower enthalpy than reactants. The change in enthalpy () is negative.

  • Endothermic reactions: Absorb heat; products have higher enthalpy than reactants. is positive.

• Entropy (): Measures randomness or disorder.

  • Increase in entropy (): System becomes more random.

  • Decrease in entropy (): System becomes more ordered.

• Free Energy (Gibbs Free Energy, ): The energy available to do work. The change in free energy () determines spontaneity:

  • Exergonic reactions: ; occur spontaneously.

  • Endergonic reactions: ; require energy input.

• Activation Energy: The minimum energy required for reactants to collide and react. Catalysts lower activation energy.

• Energy Content in Food:

  • Combustion of food molecules produces energy (Calories).

  • Fats: 9 Cal/g; Carbohydrates: 4 Cal/g; Low-calorie foods use substitutes that yield less energy.

5.2 Chemical Reactions: Kinetics

Kinetics studies the rate at which chemical reactions occur and the factors influencing these rates.

• Rate of Reaction: Measured by the amount of product formed or reactant consumed over time.

• Collision Theory: Reactants must collide with sufficient energy and proper orientation.

• Factors Affecting Rate:

  • Temperature: Higher temperature increases kinetic energy and collision frequency.

  • Amount of Reactant: More reactant increases collision frequency; rate slows as reactants are consumed.

  • Catalysts: Lower activation energy, speed up reactions, remain unchanged. Acids, bases, and metal ions often act as catalysts.

• Enzymes: Biological catalysts, usually proteins, highly specific, increase reaction rates dramatically. Defective enzymes can cause diseases (e.g., albinism).

5.3 Overview of Chemical Reactions

Chemical reactions are classified by how reactants and products interact and transform.

• Synthesis Reactions: Combine smaller molecules into larger ones; require energy; only one product.

• Decomposition Reactions: Break larger molecules into smaller ones; often release energy; only one reactant.

• Exchange (Displacement) Reactions: Involve both synthesis and decomposition.

• Reversible Reactions: Can proceed in both directions; reach chemical equilibrium when forward and reverse rates are equal.

• Irreversible Reactions: Highly exothermic; products cannot easily revert to reactants (e.g., combustion).

• Combustion: Organic molecule reacts with oxygen to produce CO2 and H2O; highly exothermic and irreversible.

• Alkanes: Undergo combustion; efficiency of combustion determines "clean burning." Incomplete combustion produces soot.

• Organic and Biochemical Equations: Show reactant, product, and reaction conditions; often coupled reactions for energy transfer.

5.4 Oxidation and Reduction

Oxidation-reduction (redox) reactions are essential for energy production and transfer in living systems.

• Inorganic Redox:

  • Oxidation: Loss of electrons (e.g., metals form cations).

  • Reduction: Gain of electrons (e.g., nonmetals form anions).

  • Redox reactions are always coupled; "OIL RIG" mnemonic: Oxidation Is Loss, Reduction Is Gain (of electrons).

• Biological Redox:

  • Metals (e.g., iron in cytochrome c) undergo oxidation and reduction in cells.

• Organic Redox:

  • Oxidation: Gain of oxygen or loss of hydrogen.

  • Reduction: Gain of hydrogen or loss of oxygen.

  • Aldehydes can be reduced to alcohols or oxidized to carboxylic acids.

• Cellular Oxidation:

  • Glucose is oxidized in cellular respiration.

  • Ethanol is oxidized to acetaldehyde by alcohol dehydrogenase.

  • NAD+ and FAD are important redox molecules in metabolism.

5.5 Organic Reactions: Condensation and Hydrolysis

Condensation and hydrolysis reactions are fundamental in biochemistry, involving the joining and splitting of molecules with water.

• Condensation Reaction: Two organic molecules join, producing water.

• Hydrolysis Reaction: Water is consumed, splitting a molecule into two smaller ones.

• Carboxylation: Addition of a carboxyl group; catalyzed by carboxylase enzymes.

• Phosphorylation/Dephosphorylation: Addition/removal of phosphate groups; condensation and hydrolysis reactions, catalyzed by phosphorylases and phosphatases.

• Hydrolyzable Lipids: Can be hydrolyzed to simpler compounds (e.g., waxes, fats, oils contain esters).

• Nonhydrolyzable Lipids: Cannot undergo hydrolysis.

• Fats and Oils: Hydrolyzed to fatty acids and glycerol (with three alcohol groups).

5.6 Organic Addition Reactions to Alkenes

Addition reactions to alkenes involve breaking the double bond and adding atoms or groups to the carbons.

• Hydrogenation: Addition of hydrogen to an alkene, converting it to an alkane. Catalysts (Pt, Ni, Pd) are used. Partial hydrogenation can produce trans fats.

• Hydration: Addition of water to an alkene double bond, often requiring a catalyst. Follows Markovnikov’s rule:

  • Hydrogen bonds to the carbon with more hydrogen atoms.

  • Hydroxyl group bonds to the carbon with more carbon groups attached.

Chapter 5 Study Guide – Key Learning Objectives

• Investigate properties of heat, energy, and spontaneity in reactions.

• Draw and interpret reaction energy diagrams.

• Calculate energy content in food.

• Examine factors affecting reaction rates and activation energy.

• Classify and predict products of synthesis, decomposition, exchange, and combustion reactions.

• Distinguish and identify oxidation-reduction reactions in inorganic and organic contexts.

• Characterize condensation and hydrolysis reactions; predict their products.

• Demonstrate addition reactions to alkenes; predict products of hydrogenation and hydration.

Table: Types of Chemical Reactions

Table: Factors Affecting Reaction Rate

Table: Oxidation and Reduction in Organic Chemistry

Table: Energy Content in Food

Relevant Image