Chapter 5: Chemical Reactions – Thermodynamics, Kinetics, and Biochemical Processes

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Chapter 5: Chemical Reactions

Thermodynamics

Thermodynamics is the study of energy and heat exchange in chemical reactions, which is essential for understanding how food provides energy and how reactions occur in living systems.

Calorie: A unit measuring the energy content of food; nutrition labels list Calories to indicate how much energy food provides.
Thermodynamics: Describes how energy and heat are transferred during chemical reactions.
Reaction Kinetics: The study of the rate or pace at which chemical reactions occur.
Example: When food is digested, molecules are broken down by acids and enzymes, releasing energy.

Heat of Reaction

Chemical reactions can either release or absorb heat, which is a fundamental aspect of their classification.

Exothermic Reaction: Releases heat to the surroundings; heat flows out.
Endothermic Reaction: Absorbs heat from the surroundings; heat flows in.
Example: Combustion of glucose in the body is exothermic.

Enthalpy and Entropy

Enthalpy and entropy are key thermodynamic quantities that describe heat exchange and randomness in chemical systems.

Enthalpy (ΔH): The heat energy exchanged in a reaction. Calculated as:
Exothermic: ΔH is negative; heat is released.
Endothermic: ΔH is positive; heat is absorbed.
Entropy (ΔS): A measure of randomness or disorder in a system.
Increase in entropy: Positive ΔS.
Decrease in entropy: Negative ΔS.

Free Energy (Gibbs Free Energy)

Gibbs free energy determines whether a reaction is spontaneous and how much energy is available to do work.

Free Energy (G): The energy in molecules available to do work.
Calculation:
Exergonic Reaction: Negative ΔG; releases energy; spontaneous.
Endergonic Reaction: Positive ΔG; absorbs energy; nonspontaneous.
Example: ATP hydrolysis is exergonic and powers cellular processes.

ATP and Energy Transfer

ATP (adenosine triphosphate) is the main molecule for energy transfer in living systems, coupling energy-releasing and energy-consuming reactions.

ATP: Transfers energy within cells.
Spontaneous Change: Free energy decreases, system stability increases.
Coupling: Energy released from one reaction is used to drive another.

Activation Energy

Activation energy is the minimum energy required for reactants to collide and react, often provided as a 'push' to start a reaction.

Activation Energy: The energy needed to initiate a reaction.
Collision Theory: Reactants must collide with sufficient energy and proper orientation.
Example: Lighting a match provides activation energy for combustion.

Energy Content of Food and Combustion

Food contains chemical energy, which is released through combustion reactions both inside and outside the body.

Combustion: Reaction of molecules with oxygen to produce carbon dioxide, water, and energy.
Calorie Burning: Refers to the energy released during combustion.
Example: Glucose combustion in cellular respiration.

Composition of Food Fuels and Calculating Calories

Different foods provide varying amounts of energy, which can be calculated from nutrition labels.

Food Fuels: Carbohydrates, fats, and proteins are primary sources of energy.
Calorie Calculation: Nutrition labels help determine energy intake.

Chemical Reaction Kinetics

Kinetics studies the rate of chemical reactions and the factors that influence how quickly products are formed or reactants are consumed.

Reaction Rate: Measured by the amount of product formed or reactant used over time.
Collision Theory: Reactants must collide with enough energy and proper orientation.
Factors Affecting Rate: Concentration, temperature, catalysts, and surface area.

Enzymes (Biological Catalysts)

Enzymes are protein catalysts that dramatically increase the rate of biochemical reactions by lowering activation energy.

Catalyst: Substance that speeds up a reaction without being consumed.
Enzyme: Biological catalyst, usually a protein.
Active Site: Region on enzyme where reactants bind and are oriented for reaction.
Rate Enhancement: Enzymes can increase reaction rates by over ten million times.
Example: Digestive enzymes break down food molecules.

Combustion Reactions

Combustion is a highly exothermic reaction involving organic molecules and oxygen, producing carbon dioxide and water.

Combustion: Organic molecule + O2 → CO2 + H2O + energy
Irreversible: Cannot easily be reversed.
Decomposition: Combustion is a type of decomposition reaction.
Oxidation-Reduction: Combustion is also a redox reaction.
Example: Burning of alkanes as fuel.

Distinguishing Chemical Reactions

Organic and biochemical reactions are often distinguished by changes in functional groups and energy transfer.

Organic Reactions: Focus on functional group changes (e.g., alkene to alkane).
Biochemical Reactions: Involve functional group changes and energy transfer (e.g., ATP hydrolysis).

Oxidation and Reduction (Redox Reactions)

Redox reactions are essential for energy production and transfer in living systems, involving electron movement or changes in oxygen/hydrogen content.

Oxidation: Loss of electrons, addition of oxygen, or loss of hydrogen.
Reduction: Gain of electrons, addition of hydrogen, or loss of oxygen.
Redox Reaction: Involves both oxidation and reduction.
Example: Cellular respiration is a series of redox reactions.

Inorganic Redox Reactions

In inorganic systems, metals lose electrons to become cations (oxidation), and nonmetals gain electrons to become anions (reduction).

Oxidized Species: Reducing agent.
Reduced Species: Oxidizing agent.
Example: Iron rusting (Fe → Fe2+).

Biological Redox Reactions

Biological systems use small organic molecules and proteins (e.g., cytochrome c) to facilitate redox cycling, crucial for ATP production.

Heme: Organic molecule involved in electron transfer.
Cytochrome c: Protein important in cellular respiration.

Organic Redox Reactions

Organic molecules are oxidized by gaining oxygen or losing hydrogen, and reduced by gaining hydrogen or losing oxygen.

Carbonyl Groups: Aldehydes can be reduced to alcohols or oxidized to carboxylic acids.
Example: Ethanol oxidation to acetic acid.

Cellular Redox Reactions

Cellular respiration involves a series of redox reactions, breaking down nutrients and transferring energy.

Combustion in Cells: Glucose is oxidized to CO2 and H2O, releasing energy.
Example: Glucose metabolism.

Condensation and Hydrolysis Reactions

Condensation and hydrolysis are common in biochemistry, involving the joining or splitting of molecules with water as a product or reactant.

Condensation: Two molecules join, producing water.
Hydrolysis: Water is consumed, splitting a molecule into two.
Example: ATP hydrolysis to ADP.

Carboxylation Reactions

Carboxylation involves adding a carboxyl group, often regulated by enzymes in cells.

Carboxylase: Enzyme adding carboxyl group.
Decarboxylase: Enzyme removing carboxyl group.
Example: Addition/removal of CO2 in metabolic pathways.

Phosphorylation and Dephosphorylation

Cells regulate pathways by adding or removing phosphate groups, which are condensation and hydrolysis reactions, respectively.

Phosphorylation: Addition of phosphate (Pi) by phosphorylases.
Dephosphorylation: Removal of phosphate by phosphatases.

Hydrolyzable and Nonhydrolyzable Lipids

Lipids are classified based on their ability to undergo hydrolysis, which is important for their biological function.

Hydrolyzable Lipids: Can be broken down by hydrolysis (e.g., waxes, fats, oils).
Nonhydrolyzable Lipids: Cannot be hydrolyzed.
Ester Functional Group: Present in hydrolyzable lipids; reacts with water to form carboxylic acid and alcohol.

Hydrolysis of Fats and Oils

Fats and oils are hydrolyzed to produce fatty acids and glycerol, which contains three alcohol groups.

Products: Fatty acids and glycerol.
Example: Digestion of triglycerides.

Organic Addition Reactions to Alkenes

Addition reactions to alkenes involve breaking the double bond and adding atoms to the carbons, forming two single bonds.

Addition Reaction: Atom/group added to double bond; double bond broken.
Example: Hydrogenation of alkenes.

Hydrogenation and Trans Fats

Hydrogenation adds hydrogen to alkenes, converting them to alkanes. Partial hydrogenation can produce trans fats, which are more stable but less healthy.

Hydrogenation: Addition of two hydrogen atoms to double bond.
Trans Fats: Formed when double bonds reform in trans configuration during partial hydrogenation.

Hydration of Alkenes

Hydration adds water to an alkene, producing an alcohol. The addition follows Markovnikov’s rule, where the –H attaches to the carbon with more hydrogens, and –OH to the carbon with more groups.

Hydration: Addition of –H and –OH to double bond.
Markovnikov’s Rule: –H bonds to carbon with more hydrogens; –OH bonds to carbon with more carbon groups.
Example: Hydration of ethene to ethanol.

Reaction Type	Main Features	Example
Exothermic	Releases heat; ΔH negative	Combustion of glucose
Endothermic	Absorbs heat; ΔH positive	Photosynthesis
Oxidation	Loss of electrons/oxygen gain/hydrogen loss	Iron rusting
Reduction	Gain of electrons/hydrogen gain/oxygen loss	Reduction of NAD+ to NADH
Condensation	Joins molecules; water produced	Formation of peptide bond
Hydrolysis	Splits molecules; water consumed	ATP hydrolysis
Addition	Atoms added to double bond	Hydrogenation of alkenes

Additional info: Academic context was added to clarify examples, formulas, and applications for each reaction type, and to ensure completeness for exam preparation.