Chemical Reactions

Law of Conservation of Matter

The law of conservation of matter states that atoms are not created or destroyed in chemical reactions. This fundamental principle underlies all chemical equations and stoichiometric calculations.

• Balancing Equations: Balance each side of the equation using coefficients (whole numbers in front of compounds or elements).

• Do Not: Change subscripts to balance equations (e.g., H2O is water, H2O2 is hydrogen peroxide).

• Example: NaCl (aq) → Na+ (aq) + Cl- (aq) (dissolved in water)

How Much Material?

Quantifying substances in chemistry often involves the mole concept, which relates the number of particles to a measurable amount of substance.

• 1 mole = 6.022 × 1023 atoms or molecules (Avogadro's number)

• Same number of atoms or molecules: 1 mole (of any substance)

• Note: Mass (g) is not the same as the number of atoms or molecules.

Stoichiometry

Stoichiometry uses reaction coefficients to determine how much of a substance reacts or is produced, usually in moles.

• Coefficients can be used to make ratios:

• Can convert to using molecular weight:

For example, H2 = 2 g/mol, O2 = 32 g/mol, H2O = 18 g/mol

• Conservation of mass: both sides share the same total mass.

Definitions

• Reactants: Substances that come together to react.

• Products: Substances produced by the reaction.

Chemical Reactions: Combustion & Redox

Combustion Reactions

Combustion is a reaction where a substance reacts with oxygen to produce energy, often as heat and light.

• Example: Methane + Oxygen → Carbon Dioxide + Water

• CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (l)

• Balance using coefficients.

• Combustion is important for biological compounds and metabolism.

Redox Reactions

Redox (reduction-oxidation) reactions involve the transfer of electrons between substances.

• Oxidation: Loss of electrons (increase in oxidation number)

• Reduction: Gain of electrons (decrease in oxidation number)

• "LEO says GER": Lose Electrons Oxidation, Gain Electrons Reduction

• "OIL RIG": Oxidation Is Loss, Reduction Is Gain

Oxidation Numbers

Oxidation numbers are assigned to determine an element's propensity to lose or gain electrons in a chemical reaction.

• Rules:

  • Oxidation number of uncombined elements = 0

  • Oxidation number of monoatomic ions = ion charge

  • O in most compounds = -2; H in most compounds = +1

  • Sum of oxidation numbers in a compound = overall charge

Oxidation Numbers for Carbon

• CO2: (+4)

• CH4: (-4)

• Metabolic reactions release energy by oxidizing carbon.

Redox in Metabolism

• Carbon is oxidized (from -4 to +4); CH4 is the reducing agent/reductant.

• Oxygen is reduced (from 0 to -2); O2 is the oxidizing agent/oxidant.

• Classic combustion reaction: burning in oxygen.

• Core reaction of metabolism: oxidation of glucose.

Definitions

• Oxidizing agent (oxidant): Reactant responsible for oxidation, is reduced.

• Reducing agent (reductant): Reactant responsible for reduction, is oxidized.

Thermodynamics: Enthalpy and Reactions

Enthalpy () and Reactions

Enthalpy () of a reaction is the thermal energy released or absorbed per mole (based on coefficients in the balanced equation).

• Combustion of glucose releases energy; exothermic reaction; negative .

• Example:

kcal/mol

• Enthalpy is the difference between the chemical energy of products and reactants:

Energy in Food

• 1 kcal = 1 Calorie; combustion is used to determine Calories in food.

• Example:

Spontaneity and Free Energy

Spontaneous Reactions

A spontaneous reaction proceeds without external stimuli. Free energy () determines if a process is spontaneous and represents energy available for work.

• Exergonic reaction: releases free energy ( negative)

• Endergonic reaction: requires free energy ( positive)

Key Biochemical Question

• Will a reaction release free energy (can the cell use it)? Is it spontaneous?

• If , the reaction is spontaneous.

What is Free Energy ()?

• Energy available for a process

• Difference between free energy of products and reactants:

• Depends on:

  • Enthalpy ()

  • Entropy ()

  • Temperature ()

What is Entropy ()?

• Measure of disorder in a system

• Temperature dependent (higher temperature, more disorder)

• Low entropy = order; high entropy = disorder

Examples of Increasing Entropy ()

• Change of state (solid → liquid → gas)

• More gas molecules in products than reactants

• Breaking large molecules into smaller particles

Spontaneity and Reaction Direction

• Whether a reaction is spontaneous depends on enthalpy, entropy, and temperature.

• Exothermic reactions (heat released) and increased disorder favor spontaneity.

• Endothermic reactions (heat absorbed) can be spontaneous if entropy increases enough.

Coupled Reactions

• A catabolic reaction is coupled to a more exergonic reaction, so the sum of the reactions is exergonic.

• Catabolic metabolic reactions break down biological molecules to release free energy.

• Anabolic reactions build up biological molecules using free energy.

• Biochemical energy is captured in the molecule ATP.

Kinetics: Reaction Rate and Activation Energy

Activation Energy

Activation energy is the additional energy necessary for a reaction to occur (to get started).

• Determines the rate of the reaction.

• Larger activation energy = slower rate.

Reaction Rate

• How fast will a reaction occur? (Kinetics)

• Measured as the change in concentration of reactants or products over time.

• For a reaction to occur, particles must collide in the correct orientation and with enough energy.

Factors Affecting Reaction Rate

1. Temperature: Increasing temperature increases reaction rate (more energy).

2. Concentration: Increasing concentrations increases reaction rate (more collisions).

3. Catalysts: A substance that increases the rate of reaction but is not used up in the reaction. It lowers the activation energy for the reaction.

4. Inhibitors: Slow down reaction rate.

Effect of a Catalyst

• A catalyst lowers the activation energy for a reaction and increases the rate of both the forward and reverse reactions.

• The size of the activation barrier (activation energy) is related to the rate of a reaction.

• Reaction rate, or kinetics, is how fast a reaction occurs (change over time).