Chemical Reactions: Energy, Enthalpy, and Reaction Rates

Posted on Nov 29, 2024 in Chemistry

Chemical Reactions and Energy

Compounds or chemicals release stored energy in several ways:

  • Thermal Energy (leads to temperature changes)
  • Mechanical Energy (movement of objects)
  • Electrical Energy (electricity from chemical reactions, like in batteries)
  • Light Energy
  • Other forms of energy (e.g., sound waves)

Origin of Energy Exchange in Chemical Reactions

Any chemical reaction involves interactions between reacting species that cause bond breaking and formation. Remember:

  • Bond Rupture: Requires energy input.
  • Bond Formation: Releases energy.

The balance between bond breaking and formation determines whether a reaction absorbs or releases energy:

  • If new bonds are stronger than broken bonds, energy is released.
  • If new bonds are weaker than broken bonds, the reaction absorbs energy.

Reaction Enthalpies and Enthalpy Diagrams

The heat of reaction is the energy exchanged with the environment as heat during a chemical reaction. Since most reactions occur in open containers at constant atmospheric pressure, we use enthalpy (ΔH) instead. Enthalpy is the energy exchanged as heat at constant pressure.

Reactions are classified as:

  • Exothermic reactions: Release heat to the environment; ΔH < 0
  • Endothermic reactions: Absorb heat from the environment; ΔH > 0

Thermochemical Equations

A thermochemical equation shows:

  • Formulas of reactants and products
  • Physical states of substances under standard conditions (P = 1 atm, T = 25°C)
  • Heat of reaction as enthalpy (e.g., C(s) + O2(g) → CO2(g) ΔH = -393.5 kJ/mol)

Two rules for using thermochemical equations:

  • The magnitude of enthalpy is directly proportional to the amount of reactant or product and can be used as a conversion factor.
  • The enthalpy of a reaction is equal and opposite to that of the reverse reaction.

Hess’s Law

Statement:

The enthalpy change of a reaction depends only on the initial and final states and is independent of the pathway.

Utility:

Allows calculation of a reaction’s heat if it can be obtained from the sum of other reactions with known heats.

Reaction Rate

Reaction rate measures how fast reactants transform into products. It’s the amount of reactant used or product formed per unit time and volume.

Collision Theory and Activation Energy

This theory states that for a reaction to occur, reactant molecules must collide. More frequent collisions lead to faster reactions. However, not all collisions are effective; only those with enough energy to break bonds result in products.

Activation energy is the minimum energy needed for a reaction to occur. It’s the energy barrier that must be overcome. At the peak of this energy barrier, an activated complex forms—a transition state where reactant bonds weaken and product bonds begin to form (10-13 s).

Effective collisions require both sufficient energy and proper orientation.

Factors Affecting Reaction Rate

  • Temperature: Increasing temperature increases particle energy and collision frequency, thus increasing reaction rate.
  • Reactant Concentration: Increasing concentration increases the number of colliding particles and reaction rate.
  • Surface Area: In heterogeneous reactions, increasing surface area increases collision frequency and reaction rate.
  • Catalysts: Substances that modify reaction rates (positive catalysis increases rate, negative catalysis decreases rate). Catalysts work by altering activation energy. A catalyst must be used in small amounts, be recoverable, and not be consumed in the reaction.