U1L13 Spontaneous Reactions

Metabolism

  • All processes of life require energy (growth, reproduction, etc.)

  • Energy: ability to do work

  • Organisms must capture, store, and use energy to function

  • Reactions that transform matter and energy in out cells occur in step by step sequences called metabolic pathways

Spontaneity of Reactions

  • Spontaneous reaction: reaction that will continue to completion without further energy input once initiated

    • Ex. oxidation of glucose

    • C6H12O6 + 6O2 → 6CO2 +6H2O

    • Reverse reaction is not spontaneous

  • Non-spontaneous reaction: reaction that can only continue as long as it receives a continual energy input

    • Ex. electrolysis of water (using an electric current to break down water into oxygen and hydrogen gas)

    • When electricity is turned off, reaction stops

  • Spontaneous reactions require less energy than non-spontaneous

Determining Spontaneity

  • Many chemical reactions that are exothermic (give off heat) occur spontaneously

  • 3 factors determine is a reaction is spontaneous or not

  1. Enthalpy (H): total value of energy of a system

    • Decreases in H tend toward spontaneity

    • When ΔH is positive, reaction is endothermic

    • When ΔH is negative, reaction is exothermic

  1. Entropy (S): a measure of randomness of a system

    • Increase in S tend toward spontaneity

  2. Temperature (T): A measure of molecular motion

    • Increase in T tend toward spontaneity

  • Gibbs free energy (G): energy in system that can do useful work

    • Ex. before combustion, free energy in gasoline is higher than products of combustion

    • Gasoline can do more useful work than carbon dioxide and water

    • ΔG decreases in this reaction

  • Spontaneity can be determined mathematically with the Gibbs free energy equation:

    • When ΔG is positive → reaction is non-spontaneous

    • When ΔG is negative → reaction is spontaneous

Biochemical Coupling

  • Non-spontaneous reactions require continual input of energy

    • Metabolically expensive

  • To conserve energy, spontaneous reactions used to “drive” non-spontaneous reactions

    • Ex. synthesizing an ATP molecule is a non-spontaneous reaction (ΔG = +31 kJ)

    • Reaction can be coupled to a spontaneous reaction (ΔG = -49 kJ)

    • ΔG = -49 +31 kJ = -18 kJ

  • Biochemical coupling of reactions occurs on surface of enzyme