Energetics Notes

Chemical Systems and Surroundings

  • A chemical system is the physical or chemical change under study.
  • The surroundings are everything outside the system.
  • Energy transfer from surroundings to the system is endothermic (+ sign).
  • Energy transfer from the system to the surroundings is exothermic (- sign).

Enthalpy

  • Enthalpy (H) is the total thermal energy stored in a compound, measured in kJ/mol.
  • Change in enthalpy (\DeltaH) can be measured, but total enthalpy (H) cannot.
  • Standard enthalpy change (\DeltaH°) is the enthalpy change when reactants in their standard states convert to products in their standard states.
  • At constant pressure, the enthalpy change equals the heat evolved or absorbed.

Exothermic and Endothermic Reactions

  • Exothermic reactions release heat energy to the surroundings.
  • Endothermic reactions absorb heat energy from the surroundings.
  • Bond breaking is endothermic; bond making is exothermic.
  • Enthalpy change is the sum of energies absorbed (bond breaking) and released (bond forming).

Reaction Mechanisms

  • Reactions occur when reactant molecules collide to form a transition state.
  • Activation energy (Ea) is the minimum energy colliding particles need for a successful collision.
  • Catalysts and enzymes lower activation energy by changing the reaction pathway.

Respiration

  • Aerobic respiration (with oxygen) and anaerobic respiration (without oxygen) are exothermic processes providing energy for biological systems.
  • Lipids serve as reserve energy stores.
  • During glycolysis, one molecule of glucose is split into two molecules of pyruvate (C3H4O_3).

Key Points

  • A system is any physical or chemical change under study, while the surrounding is everything else around it.
  • The total amount of heat energy present in a system at standard conditions is the heat content.
  • Standard enthalpy of a reaction is the enthalpy change when reactants in standard states produce products in standard states. Bonds
  • Exothermic reactions evolve heat, while endothermic reactions absorb heat.
  • A chemical reaction involves breaking and formation of chemical bonds.
  • A reaction takes place when reactant particles change into a transition state, then into products.
  • Activation energy is the energy needed by reactant particles to change into the transition state.
  • Enthalpy change is the difference between the energy of reactants and products.
  • In exothermic reactions, the energy of products is less than that of reactants and vice versa for endothermic reactions.
  • A catalyst lowers the activation energy, increasing the rate of the reaction.
  • Aerobic respiration takes place in the presence of oxygen, while anaerobic respiration takes place in the absence of oxygen.