2.4 reaction feasibility

• Thermochemistry is the study of energy changes in reactions.

  The First Law of Thermodynamics states that the total energy of the universe is constant – energy can nether be created nor destroyed. Therefore, it only changes between different types of energy.
• The standard enthalpy change is defined as the enthalpy change measured under standard conditions.

  Standard conditions are:

  ·      one mole of a substance at 

  ·      one atmosphere pressure and

  ·      any specified temperature, in Kelvin. (The temperature specified is usually 298 K (25°C)).

  The symbol used is ΔH°

  The standard state of a substance is the most stable state of that substance under standard conditions.
• The standard enthalpy of formation (ΔH°f) of a compound is the enthalpy change when one mole of a compound is formed from its elements in their standard states.

  Enthalpies of formation are used in conjunction with Hess’ law to provide standard enthalpy changes, in kJ mol l^-1

  ΔHf° = ΔHf°(products) - ΔHf°(reactants)

  If ΔHf° (products) are smaller than ΔH°f (reactants) → exothermic

  If ΔHf° (products) are bigger than ΔH°f (reactants) → endothermic
• A reaction that occurs at a particular temperature is called a feasible reaction.

  The degree of disorder in a system is called the entropy of that system. The greater the disorder, the greater the entropy.

  Entropy is given the symbol S and is measured in J K-1 mol-1
• Entropy and state

  Solid has no freedom to move and no freedom to rotate but is free to vibrate.

  Liquid has restricted freedom to move, some freedom to rotate and is free to vibrate.

  Gas has total freedom to move and total freedom to rotate and is free to vibrate.
• Entropy increases as temperature increases and that changes in state, such as melting and evaporation, involve large changes in entropy. 

  When a state change occurs there is a change in entropy without a change in temperature and this is linked to the concept of latent heat.
• The second law of thermodynamics states that the total entropy of a reaction system and its surroundings always increases for a spontaneous process.

  Heat energy released by the reaction system into the surroundings increases the entropy of the surroundings.

  Heat energy absorbed by the reaction system from the surroundings decreases the entropy of the surroundings.
• The standard entropy of a substance is the entropy for that substance in its standard state and is given the symbol S°. 

  Entropy values can change based on temperature and pressure. In general:

  ➢ the higher the molecular weight of a substance in the same state the higher the degree of disorder

  ➢Solids have less entropy that liquids, which have less entropy than gases.

  Entropy change in a reaction can be calculated from absolute entropy values.

  △S° = S°(products) - S°(reactants)

  Remember to multiply the standard enthalpy of formation or entropy by its corresponding molar coefficient.