Thermochemistry

• Thermochemistry is the study of changes in energy that occur during chemical reactions.
• Thermodynamics is the study of heat and temperature and their relation to other forms of energy and wrok.
• The first law of thermodynamics states that: energy can neither be created nor destroyed. It can be converted from one form to another while total energy remains the same.
• Enthalpy is defined as a thermodynamic quantity equivalent to the total heat content of a system.
• Actual enthalpy cannot be measured, but enthalpy changes can.
• Standard conditions are 298K and 1 atmosphere of pressure.
• The standard state of a substance is the most stable state of a substance under standard conditions.
• The standard enthalpy of formation is the enthalpy when one mole of a substance is formed from its elements in their standards states.
• The enthalpy of a single element is 0.
• The standard enthalpy of a reaction is the enthalpy change for a reaction in which the reactants and products are in their standard states at a specific temperature.
• Entropy is defined as a measure of the degree of disorder of a system.
• Greater disorder means greater entropy.
• A very regular highly ordered system at a low temperature has a low entropy.
• A highly disordered system at a high temperature has high entropy.
• The second law of thermodynamics states that: the total entropy of a reaction system and its surroundings always increases for a spontaneous process.
• A spontaneous process is one that proceeds of its own accord, but not necessarily quickly.
• The third law of thermodynamics is: the entropy of a perfect crystal at a temperature of 0 Kelvin is 0.
• At 0K, particles have minimal energy, and so will be at their most ordered. Their entropy will be 0.
• A solid has low disorder, particles are bonded in a fixed shape. As energy is supplied, the particles move faster, becoming more disordered. Bonds weaken and eventually break, forming a liquid.
• In a liquid, particles are close and attracted, but not fixed in position. As more energy is supplied, the particles break free, forming a gas.
• In a gas, particles are far apart, with no fixed position. There is a high degree of disorder.
• As the temperature of a system increases, the entropy increases.
• Dissolving will cause a decrease in entropy of the water, but the large increase in entropy of the solute will result in a net gain of entropy- more disorder overall.
• Mixtures will have a greater entropy than pure substances.
• Diffusion causes no change in enthalpy, but does cause an increase in entropy.
• As the number of moles increases, entropy and disorder increases.
• Standard entropy is defined as: the entropy of 1 mole of a substance in its standard state.
• Entropy increase is positive.
• Entropy decrease is negative.
• Common features of a spontaneous endothermic reaction are: the number of moles of product are greater than the number of moles of reactant, and the reactants are usually solid or liquid, and products are usually liquid or gas.
• Heat energy released by a system will increase the entropy of the surroundings.
• Heat energy absorbed by a system decreases the entropy of the surroundings.
• Gibbs Free Energy is the energy that is available in a system to do work.
• If Gibbs Free Energy is positive, it is thermodynamically feasible and will proceed.
• If Gibbs Free Energy is negative, it is not thermodynamically feasible, and will not proceed.
• If Gibbs Free Energy is 0, it is at equilibrium.
• For temperature dependent reactions, the point at which Gibbs Free Energy = 0 is the tipping point for spontaneity.
• The more negative the standard free energy, the more stable the compound formed.