The study of heat released or required by chemical reactions THERMOCHEMISTRY The study of heat released or required by chemical reactions Fuel is burned to produce energy - combustion (e.g. when fossil fuels are burned) CH4(g) + 2O2(g) CO2(g) + 2H2O(l) + energy

Energy due to position or condition (stored energy) What is Energy? Energy Kinetic energy (KE) Potential energy (PE) Energy due to motion Energy due to position or condition (stored energy)

i.e. it is the total energy of all the atoms and molecules in a sample Law of Conservation of Energy: the total energy of the universe is constant and can neither be created nor destroyed; it can only be transformed. The internal energy, U, of a sample is the sum of all the kinetic and potential energies of all the atoms and molecules in a sample i.e. it is the total energy of all the atoms and molecules in a sample

Systems & Surroundings In thermodynamics, the world is divided into a system and its surroundings A system is the part of the world we want to study (e.g. a reaction mixture in a flask) The surroundings consist of everything else outside the system SYSTEM CLOSED OPEN ISOLATED

OPEN SYSTEM: can exchange both matter and energy with the surroundings (e.g. open reaction flask, rocket engine) CLOSED SYSTEM: can exchange only energy with the surroundings (matter remains fixed) e.g. a sealed reaction flask ISOLATED SYSTEM: can exchange neither energy nor matter with its surroundings (e.g. a thermos flask)

UNITS OF ENERGY S.I. unit of energy is the joule (J) Heat and work ( energy in transit) also measured in joules 1 kJ (kilojoule) = 103 J 1 cal = 4.184 J

A change in internal energy can be identified with the heat supplied at constant volume ENTHALPY (H) (comes from Greek for “heat inside”) The heat supplied is equal to the change in another thermodynamic property called enthalpy (H) i.e. H = q this relation is only valid at constant pressure As most reactions in chemistry take place at constant pressure we can say that: A change in enthalpy = heat supplied

EXOTHERMIC & ENDOTHERMIC REACTIONS Exothermic process: a change (e.g. a chemical reaction) that releases heat. A release of heat corresponds to a decrease in enthalpy Exothermic process: H is negative Burning fossil fuels is an exothermic reaction

An input of heat corresponds to an increase in enthalpy Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat. An input of heat corresponds to an increase in enthalpy Endothermic process: H is positive Forming Na+ and Cl- ions from NaCl is an endothermic process Photosynthesis is an endothermic reaction (requires energy input from sun)

Measuring Heat reaction Exothermic reaction, heat given off & temperature of water rises reaction Endothermic reaction, heat taken in & temperature of water drops

All chemical reactions either release or absorb heat Reaction Enthalpies All chemical reactions either release or absorb heat Exothermic reactions: Reactants products + energy as heat (H negative) Endothermic reactions: Reactants + energy as heat products (H positive)

Entropy is a measure of disorder Low entropy = low disorder ENTROPY (S) Entropy is a measure of disorder Low entropy = low disorder High entropy = greater disorder hot metal block tends to cool gas spreads out as much as possible

= + Total entropy change entropy change of system entropy change of surroundings = + Dissolving disorder of solution disorder of surroundings must be an overall increase in disorder for dissolving to occur

( negative S , negative H) 1. If we freeze water, disorder of the water molecules decreases , entropy decreases ( negative S , negative H) 2. If we boil water, disorder of the water molecules increases , entropy increases (vapor is highly disordered state) ( positive S , positive H)

∆G° = ∆H° - T∆S° ∆G° = Gibbs free energy change no physical meaning ∆G° = positive - forward reaction is non spontaneous ∆G° = negative - forward reaction is spontaneous ∆G° = zero - reaction is in equilibrium

A non-spontaneous change is a change that occurs only when driven A spontaneous change is a change that has a tendency to occur without been driven by an external influence e.g. the cooling of a hot metal block to the temperature of its surroundings A non-spontaneous change is a change that occurs only when driven e.g. forcing electric current through a metal block to heat it

Gibbs Free Energy, G ∆G° = ∆H° - T∆S° ∆H° ∆S° ∆G° process exo(–) increase(+) - spontaneous endo(+) decrease(-) + non-spontaneous exo(–) decrease(-) ? spontaneous @ ‘low’ temp endo(+) increase(+) ? spontaneous @ ‘high’ temp