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 (stored energy) What is Energy? Energy Kinetic energy (EK) Potential energy (EP) Energy due to motion Energy due to position (stored energy)

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.

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)

e.g. kettle heats on a gas flame HEAT and WORK HEAT is the energy that transfers from one object to another when the two things are at different temperatures and in some kind of contact e.g. kettle heats on a gas flame cup of tea cools down (loses energy as heat) Work is the transfer of energy that takes place when an object is moved against an opposing force Work stimulates uniform motion Heat and work can be considered as energy in transit

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 Calorie (cal): 1 cal is the energy needed to raise the temperature of 1g of water by 1oC 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”) As most reactions in chemistry take place at constant pressure we can say that: A change in enthalpy = heat supplied 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

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 -ve) e.g. burning fossil fuels Endothermic reactions: Reactants + energy as heat products (H +ve) e.g. photosynthesis

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 < 0 (at constant pressure) Burning fossil fuels is an exothermic reaction Photosynthesis is an endothermic reaction (requires energy input from sun) 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 > 0 (at constant pressure)

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

How do we relate change in temp. to the energy transferred? Heat capacity (J/oC) = heat supplied (J) temperature (oC) Heat Capacity = heat required to raise temp. of an object by 1oC Specific heat capacity is the quantity of energy required to change the temperature of a 1g sample of something by 1oC Specific Heat Capacity (C) Heat (joules or calorie)(q) Mass (m) x change in Temp (∆ T) =

Vaporization Melting Freezing Energy has to be supplied to a liquid to enable it to overcome forces that hold molecules together endothermic process (H positive) Melting Energy is supplied to a solid to enable it to vibrate more vigorously until molecules can move past each other and flow as a liquid endothermic process (H positive) Freezing Liquid releases energy and allows molecules to settle into a lower energy state and form a solid exothermic process (H negative) (we remove heat from water when making ice in freezer)