3.2 Physical Chemistry

heat energy transferred in a reaction at constant pressure. the units of ΔH are Kjmol^-1

physical states under standard conditions

Pressure: 100kPa, Temperature: 298k

The enthalpy change when one mole of a compound is formed from its elements in their standard states

The enthalpy change for the complete combustion of one mole of a substance

The enthalpy change for the formation of 1 mol of water from neutralisation

the minimum energy required for a reaction to take place

Work at Q (q=mcΔt) convert to Kj Enthlapy change = -Q/n (n is number of moles)

the energy needed to break one mole of bonds in the gas phase, averaged over many different compounds

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions

gives out energy, ΔH is negative, bond making reactants (less stable) higher then products (more stable)

takes in energy, ΔH is positive, bond breaking Reactants (more stable) lower then products (less stable)

• Find enthalpy of combustion of a flammable liquid, by burning it • As fuel burns it heats water. Use q=mcΔt (4.18j g^-1k^-1 shc of water), -Q/n • Ideally all the heat given out by fuel as it burns would be absorbed by the water, but in practice you always lose some heat Diagram: (use calorimeter) combustion chamber, fuel inside, air flowing through, water with thermometer and stirrer on top being heated

Away

Towards

• If using bond enthalpies in calculations – these are averages, they vary in different compounds. • If measuring energy released in combustion – incomplete combustion could occur, heat could be lost. • If calculating enthalpy of formation – different reactions could occur or additional products may form. • As well as this non-standard conditions can always be a factor

Combine known quantities of acid and alkali in insulated container and measure temperature change

Total energy absorbed - total energy released

If the concentration or pressure of a chemical is increased this means there are more particles in the same volume so there will be more frequent collisions so the rate of reaction will increase

If the temperature is increased the number of collisions per second increases as does the energy of the collisions so the rate of reaction will increase

Catalysts lower the activation energy by providing an alternative path with a lower activation energy for the reaction without being used up. Increasing the rate of reaction.

Homogeneous catalysts are in the same phase as the reactants

Heterogeneous catalysts are in a different phase to the reactants

They mean lower temperatures and less energy can be used with less waste products and a higher atom economy. Reduces demand for combustion of fossil fuels, so reduction is CO2 emission