Energetics I

enthalpy change

the heat energy change measured at constant
pressure

standard enthalpy change of reaction

the enthalpy change when the number of moles of reactants as stated in the equation react under standard conditions.

Standard conditions

100kPa and 298K

standard enthalpy change of formation

The enthalpy change when one mole of a compound is formed from its elements, in their most stable states, under standard conditions.

standard enthalpy change of combustion

The enthalpy change when 1 mol of a substance is completely burned in oxygen under standard conditions, all reactants and products being in their standard states.

standard enthalpy change of neutralisation

the enthalpy change when 1 mol of water is produced as a result of the reaction between an acid and an alkali, under standard conditions.

Q=

mcΔT

bond enthalpy

the energy needed to break one mole of bonds in gaseous molecules under standard conditions

mean bond enthalpy

The Enthalpy Change needed to break one mole of bonds, averaged over different molecules

Exothermic

reaction which releases heat energy to the surroundings

Endothermic

reaction which absorbs heat energy from the surroundings

making bonds

exothermic

breaking bonds

endothermic

negative energy change

exothermic

positive energy change

endothermic

reaction profiles

Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.

exothermic reaction profile

reactants are higher than products

endothermic reaction profile

reactants are lower than products

energy level diagrams

do not include activation energy

Endothermic energy level diagram

reactants below products

exothermic energy level diagram

reactants above products

sources of error exothermic displacement reaction

heat lost to environment (energy change is less exothermic as calculated Q is smaller than true value), evaporation of water (takes energy out of solution)

source of error endothermic displacement reaction

heat taken in from environment (temperature change is smaller, so energy change less endothermic)

sources of error combustion of alcohols

not all heat energy transferred to calorimeter (temperature change is smaller so less exothermic energy change), some alcohol evaporates from spirit burner (number of moles burnt is higher so energy change is less exothermic), incomplete combustion (energy change is less exothermic as not all alcohol reacted), water in calorimeter evaporates (removes energy from calorimeter so less exothermic), some of the wick burns = LESS EXOTHERMIC

Hess's law

If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.

Dynamic equilibrium

The rate of the forward reaction is equal to the rate of the backward reaction
The concentration of reactants and products remain constant

Conditions of dynamic equilibrium

In a closed system, at a constant temperature, no material lost to surroundings

Homogenous equilibrium

The equilibrium for materials is all in the same state /phase