8. energetics I (as)
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enthalpy change (delta H) + units
the heat energy change in a reaction at constant pressure kJ/mol
exothermic reaction
give out heat energy delta H is negative temp often goes up
endothermic reaction
absorb heat energy Delta H is positive temperature often falls
enthalpy level diagrams show
the overall change of a reaction
reaction profile diagrams show
the enthalpy changes during a reaction
standard enthalpy change conditions
100kpa temp 298 kelvins
types of standards enthalpy changes
• standard enthalpy change of reaction • standard enthalpy change of formation • standard enthalpy change of combustion • standard enthalpy change of neutralisation
standard enthalpy change of reaction def
is the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation under standard conditions
standard enthalpy change of formation
is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
standard enthalpy change of combustion
is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions
standard enthalpy change of neutralisation def
is the enthalpy change when an acid and an alkali react together under standard conditions to form 1 mole of water
to find enthalpy change for a reaction you need
number of moles of the stuff that’s reacting change in temperature of the reaction
how can you directly measure the temp change of some reaction mixtures
mix reactants together stick a thermometer in reaction mixture measure overall temp change
to find enthalpy change of combustion you need a
calorimeter
calorimeter is used to
find our how much heat is given out by a reaction by measuring the temp change of some water
how to find enthalpy change of combustion of a flammable liquid
• find enthalpy of combustion of a flammable liquid by burning it • fuel burns heating the water • measure heat absorbed by the water
calculate enthalpy change equation
q = mc changeT q= heat lost or gained (joules) m= mass of water or solution (kg) c= specific heat capacity of water (4.18 J/g/k) changeT= change in temp of water (k)
hess’s law
the total enthalpy change of reaction is always the same no matter which route is taken
when reaction happens…
reactant bonds break and products bonds formed
bonds breaking is
endothermic delta H positive need energy to break bonds
bonds formed is
exothermic delta H negative energy released
bond enthalpy def
amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase
mean bond enthalpy def
the energy needed to break one mole of bonds in the gas phase averaged over many different compounds
enthalpy change of reaction = equations
sum of bond enthalpies of reactants - sum of bond enthalpies of product