Ch.9 enthalpy
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what is enthalpy (H)
a measure of heat energy in a chemical system, it can not be measured but enthalpy changes can
enthalpy change AH=
H (products) - H (reactants)
activation energy Ea
minimum energy required for a reaction to take place
standard enthalpy change AH*
enthalpy change under standard conditions
standard pressure
100Kpa
standard temp
298K (25*C)
standard conc.
1 moldm-3
standard state
state of a substance under standard conditions
standard enthalpy change of reaction ArH*
enthalpy change that accompanies a reaction in the molar quantities shown in a chemical reaction under standard conditions, and elements in their standard states
standard enthalpy change of formation AfH*
the enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions with the reactants and products in their standard states
what is the enthalpy change of an element
0 KJmol-1 it is not composed of other elements
standard enthalpy change of combustion AcH*
the enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
standard enthalpy change of neutralisation AneutH*
the energy change that accompanies the reaction of an acid by a base to form 1 mole of H2O under standard conditions, with all reactants and products in their standard states
specific heat capacity (c)
the energy required to rase the temp of 1 gram of a substance by 1K
energy change (q) joules =
mass of surroundings x spec heat capacity of surroundings x temp change of surroundings
determination of an enthalpy change of combustion
150 cm3 water in beaker init temp init weight of spirit burner light and heat breaker, stir with thermometer for 3 mins max temp of water final weight of spirit burner assuming wick has not been burnt calc change in temp, change in mass, spec heat cap of water is 4.18 use q=mcAT calc mole of spirit burnt mass/Mr q/Mol = AcH
what could make the experimental value for AcH less accurate
. heat loss to surroundings other than water . incomplete combustion results in black layer of soot on beaker . evap of spirit from wick . not in exact standard conditions
determination of enthalpy change of reaction or (neutralisation- reactants would both be solutions, same process)
excess zinc powder and 50cm3 of 1 moldm-3 copper(11) sulfate stir with thermometer find max temp and init temp temp change use q=mcAt calc mol of copper sulfate mol = 0.05 q/mol =ArH
average bond enthalpy
energy required to break 1 mole of a specified type of bond in a gaseous molecule
explain an exothermic reaction in terms of making and breaking bonds (break first)
the energy needed to break bonds was less that the energy released when the bond was created so AH is -
explain an endothermic reaction in terms of making and breaking a bond (break first)
the energy required to break the bonds was more than the energy released when the bonds where formed so AH is +
enthalpy change from average bond enthalpies = why is it not standard change
sum of bond enthalpies in reactants - sum of bond enthalpies in products average bond enthalpies use gaseous molecules (not in standard states)
hess’ law
if a reaction can take place by 2 routs, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
using enthalpy change of formation ArH =
sum of AfH products - sum of AfH reactants