Summary
Summary Module 3.2.1 Chapter 9 Enthalpy
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notes on: 1 - enthalpy changes 2 - measuring enthalpy changes 3 - bond enthalpies 4 - Hess' law and enthalpy cycles
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9.1 enthalpy changesEnthalpy + - Enthalpy – measure of heat E in chemical system, E stored within bonds that cant be measured.
change - Enthalpy change – change in enthalpy of reactants and products (HP –HR)
- Can be –ve or +ve = more or less E than reactants.
Conservation - Heat E transferred between system and surroundings in chemical reaction.
of energy - System = chemicals = reactants and products.
- Surroundings – apparatus, laboratory etc.
Heat in or out - Enthalpy change can be measured with transfer between system and surroundings.
- From system to the surroundings – exothermic change.
1) releases heat E to the surroundings – change in H is negative = temp of surrounding increase if E is
gained.
2) any energy loss by chemical balanced by energy gain by surroundings.
3) on graph - R is above and P below with arrow downwards to join = change in H
- From surroundings to system – endothermic
1) change in H is positive as chemical system gains energy.
2) surroundings lose E = temp of surrounding decreases.
3) on graph – R is below and P is above with arrow upwards to join = Change in H.
Activation E - Minimum energy required to break bonds and start a reaction.
- Atoms and ions held by chemical bonds that need to be broken by energy.
- Small Ea – very rapidly = E required to break bonds are readily available from surroundings.
- Large Ea – large E barrier = extreme slow as E required is large and not as available.
Standard - Standard pressure – 100kPa
enthalpy - Standard temp – 298K or 25C
- Concentration – 1moldm-3
- Standard state – physical state of a substance under STPC.
Enthalpy - Enthalpy change accompanying a reaction in molar quantities shown in a chemical equation under
change of STP – so all R+P are in standard states. = E shown in kJmol-1
reaction - A stated equation and value depends on the balancing numbers.
- An equation is given to you, find difference in balancing numbers and use that as your multiplier.
Enthalpy - Enthalpy change when 1 mole of compound is formed from its elements under STPC – all in states.
change of - Compounds: - fractional balancing of reactants and not products - to suit 1 mole of compound.
formation - Elements – formation of element from its element – no change = 0kJmol-1
Enthalpy - Enthalpy change when 1 mole of substance reacts completely with oxygen under STPC in states.
change of - Reactant stays the same and the rest are all balanced. (O2 in fractions)
combustion - P = CO2 + H2O all balanced.
Enthalpy - Energy change accompanying reaction of acid by a base to form 1mol of H20 under STP with
change of states.
neutralisation - Product – H2O has to stay as 1 mole - value for H is same for all neutralisation reactions.
9.2 measuring enthalpy changes
kelvin - 0K = -273C, add 273 to C to get K.
- 1k rise is same as 1C rise.
Calculating E - Mass, SHC and temp change.
change - Mass of surrounding – weighing in g.
- SHC – E required to raise temp of 1g of substance by 1K – insulators have high SHC, water – 4.18.
- Temp chance – final – initial.
combustion - Q = MCT - vol can be M and in kJ.
- Mole – M/Mr
- H = Q/n = ___kJmol-1 (ROUND AT THE LAST PART ONLY)
Accuracy;
- Heat can be lost to surrounding other than water.
- Incomplete combustion or evaporation can happen OR non standard conditions.
reaction - Q = MCT (vol used as M) in kJ so convert.
- N= CV(in dm3)
- H = Q/n in kJmol-1
neutralisation - Use total volume of all solutions = M
- Q = MCT in kJ
- N=CV(dm3)
- H = Q/n in kJmol-1
9.3 bond enthalpies
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