4) Energetics
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define enthalpy change
heat changes measured at constant pressure
state what endo and exo reactions are in terms of heat changes and ΔH
EXOthermic = takes in heat energy - so ΔH is negative ENDOthermic = gives out heat energy - so ΔH is positive
state the standard conditions for enthalpy changes, include symbol
θ - pressure 100KPa - temperature 298K - if conc. - 1 moldm-3
define standard enthalpy of formation
ΔHf = the enthalpy change when 1 mole of a substance is formed from its elements, with all reactants and products in standard states, under standard conditions
define standard enthalpy of combustion
ΔHc = the enthalpy change when 1 mole of a compound burns completely in oxygen, with all reactants and products in standard states, under standard conditions
state Hess law
enthalpy change in a reaction is independent of the route taken
define mean bond enthalpy
enthalpy change when 1 mole of gaseous covalent bonds is broken, averaged over a range of compounds
how do you find enthalpy change using bond enthalpies
bonds broken - bonds made = reactants - products
state why the values calculated by bond enthalpies is not the same as predicted by a hess cycle
• as mean bond enthalpies are not specific to a molecule
state the equations needed to calculate enthalpy change from calorimetry data
q = mcΔt ΔH = q/n
required practical 2
they are always endothermic in one direction and exothermic in the other direction
state why combustion of a fuel may lead to inaccurate enthalpies calculated
1 - heat loss to surroundings 2 - may be incomplete combustion
describe a method used to calculate enthalpy change for 2 solutions
1 - measure temperature at regular intervals and stir 2 - add second solution at 4th min, stir, do not record temp 3 - record temp from 5th min onwards until a constant temp reached 4 - plot a graph of temp against time and extrapolate to find ΔH - use a polystyrene cup and lid to insulate and reduce heat loss - extrapolate lines as there is a big temp change in 4th mins which would not be accurate