Energetics (All 3)
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Define enthalpy change
change in energy during a chemical reaction at a constant pressure.
Standard conditions of enthalpy change
pressure always 100 kPa temp always 298 K 1 moldm-3 conc for all reacting solutions
What does BEND + MEX stand for
breaking bonds = endothermic (requires heat) making bonds = exothermic (releases heat)
Define mean bond enthalpy
energy required to break 1 mole of a covalent bond into gaseous atoms averaged over a range of compounds.
Mean bond enthalpies are always…
endothermic (+)
Define standard enthalpy of formation
enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions with all reactants and products in their standard states H2(g) + 0.5 O2(g) –> H2O(l)
State why the enthalpy of formation of Na(s) is 0
Na is an element in its standard state.
Define standard enthalpy of combustion
enthalpy change when 1 mole of a substance is burnt in excess oxygen under standard states and conditions CH4(g) + 2O2(g) –> CO2(g) + 2H2O(l)
What is Hess’s law?
states that the enthalpy change for a reaction is the same whatever route is taken from reactants to products.
To calculate bond enthalpies…
BRP reactants - products
Why is the bond enthalpy calculated using Hess’s law different to the bond enthalpy in the data book?
Hess’s law is an average taken of different samples. the data book value is averaged over a range of different compounds
To calculate enthalpy change of formation…
use Hess cycle or FPR products - reactants
To calculate enthalpy change of combustion
use Hess cycle or CRP reactants - products
2 equations to calculate enthalpy change
Q= Mc delta T Q= energy (J) M= mass of water (g) c= specific heat capacity delta T = change in temp (K or C) delta H = Q/N delta H = enthalpy change KJmol-1 Q= energy in KJ (/1000) N= moles
Process of calculating type 1 - combustion reactions
the mass of water would be equal to the volume given if mass not stated use the equation to calculate Q divide Q by 1000 and then use the delta H = Q/N if temp increases delta H = negative if temp decreases delta H = positive
Process of calculating type 2 - 2 solutions reacting
the mass would be equal to the TOTAL volume of reactants if mass not stated use the equation to calculate Q divide by 1000 and use the delta H = Q/N
Process of calculating type 3 - adding a solid to a solution
the mass would be equal to the volume if mass not stated use equation to calculate Q divide by 1000 and use delta H = Q/N use LIMITING MOLES
To reduce heat loss in calorimetry…
put a lid on calorimeter use polystyrene cup cover with sleeve
To determine a more accurate value of enthalpy change ( 6 marker)
use a polystyrene cup with a lid to reduce heat loss record temp at regular time intervals to plot a graph of temp vs time extrapolate graph back to point of addition to establish the max temp