Energetics (All 3) Flashcards
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
