m3 and m5 - enthalpy and rates of reaction Flashcards
enthalpy def
(H)
the thermal energy stored in a chemical reaction
Ea def
(activation energy)
minimum energy required to start a reaction
by breaking bonds in the reactants
enthalpy diagrams
exo: —__ (giving energy to surroundings so atoms lose it)
endo: __— (taking in energy from surroundings)
add: curvy arrow for activation energy, arrow for delta H
standard conditions and standard states
conditions:
temp in K usually 298K
pressure 100kPa
solutions 1mol/dm3
states: physical state they would be at the standard conditions
enthalpy change of reaction (ΔHr)
enthalpy change between molar quantities of reactants, under standard conditions to give products in their standard states
enthalpy change of formation (ΔHf)
enthalpy change when one mole of a compound is formed from its elements under standard conditions, everything is in their standard states
enthalpy change of combustion ΔHc
enthalpy change when one mole of an element/compound reacts by complete combustion with excess oxygen under standard conditions
enthalpy change of neutralisation ΔHn
enthalpy change when 1 mole of water is formed from a neutralisation reaction
q = mcΔT
m = mass of water usually
c = specific heat capacity 4.18
ΔT = change in temp of water usually
average bond enthalpy def
breaking of 1 mol of bonds in gaseous atoms
exothermic reactions in terms of bond energies (bendomexo)
more energy required to form the new bonds, than to break existing bonds
is breaking bonds endo or exo
endo
BENDOMEXO
bond enthalpy changes
sum of reactant bond enthalpy - sum of product bond enthalpy
(left hand side - right hand side)
Hess’ law def
the total enthalpy change of a reaction is independent of the route taken
(same enthalpy change of a reaction even if it’s done in different steps basically)
hess law in a triangle diagram
A —ΔH1—> B
\ /
ΔH2 ΔH3
\ /
C
ΔH1 = ΔH2 + ΔH3
USING enthalpies of combustion: calculate ΔHf C3H8 using ΔHc data
3C (s) + 4H2 (g) -> C3H8 (g)
\ /
3xΔΗc of C | 4xΔΗc of H2 ΔΗc of C3H8
\ (arrow down) / (arrow down)
CO2 (g) + H2O (l)
ΔΗf = 3ΔΗcC + 4ΔΗcH2 - ΔΗcC3H8
USING enthalpies of formation: calculate ΔΗr using ΔΗf data
ZnCO3 —ΔΗr—> ZnO + CO2
\ (arrow up) / /
ΔHf ZnCO3 ΔHf ZnO | ΔΗf CO2
\ / /
[ Zn (s) + C (s) + O2 (g) ]
ΔΗr = -ΔΗf of ZnCO3 + ΔΗf of ZnO + ΔHf of CO2
= +812 - 348 - 393 = +71kJmol-1 (endothermic)
collision theory
for a reaction to occur particles must:
- collide with enough energy to react (above Ea)
- be in the correct orientation
how does temp effect ROR
increased kinetic energy so increased movement, more successful collisions per second, more likely to have sufficient activation energy
how does concentration/pressure affect ROR
increases, particles are closer together, so frequency of collisions increase
how does surface area affect ROR
increases, more points of contact so frequency of collisions increase