Energetics and Reversible Reactions Flashcards
Exothermic and Endothermic
Mexo - Heat energy given out (more energy released from making bonds than breaking bonds)
Bendo - Heat energy taken in (more energy taken in to break bonds than making bonds)
MEXO > BENDO (Energy released making bonds > Energy needed to break bonds = EXO)
Displacement reaction (Calorimetry)
Magnesium displacing copper from copper(II) sulfate
Why polystyrene?
Mg (s) + CuSO4 (aq) -> MgSO4 (aq) + Cu (s)
50cm^3(50g) copper sulfate in polystyrene cup
Record initial temp of solution
Add magnesium (then cover lid)
Record final temp of solution
Polystyrene because it is an insulator which reduces heat loss, more accurate results
Combustion reaction (Calorimetry)
Ethanol is burnt in a small spirit burner
C2H5OH + 3O2 -> 2CO2 + 3H2O
Measure initial mass of spirit burner (including ethanol)
100cm^3 water into copper container, measure initial temp of water with thermometer inside
Light burner until temp rises by 30°C
Extinguish flame, measure final temp of water
Measure final mass of spirit burner (initial - final = mass of ethanol burnt)
Equation to calculate heat energy transferred in calorimetry experiment
Then molar enthalpy change
Q (J) = m (g) x c x DT (°C)
Heat gained/lost = mass of solution x heat capacity of solution x change in temp
ΔH = Q/1000 (converting to kJ) divided by number of moles (mol)
MAKE SURE TO CHECK IF REACTION IS EXO/ENDO FOR NEGATIVE SIGN
Indicator of a reversible reaction
⇌
Dehydration of hydrated copper(II) sulfate (reversible reaction)
CuSO4·5H2O (s) (blue crystals) ⇌ CuSO4 (s) (white powder) + 5H2O (l)
-> is endothermic (heat), <- is exothermic (add water)
Decomposition of ammonium chloride (reversible reaction)
NH4Cl (s) (white) ⇌ NH3 (g) + HCl (g)
-> endothermic (heat), <- is exothermic (ammonia and hydrogen chloride cool and mix)
Conditions for dynamic equilibrium
Sealed container - closed system
Dynamic equilibrium definition
When the rate of the forward reaction equals the rate of the backwards reactions;
And the concentrations of the reactants and products remain constant;
Le Chatelier’s principle
If a system at equilibrium is subjected to some change, the position of equilibrium will shift in order to minimise the effect of the change
Effect of a catalyst on the position of equilibrium
No effect on the position of equilibrium
A catalyst speeds up both the forward and backward reactions
However, the system will reach dynamic equilibrium more quickly
Effect of increasing temperature on the position of equilibrium
Equilibrium shifts towards endothermic side
e.g. A + B ⇌ C + D ΔH = -206kJ/mol (shifts to left as backwards reaction is endo.)
Effect of increasing pressure on the position of equilibrium
Equilibrium shifts towards the side with fewer moles/molecules
e.g. 2A + B ⇌ C + D (shifts to left as there are 3:2 moles)
