C5.3 Flashcards
what are reversible reactions?
a reaction in which the products can react together to form the reactants
what is the symbol used in reversible reactions?
and what does it mean
⇆
means forward reaction
ie. reactants to products
how can a reversible reaction occur to produce either a product or a reactant? (give examples)
altering the reaction conditions
ie. adding water to a product, or heating it up
what is dynamic equilibrium?
describe an analogy
when the forward and backward reaction happen at the same rate - so there is no observable change in amount of product/reactant
The concentration of all the reacting substances remain constant
(digging a hole, you dig it and don’t remove dirt, so the hole does not change)
in what condition can dynamic equilibrium occur?
ONLY in a closed system (so no substances can enter or leave)
is a substance stationary if it is in dynamic equilibrium
no, the forward and backward reactions are still happening, but they both flip sides at the same ‘rate’, so it looks like nothing is changing
do reversible reactions happen by themselves?
sometimes they can be spontaneous but other times reaction conditions have to be changed (ie. heat it up, change conditions etc.)
what is the equilibrium position?
a symbol which gives the idea of the ratio of the equilibrium concentration of products to reactants
ie. shows which one is more or less
how do you calculate the effect of changing pressure on the equilibrium position?
1) calculate moles (of gas) on each side
2) therefore, the position would shift to side with least moles
IN ORDER TO TRY AND GO BACK TO EQUILIBRIUM
if correct attempt question on p188 - example one
why does the equilibrium position shift in the first place?
since one of the conditions have changed, and in order to try and get the amount of products vs reactants at a dynamic equilibrium
how do you calculate the effect of changing concentration on the equilibrium position?
(as it is usually in terms of increasing the concentration of one reactant)
- the equilibrium position shifts away from that substance,
ie. if sulfuric acid is on left, equilibrium position shifts to the right
describe the concept behind the effect of changing concentration on the equilibrium position
- ultimately, this reversible reaction wants to go back to equilibrium right?
- therefore, is one (ie. product’s) concentration increases, then more product is made to counter that in order to balance both substances out
how do you calculate the effect of changing the temperature on the equilibrium position?
1) look at the enthalpy number ie. (∆H = -58kJ/mol)
2) so then write out where the heat is (ie. at reactant side or the product side)
3) the equilibrium position shifts to opposite side
where the heat is
OR
equilibrium shifts to the direction of the endothermic reaction
whole idea is which way would make the temperature go back to equilibrium
how would the enthalpy number look for a
(a) exothermic reaction
(b) endothermic reaction
(a) (∆H = -58kJ/mol)
MINUS NUMBER as overall heat is lost
(b) (∆H = +58kJ/mol)
POSITIVE NUMBER as overall heat is gained
Equilibrium yield
The amount of desired product present in a reaction at equilibrium.
Depends of pressure, temperate, conc. of reactants
Le Chatelier’s principle
‘When a change is made to a reaction at equilibrium, the position of equilibrium moves to oppose the change’
Choosing reaction conditions: high pressure
High pressure = equilibrium yield (in producing methanol)
BUT
High pressures need:
-expensive equipment to compress the gasses,
-a lot of energy to run,
-tough reaction vessels to withstand the pressure
SO compromise pressure chosen : high enough to achieve reasonable equilibrium yield, but not so Hugh to be expensive or hazardous
Choosing reaction conditions: temperature
In Manufacture of methanol, forward reaction is endothermic - so high temps decreases equilibrium yield
BUT
Too low temperature means rate of reaction too low to produce methanol quickly enough to be profitable
SO comprise temperature chosen: low enough to achieve reasonable equiliburm yield, but high enough to achieve a reasonable rate of reaction
Spec - recall something about dynamic equilibrium,
C5.3b recall that dynamic equilibrium occurs in a closed system when the rates of forward and reverse reactions are equal
What is a reversible reaction - pb
One in which the products are able to react with each other to change back into the reactants WITHOUT ANY ADDITIONAL CHANGES
Pb - define equilibrium
When the forwards rate of reaction is equal to the backwards rate of reaction, in a reversible reaction
