3.2.3 chemical equilibrium Flashcards
What does an arrow represent in an equation
The reaction going to completion (all reactants used up)
Why do some reactions not go to completion
As they’re reversible - products can’t reacts to re-form original reactants
What’s dynamic equilibrium
When rate of forward + reverse reaction are equal and concentrations of reactants and products are constant
Where will dynamic equilibrium only occur
In closed systems (nothing allowed to escape from reaction vessel)
E.g of a homogenous equilibrium reaction (all products and reactants in same physical state)
Esterification reactions
Gas phase reactions -> the haber process
E.g of a heterogenous equilibrium reaction (products and reactants in different physical states)
Production of quicklime CaO, used on building industry
What is Le Chatelier’s Principle
If any factor is changed which affects an equilibrium, the position of the equilibrium will shift so as to oppose the change
3 factors that can affect the position of equilibrium
Pressure (gases)
Temperature
Concentration
2 factors that surprisingly don’t affect the position of equilibrium
Catalysts
Surface area
What tells you the pressure of a molecule
Big number in front
What does the equilibrium do if pressure is increased (when more moles on left)
Forward reaction is favoured as this will reduce number of moles and pressure
This restored the pressure to its original lower value
so the equilibrium shifts to the right
More moles on left -> pressure lowered by equilibrium shift to the right
What does the equilibrium do if pressure is decreased (when more moles on left)
Reverse reaction is favoured as this will increase moles + pressure
Equilibrium shift left
If each side has equal number of moles and pressure is increased what is the effect
No effect as each side has equal number of moles
What does the equilibrium do if temperature is increased (when reaction is exothermic)
Backward (endothermic reaction) is favoured as will absorb added heat + oppose temp increase
Equilibrium shifts left
What does the equilibrium do if temperature is decreased (when reaction is exothermic)
Forward (exothermic) reaction is favoured as it releases heat energy, opposing the temp decrease
Equilibrium shifts right
What does the equilibrium do if concentration is increased (of a reactant)
Equilibrium shift right
Causes removal of added reactant , restoring it to original lower concentration
What does the equilibrium do if concentration is decreased(a reactant removed)
Equilibrium shift left
System attempts to replace removed reactant
What does a catalyst do to both forward and backward reactions
Increases rate of them both equally
Why does a catalyst have no effect on position of equilibrium
However what does it affect
Has no effect as both the reaction rates are equally increased
But it will increase rate at which equilibrium is reached
What is the aim for industries
To produce the best yield of product with minimum costs and time
What is the problem with high pressure in reaction vessels in the haber process
Reaction vessel have to be very strong (costly)
Expensive to maintain such high pressure
Risk of explosion
What’s the pressure used for the haber process as a compromise
200atm
What’s the temperature used for the haber process as a compromise
Why
450*c
As a low temperature would cause a very slow rate
This is high enough to give acceptable rate but not too high that equilibrium would be driven too far left, reducing yield
What pressure and temperature is most efficient for most reactions
High pressure
Low temperature
What’s ethanol a greener alternative for
Petrol
What’s pure methanol used as
A fuel in Indy Car Racing
What’s the general reaction for the equilibrium constant, Kc
aA + bB xX + yY
What’s the equation for Kc
Kc = [products/reactants]
[X]x [Y]y / [A]a [B]b
What do the [ ] tell us in Kc
That the concentration of that substance is in moldm-3
How do we work out the overall moldm-3 concentration in Kc
By cancelling out the moles
What always goes on top of the Kc fraction
The products
For an exothermic reaction, what does increasing the temp do to Kc
Decreases it -> equilibrium move left
For an endothermic reaction, what does increasing the temp do to Kc
Increases the Kc -> equilibrium move right
2 things Kc isn’t affected by
Concentration
Catalysts
What is the Kc of the reverse reaction
The inverse
What is said to be happening in reactions with Kc greater than 10*10
Going to completion e.g combustion’s
What is said to be happening in reactions with Kc less than 10*-10
They arent taking place at all (non-existent)
