Equilibrium Flashcards
What is dynamic equilibrium?
A reversible reaction in which the rate of the forward reaction equals the rate of the reverse reaction, in which the concentration of the reactants and the products do not change
How is dynamic equilibrium established?
In the early stages of the reaction, the concentration of reactants is high and products is low, so the rate of forward reaction is much greater than the rate of the reverse reaction, but, as the reaction proceeds, the concentration of reactants decreases and products increases, so the rate of the forward reaction decreases while rate of backwards reaction increases. Eventually, when the reactants and products are reacting at the same rate then dynamic equilibrium will be reached
Why is the equilibrium of water and sodium nitrate dynamic rather than static when there is an excess of sodium nitrate?
There is an interchange of some solute between the solution and the solid - some sodium nitrate dissolves and some crystallises
What happens to the position of equilibrium when more reactant is added?
Equilibrium moves to the right hand side - more products are able to form
What happens to the position of equilibrium when more product is added?
Equilibrium moves to the left hand side
What happens to the position of equilibrium when pressure is increased?
An increase in pressure moves the equilibrium in the direction which produces the fewer moles of gas, no gas = no change
What happens to the position of equilibrium when temperature is increased?
An increase in temperature moves the equilibrium in the endothermic direction
What happens to equilibrium when a condition is changed?
If the conditions of a chemical system initially at equilibrium are changed then the equilibrium will shift in the direction that tends to annul the change, if possible
What happens to the position of equilibrium when a catalyst is added?
A catalyst has no effect on the position of equilibrium as it is a reversible reaction. It speeds up the forward and reverse reactions equally, so equilibrium is reached sooner
What do square brackets mean ( [X] )?
Concentration of X
What is Kc?
Equilibrium constant, expressed in terms of concentration
What is Kc affected by?
Only affected by temperature
How would Kc for a general reaction xA + yB ⇌ nC + mD be calculated?
[C]^n[D]^m / [A]^x[B]^y
Products over reactants
What does a large Kc represent?
A high proportion of products
What does a small Kc represent?
A high proportion of reactants
What does Kc tend towards if the reaction goes to completion?
Infinity
What does Kc tend towards if there is ‘no reaction’?
0
How does adding extra reactants or products affect Kc?
Does not change the value of Kc - the relative proportions will change to re-establish the conditions for Kc to be constant
What are the units of Kc?
Where the number of particles remain constant, the concentration units cancel, so Kc has no units (e.g. in H2 + Br2 ⇌ 2HBr)
For other reactions, Kc has units which must be stated (e.g in 2SO2 + O2 ⇌ 2SO3 where the concentration units do not cancel, so the units for this reaction are mol^-1 dm3 according to the Kc equation)
Can the rate at which equilibrium is reached be seen in the value of Kc?
No.
How does an equation with heterogeneous equilibrium change how Kc is calculated?
If there is a solid in the reaction, then they shouldn’t be included when calculating Kc as they have a constant concentration
Why are catalysts useful in a reversible reaction in industry?
For some reactions (e.g the Haber process) the conditions of the reaction do not favour high yield and efficient rate. which makes a process uneconomic, particularly if a reaction is reversible and exothermic. Because of this, a catalyst can be used to allow a reaction to take place efficiently at a lower temperature so an acceptable yield can be produced at a reasonable rate. This is referred to as a ‘compromise’ temperature as it balances an economic yield with an economic rate
