Equilibria Flashcards
What is a reversible change?
A change that can take place in both directions
What happens when a reaction is in a dynamic equilibrium?
The forward and reverse reactions of reactants and products occur at the same rate and the concentrations are constant
Give three examples of reversible reactions and their equations
• Ammonia manufacture: Haber process
3H2 + N2 2NH3
• Co2 reacting in sparkling mineral water
CO2 + H20 HCO3- + H+
• Haemoglobin
Haemoglobin + oxygen oxyhaemoglobin
Describe an experiment to monitor forward and reverse reactions
•Hydrogen and iodine reacy in a reversible reaction to produce hydrogen iodide
H2 + I2 2HI
• two experiments are carried out one starting with hydrogen iodide and the other starting with hydrogen and iodine. In both cases the gasses are sealed in a glass bulb and kept at a constant temperature of 445°c and the reaction is stopped at regular intervals by cooling the bulb rapidly and the contents analysed
• Experiment A: 1 mol hydrogen iodide was put in the flask after 84 mins 0.78 mol hydrogen iodide was present and no further change took place
• Experiment B: 0.5 mol hydrogen and 0.5 mol iodine put in the flask. After 80 mins there were 0.79 mol of hydrogen iodide and the number remained the same.
The equilibrium stayed the same in both experiments
Why does the forward reaction happen quicker than the reverse reaction initially in a closed system?
Because the reactants are at their highest concentrations
What is the equilibrium constant and what’s the symbol for it?
Kc
It is the concentration of all the substances measured if a reversible reaction is allowed to reach equilibrium combined into an expression
What is the equation for the equilibrium constant? And explain it
Kc = [C]c [D]d/ [A]a [B]b
Where the letter on the outside of the bracket is the number of moles of the substance
The right hand side of the equation goes on the top line and the left on the bottom
The square brackets indicate concentrations in mol dm-3 in the equilibrium mixture
What is a closed system?
Where no substances are exchanged with the surroundings and so no reactants or products were lost
What has to be kept constant for an equilibrium to be maintained and why?
The equilibrium concentrations of reactants and products
Because if a reactant is allowed to escape its concentration decreases and the rate of forward reaction decreases do the original state of equilibrium no longer exists
What is the difference between a homogenous and heterogeneous equilibria?
In a homogenous equilibria all the substances are in the same phase (e.g. all gases) in a heterogeneous equilibria they are in different phases
What would be the equation for the equilibrium constant of the following reaction?
2SO2 + O2 2SO3
Kc = [SO3]2/ [SO2]2 [O2]
What is the constant that uses partial pressures rather than concentrations and is used for gaseous reactions?
Kp
How are the units of Kc found?
By substituting the concentration units in the equation for Kc and then cancelling
Calculate these concentration units:
Mol dm-3)2/ (moldm-3)2 (moldm-3
1/ mol dm-3 = mol -1 dm3
What is the acronym in the table you need to draw in order to do calculations involving Kc and what do they stand for?
I= initial moles/ concentration C = change in moles / concentration E = equilibria moles / concentration
In a Kc equation what should you look for if you are only given moles and not volume
That there are equal number of moles of reactants and products and then you dont need the volume
If you are not given the equilibria concentrations how should you solve the equation?
Algerbaically
What does the Kc value tell us?
- If it is 1 or near to one it shows that the concentrations of reactants and products are similar and the position of equilibria lies near the centre
- if the it is greater than 1 the position of equilibria lies towards the products
- if it is less than 1 the position of equilibria lies towards the reactants
What does the position of equilibrium in a reaction tell us and in what occasion would it lie to the right?
The position of equilibrium tells you the proportion of products to reactants
If products concentrations at equilibrium are high and reactant concentrations low then the position of equilibrium lies to the right
What is the position of equilibrium changed and not changed by?
It can be changed by changes in temperature, pressure and concentration
It is not affected by catalysts
What is le Chatelier’s principle?
The position of the equilibrium of a system moves to minimise the effect of any imposed change in conditions
How can le Chateliers principle be applied to the haber process?
N2+3H2 2NH3
Delta H = -92 KJ mol -1
The reaction is exothermic in the forward direction so under heating the reaction moves to the equilibroum position to the left maximising the yield of the reactants. This happens because raising the temperature shifts the equilibrium in the direction of the endothermic change so that the system can absorb the energy.
However in the Haber process we want the products to be made but we cant lower the temperature because although that would increase the yield it would decrease the rate of reaction
What is the effect of inceeasing pressure on equilibria with the following reaction and why?
N2 + 3H2 2NH3
It shifts the equilibria to the right because there are fewer miles of gaseous product on the right than the left and in a gas the pressure depends on the number of molecules colliding and exerting a force. When pressure is increased the equilibrium shifted to the right as the reaction opposed the change imposed on it.
What is the effect of concentration on equilibria?
If the concentration is increased the equilibria will shift to the side with fewer moles to oppose the change
What is the effect of a catalyst?
- it increases the rate of reaction by providing an alternative route with a lower activation energy
- it does not alter the position of the equilibrium because the concentrations of the products and reactants remain exactly the same and the value of Kc does not change but equilibrium is reached more quickly
What catalyst is used in the haber process?
Finely divided iron
Why cant high pressures of temperatures be used in the haber process and what is used instead?
- They are both expensive
- a high temperature decreases ammonia yield
- a compromise temperature of 700 K is used
- a compromise pressure between 5000 and 20 000 kpa is used
