Topic 10 Equilibria I Flashcards
What is a reversible reaction?
They are reactions that can go backward and forward
Forward reaction - initially reactants are used up quickly but then slow as their concentration drops
Backwards reaction - Initially reactants are reformed slowly but then speed up as the concentration of products increases
When dynamic equilibrium is reached, the forward equals the backward reaction. The concentration of each substance remains constant
Only occurs in closed systems
What is La Chateliers principle?
If a reaction at equilibrium is subject to a change in pressure, temperature or concentration, the position of equilibrium will move to counteract the charge.
If the change in condition results in equilibrium shifting to the left we make more reactant
If the change in condition results in equilibrium shifting to the right
Changing concentration:
If we increase the concentration of a reactant or a product the equilibrium will shift to try to reduce the concentration. (The opposite will happen if concentration is decreased)
3H2(g) + N2(g) <—–> 2NH3(g)
If we increase the concentration of H2, The equilibrium will shift to the right to use it up and reduce the concentration. More NH3 will be produced
If we increase the concentration of NH3, equilibrium will shift to the left to use it up and reduce the concentration. More H2 and N2 will be produced.
THIS ONLY WORKS IN HOOGENOUS EQUILIBRIA (reactants and products in the same state)
Changing pressure:
If we increase the pressure the equilibrium will shift to try to reduce the pressure.
3H2(g) + N2(g) <—–> 2NH3(g)
If we increase the pressure, equilibrium will shift to the side with the fewest number of gas particles. This will reduce the pressure. More NH3 will be produced.
If we decrease the pressure, equilibrium will shift to the side with the most gas particles, this will increase the pressure. More N2 and H2 will be produced
Changing temperature:
If we increase temp, the equilibrium will shift to try to reduce the temp.
3H2(g) + N2(g) <—–> 2NH3(g)
If we increase the temp, equilibrium will shift in the endothermic direction, this will reduce the temperature. More N2 and H2 will be produced
If we decrease the temp, equilibrium will shift in the exothermic direction, this will increase the temp. More NH3 will be produced
Catalysts:
They have no effect on the position of equilibrium. A catalyst will speed up the rate if the forward and backward reactions equally.
A catalyst will speed up the rate at which equilibrium is reached but have no effect on yield
Le Chatelier’s principle making ethanol
Pressure = 60 atmospheres
Temperature = 300 degrees
Catalyst = Phosphoric acid (H3PO4)
C2H4 + H2O <—-> C2H50H
Temp:
Forward reaction is exothermic so decreasing temp will mean equilibrium shifts to the right producing more ethanol. However, a lower temp means a lower rate of reaction. 300degrees is a compromise between yield and rate.
Pressure:
High pressure means equilibrium shifts right producing more ethanol. Less moles of gas on the right so equilibrium shifts right to reduce pressure. Higher pressure increases the rate too. However, higher pressure environments are expensive due to thicker more robust vessels and pipes needed. This is a compromise between yield/speed and cost
What is Kc - The equilibrium constant (Homogenous)
Kc expression can be worked out from the molar concentration in a reaction
Kc = [C][D] / [A][B]
= products / reactants
This is a homogeneous reaction so all reactants and products are included in the final expression (all in the same state)
What is Kc - The equilibrium constant (Heterogeneous)
Kc can be worked out from the molar concentration in a reaction
This is a heterogeneous reaction so we don’t include solids and pure liquids in the final Kc expression. Only gases. This is due to their concentrations remaining constant.
What affects the value of Kc?
Adding a catalyst has no effect on the value of Kc
A catalyst speeds up the forward and reverse reaction
A catalyst speeds up the rate at which equilibrium is established
