equilibria Flashcards
What is a reversible reaction?
● A reaction in which reactants form products and products reform reactants. ● This occurs until an equilibrium is reached.
What is dynamic equilibrium?
The rate of the forward reaction equals
the rate of the reverse reaction, hence
the concentrations of the reactants and
products remains constant.
What is Le Chatelier’s principle?
If a system in dynamic equilibrium is
subjected to a change, then the position
of equilibrium will shift to minimise this
change.
How does increasing the temperature affect the
position of equilibrium?
For an equilibrium where the forward reaction is
exothermic, increasing the temperature will shift the
equilibrium left (so more endothermic reactions occur) to
take in more heat energy and reduce the temperature.
The reverse is true when the forward reaction is
endothermic.
How does decreasing the temperature affect the
position of equilibrium?
For an equilibrium where the forward reaction is exothermic,
decreasing the temperature will shift the position of
equilibrium to the right (so more exothermic reactions occur)
to release more heat energy and increase the temperature.
The reverse is true when the forward reaction is endothermic.
How does the concentration of reactants affect the
position of equilibrium?
Increasing the concentration of reactants causes the
position of equilibrium to shift right in order to reduce
the concentration of reactants and form more products.
The reverse occurs if the concentration of reactants is
decreased.
How does pressure affect the position of equilibrium?
● Increasing the pressure will cause the position of
equilibrium to shift to the side with the fewest gaseous
molecules in order to increase the pressure.
● The opposite occurs if pressure is decreased.
● If there is an equal number of gaseous molecules on
both sides of the equation, changing the pressure will
have no effect on the position of equilibrium.
How does the presence of a catalyst affect the position
of equilibrium and the magnitude of the equilibrium
constant?
● The presence of a catalyst doesn’t affect the position
of equilibrium.
● The magnitude of the equilibrium constant therefore is
unaffected.
● It does however increase the rate of the forward and
reverse reactions so equilibrium is established sooner.
How does temperature affect the magnitude of the
equilibrium constant
● If the forward reaction is exothermic, increasing the
temperature shifts the position of equilibrium to the left
so Kc decreases.
● If the forward reaction is endothermic, increasing the
temperature shifts the equilibrium to the right so Kc
increases.
● The reverse is true if temperature is decreased.
How does pressure affect the magnitude of the
equilibrium constant?
Kc remains the same:
● Doubling the pressure will double both the partial pressures and
concentrations of the species on both sides of the equation.
● The system is no longer in equilibrium so partial pressures of
reactants and products must change to keep Kc the same.
● New equilibrium position will be reached whereby Kc is restored (the
ratio of the Kc expression is the same as before).
How does concentration affect the magnitude of the
equilibrium constant?
● Changing the concentration of a reactant or product
means that the system is no longer in equilibrium.
● The concentrations of the reactants and products
now must change so the ratio and hence Kc is
restored.
● Kc is therefore unaffected by concentration changes.
What does the equilibrium constant tell you?
● The position of equilibrium of a reaction. ● The magnitude indicates whether there are more reactants or products in an equilibrium system.
What is Kp?
● The equilibrium constant for reactions
in the gaseous phase.
● Similar to Kc but it uses partial
pressures instead of concentrations.
How do you calculate the mole fraction of a gas?
(number of moles of gas)/(total number of moles of all present species
How do you calculate the partial
pressure of a gas?
mole fraction x total pressure
What is the Haber process? Write an
equation for this reaction and state the
source of the reactants
● It is a process that produces ammonia:
N2(g) + 3H2(g)⇌2NH3(g) ΔH = -92kJmol^-1
● Nitrogen - fractional distillation of air
● Hydrogen - natural gas
What conditions are used for the Haber process?
● 400 - 450℃
● 200 atm
● Iron catalyst
What temperature would you expect to be used for
the Haber process and why? Why is the temperature
used in industry a compromise?
● The forward reaction is exothermic so a low
temperature would give the greatest yield.
● However, a low temperature gives a slow rate of
reaction and so a higher temperature is normally
used to strike a balance between yield and rate.
What pressure would you expect to be
used for the Haber process and why?
Why is the pressure used in industry a
compromise?
● According to Le Chatelier’s principle, a high
pressure would give the greatest yield of ammonia.
● However, high pressures can be dangerous and
expensive to maintain so a lower pressure may be
used.
What is the Contact process? Write an
equation for this reaction and state the
source of the reactants
● The process for creating sulfur trioxide:
2SO2(g) + O2(g)⇌ 2SO3(g)
ΔH=-196kJmol^-1
● Sulfur dioxide - heat sulfur in air
● Oxygen - air
What conditions are used for the Contact
process?
● 400 - 450℃
● 1 - 2 atm
● V2O5 catalyst
What temperature would you expect to
be used for the Contact process and
why? Why is the temperature used in
industry a compromise?
● The forward reaction is exothermic so a low
temperature would give the greatest yield.
● However, a low temperature results in a slow rate
of reaction and so a higher temperature may be
used to strike a balance between yield and rate
What pressure would you expect to be used for the
Contact process and why? Why is the pressure used
in industry a compromise?
● According to Le Chatelier’s principle, a high pressure would
give the greatest yield of sulfur trioxide.
● However, even at pressures close to atmospheric pressure,
99.5% of SO2 is converted into SO3 so increasing the
pressure would only see a minute improvement in yield that
wouldn’t be economically worthwhile.
What is a Brønsted-Lowry acid?
A proton donor
What is a Brønsted-Lowry base?
A proton acceptor
What is a conjugate acid-base pair?
A conjugate acid-base pair is two species that
differ from each other by a proton (H+ ion)
What is a strong acid?
A strong acid is an acid that completely
dissociates in solution
What is a strong base?
A strong base is a base that completely
dissociates in solution
What is a weak acid?
A weak acid is an acid that only partially
dissociates in solution
What is a weak base?
A weak base is a base that only partially
dissociates in solution
