topic seven/seventeen Flashcards
in homogenous equilibria, the reactants and products are present in the same
phase
four key characteristics of a reactiona t equilbrium
- The forward and reverse reactions occur at the same rate.
- The concentrations of reactants and products at equilibrium are constant.
- Equilibrium requires a closed system.
- There is no change in macroscopic properties at equilibrium.
The same characteristics apply for physical equilibrium as for chemical equilibrium:
Equilibrium is dynamic – the liquid is continuing to evaporate and condense.
The rate of the forward process (evaporation) is equal to the rate of the reverse process (condensation).
The volume of liquid at equilibrium remains constant.
Physical equilibrium can only occur in a closed system.
At equilibrium, there are no changes in macroscopic properties such as colour.
what is Kc
a constant for a given temperature, therefore, it is temperature-dependent. A change in temperature would result in a change in the value of Kc for a reaction (this is discussed in more detail in a later section).
equilibrium constant expression
aA + bB ⇌ cC + dD
Kc=[C]c[D]d / [A]a[B]b
If the Kc value at a given temperature is high, this indicates that
the reaction has almost gone to completion and a high proportion of reactants have been converted to products.
A high value of Kc means that there is a higher concentration of products than reactants in the equilibrium mixture
If the Kc value at a given temperature is low, this indicates that
the forward reaction has barely proceeded and only a small proportion of the reactants have reacted to form products.
A low value of Kc means that there is a higher concentration of reactants than products in the equilibrium mixture
value of Kc when Forward reaction hardly proceeds
Kc < 10^–10
value of Kc ‘Reactants’ predominate at equilibrium
Kc = 0.01
value of Kc when Equal amounts of reactants and products
Kc = 1
value of Kc when ‘Products’ predominate at equilibrium
Kc = 100
value of Kc when Reaction almost goes to completion
Kc > 10^10
what is the value of Kc when the reaction has gone to completion
Kc is v much over 1
what is the value of Kc when the reaction hasnt gone to completion
Kc is v much less than 1
if a reaction is reversed, what happens to the kc
it will be the reciprocal of that value
if the reaction coefficents are doubled, the Kc is
squared
if the reaction coefficients are halved, the kc is
square rooted
what is the reaction quotient Q
a measure of the relative amounts of reactants and products for a reaction that is not yet at equilibrium. The reaction quotient is useful for predicting in which direction a reaction will proceed to reach equilibrium.
what is the different in reaction quotient expressiona dn equilibrum constant expression (q and kc)
kc uses equilibrium values
when Q is less than Kc
the reaction is not yet at equilibriujm
forwards reaction favoured
when Q = Kc
the reaction is at equolibrium
when Q is bigger than Kc
the reacton has exceeded equilibrium
reverse reaction favoured
Le Châtelier’s principle states that:
‘When a system at equilibrium is subjected to a change, the system will respond to minimise the effect of the change.’
The effect of changes in conditions on the position of equilibrium
The chemical process that produces ammonia gas involves the following reaction:
N2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔH = –92 kJ mol–1
the haber process: This means that increasing the pressure will shift the equilibrium position to the right, yielding more what
ammonia
which reaction in the haber process is exothermic
forward (-92kH mol^-1)
As a result the production of ammonia is favoured by lower temperatures. however uually temp of 450 is used for quick reaction time
what catalyst is used in the haber process
iron
what does contact process create
sulfuric acid
main uses of contact process
Fertilisers.
Paints and pigments.
Detergents and soaps.
Dyestuffs.
temperature in the contact process
Applying Le Châtelier’s principle suggests the highest yield of sulfur trioxide would be obtained using low temperatures and high pressures. However, in actuality, very low temperatures cannot be used, as the rate of reaching equilibrium would be too slow, and therefore uneconomic. A further factor to be considered here is that the vanadium(V) oxide catalyst becomes effective only at temperatures above 400°C. The percentage conversion is improved by passing the reacting gases through a sequence of four separate catalyst beds. This repetitive technique produces a conversion of sulfur dioxide to sulfur trioxide that is 99.5% efficient.
pressure in the contact process
Given this high rate of conversion, the use of high pressure is unnecessary, even though it would increase the yield of sulfur trioxide, SO3. The cost of using high pressures is uneconomic and introduces dangers of explosion. A pressure of 1-2 atmospheres, slightly above atmospheric pressure, is used to ensure the gases circulate freely through the catalyst beds.
how to work out Kc from equilibrium concentrations
use equilibrium cosntant equation
how to work out Kc from initial concentrations
the initial concentration, I
the change in concentration, C
the equilibrium concentration, E.
Using the balanced equation for the reaction, we can calculate the equilibrium concentrations of each reactant and product and then find the value of Kc.
the form of the equilibrium expression for Kc,
Kc = kf / kr
If the rate constant for the forward reaction (kf1) is greater than that of the reverse reaction (kr1), the equilibrium constant will be
large, and the forward reaction is favoured
if the rate constant for the backward reaction (kr2) is greater than that of the forward reaction (kf2), the equalibrium constant will be
small and the reverse reaction is favoured
Gibbs free energy change must be what for a spontaneous reaction
negative
The equilibrium constant, K, is related to the standard Gibbs free energy change, ΔG⦵, for a reaction by the equation below:
ΔG⦵ = −RT ln K
R is the universal gas constant (8.31 J K−1 mol−1)
T is the absolute temperature in kelvin (K)
ln K is the natural log of the equilibrium constant, K.
You can see that the more negative the value of the standard Gibbs free energy change, the
larger the value of the equilibrium constant, K. This means that the more spontaneous a reaction is, the further the position of equilibrium lies to the right
if gibbs is negative, ln k is positive and Kc is less than 1, waht is the pos of e?
Lies to the right – favouring the products
if gibbs is zerio, ln k is zerio and kc is 1, what is the pos of e
Equal proportions of reactants and products
if gibbs is positive, ln k is negative and Kc is over 1, waht is the pos of e
Lies to the left – favouring the reactants
Change in Gibbs free energy for a spontaneous reaction.
at equilibrium, gibbs is at a min or max and entropy is at a min or max
Gibbs free energy is at a minimum and entropy is at a maximum.
