CHEM UNIT 4: EQUILIBRIA

At equilibrium the amounts of reactants and products stay the same

• as reactants get used up, the forward reaction slows down - and as more product is formed the reverse reaction speeds up. After some time, both forward and backward reactions will be going at exactly the same rate
• dynamic equilibrium - amount of reactants and products not changing because the reactions are going at the same rate
• can be set up in physical systems - when bromine (l) is shaken in a closed flask, some of it changes to orange bromine (g). after a while equilibrium is reached - bromine (l) still being turned into gas and bromine (g) is turning into liquid but at the same rate
• can be set up in a chemical system - if hydrogen(g) and iodine (g) are mixed together in a closed flask, hydrogen iodide is formed
• can only happen in a closed system at a constant temp.

many important industrial processes are reversible

• contact process to make sulfuric acid for dyes, med and batteries, one stage is 2SO2(g)+O2(g)       2SO3(g)
• haber process to make ammonia for fertilisers and other nitrogen based compounds N2(G) + 3H2(g)       2NH3(g)
• economically important

Equilibrium concentrations of reactants and products are related

calculate the ration of product conc to reactant conc for an experiment, you will always end up with the same value.

[HI] / [H2][I2]

If you repeated this experiment at 793K using diff starting concs of the reactants or products and put them in the same expression, you would walys end up with the same value. This value is the equilibrium constant, Kc, and it is always constant for a particular reaction when measured at the same temp.

Kc is the equilibrium constant - ratio of products against reactants

aA + bB         dD + eE, Kc = [D]  [E]    /   [A]  [B]

• only applies to homogeneous equilibria (one where all the reactants and products are in the same phase]
• if mixture involves solids and liquids or solids and gases you leave out the conc of the solids. if you've got gases and liquids you need to use Kp instead

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total pressure = sum of partial pressures of individual gases

partial pressure - pressure each individual gas exerts

partial pressures can be worked out from mole fractions mole fraction - proportion of a gas mixture that is a particular gas so if you have 4 moles pf gas in total, and 2 of them are of gas A, then the mole fraction of gas A is 1/2

mole fraction of a gas in a mixture = no. moles of gas / total no. moles of all gases in mixture

partial pressure of a gas = mole fraction of a gas x total pressure of the mixture

equilibrium constant, Kp, is calculated from partial pressures

for aA(g) + bB(g)        dD(g) + eE(g) : Kp = p(D)  p(E)   /   p(A)  p(B) 

Kp can be used to find partial pressure

Kp for heterogeneous equilibria still only include gases - dont include solids or liquids

Total entropy change is related to the equilibrium constant, K

a spontaneous reaction produces lots of products sp it will have a high equilibrium constant. Its spontaneity depends on the total entrop change for the reaction, Stotal

Stotal = RlnK (R = gas constant = 8.31 J/K/mol

Size of K tells you how far a reaction has progressed

• higher the value of K, the greater the conc of product and therefore the further the forward reaction has progressed - make the conc. of reactants low
• low value of K = very little product formed
• reaction with equilibrium constant of less than 10^-10 does not take place
• reaction with equilibrium constant greater than 10^10 goes to completion
• reactions with intermediate values are reversible
• total entropy change for a reaction is positive - reacion will be spontaneous

Changing the temp of a reaction affects Stotal

When you increase the temp, the value of   H/T decreases

• for an endothermic reaction - increasing temp will increase Stotal
• for exothermic reaction, increasing temp will decrease Stotal

-Predicts what will happen if conditions are changed, shows how the position of equilibrium will change

• If position of equilibrium moves ot the left, you'll get more reactants
• if it moves to the right you get more products 
• if there's a change in temp or pressure, the equilibrium will move to counteract the change
• raise temp - position will shift to try to cool things down
• raise the pressure - position will shift to reduce it again

Temp. alters Kc and Kp - pressure doesn't

• pressure - (only affect equilibria involving gases) - increasing pressure shift the equilibrium to side with fewer gas moles. Kp stays the same
• temp - increase temp (add heat) equilibrium shifts to endothermic side (positive   H) to absorb this heat
• catalysts have no effect on the position of equilibrium - can't increase yield but do mean that equilibrium is reached faster

Both temp and pressure affect the rate of reaction

• increase temp, particles move around more and their average kinetic energy is higher - collide more often and are more likely to react when they do - speed up reaction
• only affect gaseous reactions, increase pressure, gas particles will be pushed closer together - increases the chances of particles colliding and reacting so the reaction speeds up

The Haber process

temp chosen is a compromise

• exothermic reaction - lower temps favour the forward reaction
• but lower temp means a slower rate of reaction
• temp chosen is a compromise between maximum yield and a faster reaction
• another way of looking at it is to consider the entropy changes involved

High pressure = big yield but it'd be expensive

• higher pressure = favour the forward reaction
• increasing pressure = increases rate of reaction
• really expensive to produce - need strong pipes and containers to withstand the high pressure so to minimise costs you have to make a compromise with the pressure

during this industrial process the reaction never actually reaches equilibrium - does not take place in a closed system. Gas mixture continually leaves the reactor and is liquefied so that the ammonia can be removed - unreacted nitrogen and hydrogen are recycled  

designed to maximise atom economy

% atom economy = mass of atoms in product / mass of atom in reactants x 100

• recycling unreacted materials  
• finding an alternative route for synthesis - decreasing the steps involved in making it

industrial processes need to be controlled

• will the reaction happen? less than Stotal = -100J/K/mol are unlikely to go on even with changes to temp and pressure, if Stotal = >0J/K/mol reaction should work without interference
• how fast is it? need to be fast to be economical - changes to temp, pressure and adding a catalyst can speed this up
• can atom economy be increased? will help reduce waste and keep costs down
• any ways of reducing energy consumption? e.g. by using heat exchangers - keep production costs down
• safety procedures? if high pressures and temps are used there need to be safeguards in place to protect the workforce and the environment - same if any of the waste products are toxic or highly flammable