ch3 ammonia production

7/30/2019 CH3 Ammonia Production

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Reversible reactions are exothermic (give

out heat) in one direction and endothermic

(take in heat) in the other. The same

amount (Joules) of heat energy is given outin one direction and taken in in the other

direction.

Energy Changes in reversible reactions

When the forward and the backward reaction are happening at the same time, an equilibrium

can be set up.

At equilibrium there will be both products and reactants present in a mixture – the concentrations of each will not be changing

even though reactions are constantly taking place.

The rate of the forward reaction will be the same as the

rate of the backwards reaction. (Just like the runner on the treadmill -

the runner and the belt are moving at the same speed but in opposite dircetions, so the runner is staying in the same place)

To get an equilibrium, none of the reactants or products must

escape – you need a closed system.



How much is there at equilibrium ?

The amounts of the products and the reactants at equilibrium depends on the conditions(e.g. temperature, pressure).

Changing the conditions will change the amounts of reactants and products present in

the mixture - a new equilibrium gets set up.

We call this changing the position of equilibrium.

- if changing the conditions causes the position of equilibrium to move in the forward

direction (to the right) we get more products, less reactants in the equilibrium mixture

- if changing the conditions causes the position of equilibrium to move in the backwards

direction (to the left) we get less products, more reactants in the equilibrium mixture

N2 + 3H2 2NH3

position of equilibrium towards the right – more ammonia and less

nitrogen and hydrogen at equilibrium

N2 + 3H2 2NH3

position of equilibrium towards the left – less ammonia and morenitrogen and hydrogen at equilibrium



Check your understanding : True or False ?

• All chemical reactions are reversible.

• The mass can change if a solid product is formed in a closed system.

• Equilibrium can only happen in closed systems.

• In a reversible reaction, reactants can form products and products can form reactants.

• A⇌ symbol shows that an equilibrium has been set up.

• Once at equilibrium no more products are formed.

• Once at equilibrium, the forward and backward reactions stop.

[Answers at the end]



Controlling the position of equilibrium - making more product

Whenever we change the conditions, the position of the equilibrium will shift to try and

oppose the change. (This is called LeChatelier’s Principle).

To know the effect of changing temperature on the position of equilibrium, we need to knowwhich direction is exothermic and which direction is endothermic:

The effect of increasing the pressure is to move the position of equilibrium to whichever side

of the equation has fewer moles of gas (look at the numbers in front for the gaseous products

and reactants) thus reducing the effect of the pressure increase:



Raw materials:- nitrogen (N2) which is obtained from the air

(by fractional distillation of air)

- hydrogen (H2) which is mainly obtained from natural gas and steam

(CH4(g) + H2O(g) CO(g) + 3 H2(g))

Equation:

N2 + 3 H2 2 NH3 nitrogen + hydrogen ⇌ ammonia

Conditions in the ammonia reactor:iron catalyst; moderate temperature of about 450°C; high pressure of about 200 atmospheres

Uses of ammonia: Ammonia is a very important substance, used in huge quantities for manufacturing other

chemicals: dyes medicines fertilisers explosives

Case Study: The Haber Process.Use: industrial manufacture of ammonia



How the Haber process works:

By removing the ammonia and recycling the unreacted gases:

- Reactants are used efficiently, not wasted

- Energy used to make reactants is not wasted

- a reasonable yield of ammonia is made as quickly as possible



Why do we use a temperature of 450ºC for the Haber process ?Heat energy is given out when ammonia is formed – the forward reaction is exothermic

N2 + 3H2 2NH3

Should we do the reaction at a high or low temperature to shift the position of equilibrium to

make more NH3 ? If we do the reaction at a low temperature then we should get moreammonia in the reaction mixture at equilibrium.

BUT the rate of reaction is too slow at low temperatures. We therefore use a moderate

temperature as a compromise in order to make ammonia quickly enough to be economical.

We also use a catalyst to speed up the reaction (rather than using more heat), so the process

is more energy efficient.

Why use a pressure of about 200 atmospheres ?4 molecules of gas (N2 and 3 H2) turn into 2 molecules of ammonia gas when ammonia is

formed:

N2 + 3H2 2NH3

What should we do to the pressure to get more ammonia in the equilibrium mixture ?

If we carry out the reaction at high pressure then we will get more ammonia in the reactionmixture at equilibrium.

BUT high pressures take a lot of energy to create, and require greater safety precautions to

prevent leaks or explosions.

We therefore use a moderately high pressure as a compromise.



Answers : True or False ?

• All chemical reactions are reversible. FALSE

• The mass can change if a solid product is formed in a closed system. FALSE

• Equilibrium can only happen in closed systems. TRUE

• In a reversible reaction, reactants can form products and products can form reactants. TRUE

• A⇌ symbol shows that an equilibrium has been set up. FALSE (see note 1)

• Once at equilibrium no more products are formed. FALSE (see note 2)

• Once at equilibrium, the forward and backward reactions stop. FALSE

Note 1: The symbol indicates a reversible reaction, not that an equilibrium has been

established.

Note 2: Products are being formed continually, its just that they are being used up (turned back

into reactants) at the same rate, so the amount of products present is not changing.

ch3 ammonia production

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