Chemistry - Chemical reaction (PART 2)

A reversible reaction is a reaction which…

you can change the direction of the reaction by changing the reaction conditions

Equilibrium

Means there is no overall change, the amounts of the substances remain steady

Dynamic

There is a continual change at particle level - products break down while the reactants form

A reversible reaction in a closed system is at equilibrium when…

The foward and reverse reactions take place at the same rate, the concentrations of reactants and products are no longer changing.

Shifting the equilibrium

When a reversible reaction is in equilibrium and you make a change, the system acts to oppose the change and restore equilibrium. A new equilbirum mixture forms. By changing the conditions you can shift the equilibrium

What does it mean by the system acts to oppose the change and therefore shifts the equilibrium?

The reaction shifts in the direction that reduces the effect of the change and eventually reaches new equilibrium.

What are four changes to shift equilibrium?

Change the temperature, change the pressure and change the concentration and using a catalyst.

How does changing the temperature shift the equilibrium?

E.g Ammonia, the foward reaction is exothermic, reverse reaction is endothermic. Heating speeds up both reactions but it favours the endothermic reaction. More ammonia breaks down in order to use the heat, so the reaction reaches equilibrium faster but the new equilibrium mixture has less reactants

How does changing the pressure shift the equilibrium?

By changing the pressure more of the gas mixture is forced into a space and more molecule are present. This change favours the foward reaction has it will reduce the the number of particles present. Equilibrium shifts to the right

How does changing the concentration shift the equilibrium?

Equilibrium mixture is a balance between nitrogen, hydrogen and ammonia. If ammonia is removed, more nitrogen and hyrdogrn will react to form ammonia to restore equilibrium. Yield increases

What is the effect of a catalyst on equilibrium?

Speeds up reaction to reach equilibrium, doesn't change the position of the equilibrium

How to choose the optimum conditions

Use high pressure and remove ammonia (product) to improve yield, use moderate temperature and catalyst to get a decent rate

How is yield increased by changing the concentration?

By adding more reactant or removin products

How is yield affected by changing the temperature?

If foward reaction is exothermic and temp is increased, yield decreases. If foward reaction is enothermic and temp is increased, yield is increased

How is yield affected by changing pressure>

If there are fewer molecules on the right of the equation, increasing pressure, means increasing yield. If there is fewer molecules on the left of the equation, increasing pressure means decreasing yield

What is the effect of using a catalyst on yield?

Nothing, only speeds up reaction

What is the haber process used for?

manufacturing ammonia

What is the symbol equation for the Haber process?

N₂ + 3H₂ ⇋ 2NH₃

Where is the sources of nitrogen from in the haber process?

From the air which is 80% nitrogen and 20% oxgyen. Oxgyen is removed by burning it with hydrogen to form water.

Where is hydrogen obtained for the haber process?

From reacting natural gas (methane) with stem

CH4 + H2O --> CO2 + 4H2

What are the steps for the haber process

• Gases are mixed and scrubbed to remove impurities

• Mixture is compressed, more gas is pumped in until pressure reaches 200 atmospheres (to maxmise yield and speed up reaction)

• Compressed gas flows to converter - round tank with beds of iron at 450 celisus where the key reaction occurs. Iron is the catalyst, but less than 30% of the mixture leaving is ammmonia

• Mixture is cooled until ammonia condenses to liquid, nitrogen and hydrogen are recycled to the converter for another chance to react. Steps 3 and 4 are continually repeated

• Ammonia is run into tanks and stored as liquid under pressure

Chosen conditions for Haber process

Conditions for highest yield is at 350 celsius and 400 atmosphere, however considering safety, speed and cost, haber process is at 450 celsius and 200 atmosphere.

Why isn't the best conditions used for the haber process?

At 350 celisus th reaction is too slow (consumers are waiting), maintaining 400 atmosphere takes a lot of energy and therefore money and it isn't that safe.

What is the contact process used for?

Manufacturing sulfuric acid

Raw materials used in the contact process

Sulfur or sulfur dioxide, air and water. Sulfur is mined and sulfur dioxide is produced when metal sulfde ores are roasted in the air

What is the symbol equation for the conversion of sulfur dioxide to sulfur trioxide in the contact process?

2SO2(g) + O2(g) ⇌ 2SO3(g)

What is the steps for the contact process?

• Sulfur is burned in the air to form sulfur dioxide

• Sulfur dioxide is then mixed with more air and passed through four beds of catalyst (pellets of vanadium V) at 450 celisus to form sulfur trioxide. Unreacted sulfur dioxide is recycled for another chance to react

• Sulfur trioxide gas is then dissolved in concentrated sulfuric acid. (sulfur trioxide does react with water to produce sulfuric acid but produces deadly mist of acid)

• Solution is mixed carefully in water to form concentrated sulfuric acid

What is the symbol equation for making sulfuric acid?

H20 + SO3 -> H2SO4

What is the chosen conditions for the contact process?

At 450 celisus, (as the catalyst is essential but inactive below 400 celisus, but works better at higher temp, 450 celisus is a compromise), at 2 atmosphere of pressure (which is low but yield is acceptable, raising pressure would cost more), removal of sulfur trioxide and using a catalyst of vanadium (V) oxide