Chemistry Unit three 2:3:14 - 2:3:15

Revisible reactions

A reactive metal reacts with an excess of acid to produce a salt and hydrogen.

When an reaction is reversible it can take place in either the forward or reverse direction.

Dynamic equilibrium

When nothing seems to be happening, the system is called dynamic.

Because as fast as the reactants are convered into products, the products change back.

You can't see anything but the reaction is still happening in both directions and the rate of reaction is equal for both ways. 

Factors affecting the position of equlibrium

- concentration of the reactants or products 

- pressure in reactions involving gas

- temperature

Le Chatelier's principle 

 states that when a system in dynamic equilibrium is subjected to a change, the position of the equilibrium will shift to minimise the change.. 

The effect of concentration on equilibrium 

Increasing the concetration of a reactant; the system opposes the change by decreasing the concentration of the reactant by removing it; the position of equilibrium moves to the right-hand side, forming more products 

Increasing the concentration of a product; the system oppsose the change by decreasing the concentration of the product by removing it; the position of equilibrium will move to the left-hand side, forming more reactants. 

The effect of pressure on equilibrium (gases only)

Increasing the total pressure will move the position of the equilibrium to the side with less gas molecules, as this will decrease the pressure.

Decreasing the total pressure will move the positio of the equilibrium to the side will more gas molecules, as this will increase the pressure. 

The effect of temperature on equilibrium (depends on the enthalpy sign)

Increasing the temperature moves the position of equilibrium to move in the direction that decreases the temperature, the endothermic direction. 

Decreasing the temperature moves the position of equilibrium to move in the direction that increases the temperature, the exothermic direction. 

The effect of a catalyst on equilibrium

A catalyst increases the rate at which equilbrium is established, and equally speeds up both sides. 

Equlibrium and industry 

The haber process; (prepartion of ammonia)

The raw materials are readily available; Nitrogen is obtained from the air by fractional distillation; Hydrogen is prepared by reacting together methane and water   The favourable conditions for the production of ammonia; forward reactions needs high pressure; forward reaction needs low temperature.                                         However; low temperature means slow reaction, only a few of the nitrogen and hydrogen molecules have energy equal to, or greater than, the required activation energy. Large quantities of energy are required to compress the gas at high enough pressure adding to the cost.

The modern ammonia plant (compromise)

Temperature - high enough to allow reaction at an acceptable eqilibrium yield (400 - 500)

Pressure - high pressure but not dangerous (200 atomspheres)

Catalyst -  an iron catalyst is added to speed up, less energy is used to generate heat,   reducing cost