Chemical Equilibrium Chemical Equilibrium Static Equilibrum n The

Chemical Equilibrium

Chemical Equilibrium Static Equilibrum n The entire system is not moving n Eg: A meter stick that is suspended at its centre pf gravity. Dynamic Equilibrum n The system is in motion. n Two opposing motions balance each other out. n Eg: a man on a threadmill.

Chemical Equilibrium n n n Some reactions go to completion. Eg: Magnesium burning in air. 2 Mg + O 2 → 2 Mg. O n n n Many chemical reactions don’t go to completion (equilibrum reactions). Take place in both directions (reversable) Eg: formation of ammonia from H 2 + N 2 3 H 2 + N 2 2 NH 3 Dynamic equilibrium

Chemical Equilibrium n n At equilibrum the rate of the forward reaction equals the rate of the reverse reaction. The conc at equilibrum remains constant

Chemical Equilibrium n Definition: Chemical Equilibrum is a state of dynamic balance where the rate of the forward reaction equals the rate of the reverse reaction.

Le Chatelier’s Principle n n 1888 Henri Le Chatelier put forward a rule which allowed chemists to predict the direction taken by an equilibrum rxn when the conditions of the rxn were changed. RULE: If a stress is applied to a system at equilibrum, the system readjusts to relieve the stress applied. Stress = change in condition of the reaction eg temp/conc/pressure Catalyst=reaches equili quicker

Le Chatelier’s Principle n n When a stress is applied the system will react to minimise the stress Ie: return to its original equilibrum

Le Chatelier’s Principle n Consider:

Each test tube is at equilibrum

Each test tube is at equilibrum n What happens when palced in ice? Light Yellow (more dinitrogen tetraoxide made) n What happens when placed in hot water? n Darker Brown (more nitrogen dioxide made) n

What were the stresses? n The change in temperature hot and cold. n n n When placed in hot water it carried out the reaction which would absorb heat When placed in cold water it carried out the reaction which would create heat This is to minimise the effect of the stress

Effects of Changing Pressure of Gases in rxns n n If pressure is increases the equilibrium is shifter the side of the equation which has the smaller number of molecules. Why? n The smaller number of molecules will occupy less volume thus reduce the pressure

Le Chatelier’s Principle n State Le Chatlier’s Principle: n Le Chatlier’s Princilpe states that if a stress is applied to a system at equilibrum the system will readjust to relieve/minimise the applied stress

Le Chatelier’s Principle Predicts: n n That in an all gaseous rxn an increase in pressure will favour the rxn which takes place with reduction in volume ie, towards the side with the smaller number of molecules Only affects equili rxns with unequal no’s of gaseous reactants and products

Equilibrium and Catalysts n A catalyst speeds up the rate at which equilibrium is reached but does not change the position of equilibrium. n Increases both the rate of the forward and reverse reactions

Mandatory Experiment n n To demonstrate Le Chatelier’s Principle: A) Effect of n n Temp Conc changes

n What happens when n Add HCl n n Add water n n Turns blue Turns red Place in hot water n Turns blue

To demonstrate Le Chatelier’s Principle: n n n B) Effect of Concentration Originally the soln is orange. Where does the equilibrium lie?

What happens when n Add dilute Na. OH n n Turns yellow Add dilute HCl n Turns orange

To demonstrate Le Chatelier’s Principle: n n n C) Effect of Concentration Originally the soln is red. Where does the equilibrium lie?

What happens when n Add dilute HCl n n Turns blue Add iron(III) chloride n Turns red

Industrial Applications of Le Chatelier’s Principle n n Manufacture of ammonia by the Haber Process Manufacture of sulfuric acid by the contact process

The Haber Process n n n Use: Fertilisers (80%), explosives Cleaning agents Objective: Produce as much max amount of NH 3 as cheaply as possible. What used: n n H 2 from Natural Gas N 2 from Air

The Haber Process n The amount of NH 3 produced depends on: n n Temperature Pressure n Catalyst (Iron)

Achieving max yield Pressure: n Increased pressure at equilibrium favours the production of NH 3 n Carried out at high pressure (200 atmospheres) n Pressure cant be too high: n n Expensive to build high pressure plants Problems like safety

Temperature: n A lower temp will cause more ammonia to be produced n Temp cant be too low: n n n Rate of reaction will be to slow Not enough collisions having energy = activation energy Temp used = 500°C

Catalyst: n In absence of catalyst the rxn has a high activation energy. n Catalyst used because: n n Brings system to equilibrium faster by lowering Eact Rxn can now proceed at a lower temp reducing fuel costs

Thus: n Le Chatelier’s Principle tells us that the best conditions to produce NH 3 in the Haber process is under conditions of: n n High pressure Low temperature

Manufacture of sulfuric acid by the contact process n n n Manufactured by the Contact Process H 2 SO 4 used in the manufacture of paints, detergents, fertilisers, plastics, fibres, car batteries. Sulfur trioxide is the desired product as it dissolves in water to give H 2 SO 4.

The Contact Process n n Called the contact process as very close contact must be made between the two reactant gases and the catalyst. The amount of SO 3 produced depends on: n n Temperature Pressure n n Catalyst Concentration

Temperature: n A lower temp will cause more SO 3 to be produced n Temp cant be too low: n n n Rate of reaction will be to slow Not enough collisions having energy = activation energy Temp used = 450°C used as found the catalyst works best at this temp.

Pressure: n Increased pressure at equilibrium favours the production of NH 3 n Carried out at a pressure just higher than atmospheric pressure. n Pressure cant be too high: n n Expensive to build high pressure plants Problems like safety

Concentration n By removing the SO 3 as it is formed reduces its conc thus shifting the equilibrium to the right

Catalyst n Brings the reaction to equilibrium faster by reducing the activation energy.

The Equilibrium constant n Calculations to find the conc of products & reactants at equilibrm n n System must be at equilibrium Temperature must stay constant

The Equilibrium Law

The Equilibrium constant n n Every equilibrum rxn has its own value for Kc at a particular temperature The larger the value for Kc the futher the equili is pushed towards the products. The smaller the value for Kc the futher the equili is pushed towards the reactants. We do not need to know the unit of Kc.