Reversible Reactions

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Catalysts

You can increase the rate of chemical reaction by using a catalyst

A catalyst:

-reduces the amount of energy needed for a collision to be successful

-speeds up the reaction

-provides a surface for the molecules to attach to so they are more likely to bump into each other

Enzymes are biological catalysts

Different reactions need different catalysts e.g

Cracking  hydrocarbons uses broken pottery

Iron is used n the production of ammonia

In industry, reducing the rates of reaction reduces costs.

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Reversible Reactions

If a reaction is reversible, the products can change into the reactants and vice versa - the reaction can go forwards and backwards

If you change the conditions, the direction of the reversible reaction can be changed

If a reaction is exothermic in one direction, then in the opposite it will be endothermic

The amount of energy given out in the exothermic reaction is equal to the amount taken in during the endothermic reaction

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Closed Systems

If no reactants are added and no products are removed, it is a closed system

In a closed system, a reversible reaction reaches an equilibrium which is when the rate of the forward reaction is equal to the rate of the backwards reaction

The specific amounts of the reacting substances at equilibrium depend on the conditions of the reaction within the system 

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Changing Reaction Conditions

Le Chatelier's Principle - if a system in equilibrium is subjected to changes in condition, the system will shift to resist the change

In an exothermic reaction:

-the yield decreases when the temperature is increased

-the yield increases when the temperature is lowered

In an endothermic reaction:

-the yield increases if the temperature is raised

-the yield decreases if the temperature is lowered

In reactions involving gases:

-the reaction that produces the least number of gas molecules is favoured by increased pressure

-the reaction that produces the most number of gas molecules is favoured by decreased pressure

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Changing Reaction Conditions 2

If the concentration of a reactant or product is changed:

-the system is no longer in equilibrium

-the system adjusts until it can reach an equilibrium again

If one reactant increases concentration, the position of the equilibrium shifts so that more products are formed and equilibrium is reached again

In contrast, if a reactant decreases concentration, the position of equilibrium shifts so that more reactants are formed and it has reached equilibrium again

These factors, plus reaction rates, determine the optimum conditions in industrial process

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