Chemistry P2*

CHEMISTRY PAPER 2 

Collision theory - Chemical reactions can only occur when reacting particles collide with each other and with sufficient energy. 

Activation energy - the minimum amount of energy that particles must have before they react. 

Increasing the below increases the frequency of collisions the rate of reactions. 

Concentration of reactants 

• The more of a reactant there is, the more likely the particles are to collide and react. 

• Pressure of reacting gases 

• Increased pressure pushes gas particles more closely together, causing more collisions and reactions. 

• Surface area of solid gases 

• The larger the SA: Volumeratio, the faster the reaction 

• Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, ∴ increasing the rate of reaction.  

• This is because a higher proportion of particles have energy greater than the activation energy. 

Catalysts 

 - change the rate of chemical reactions without being used up during the reaction. 

Different reactions need different catalysts. Most use transition elements such as iron (used in the making of ammonia), and platinum (used in the making of nitric acid). 

Enzymes act as catalysts in biological systems. 

Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy. 

  

  

In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions. 

E.g: 

A+B⇌C+D 

The direction of the reversible reaction can be changed by changing the conditions. 

E.g: 

with heat 

ammonium chloride ⇌ ammonia + hydrogen chloride 

  cooled  

If a reversible reaction is exothermic in one direction, it is endothermic in the other. The same amount of energy is transferred in each case. 

  

E.g  

      endothermic 

Hydrated copper sulfate ⇌ anhydrous copper sulfate + water 

    (blue)       exothermic         (white) 

  

Equilibrium is reached when a reversible reaction occurs in apparatus which prevents the escape of reactants, and the forward and reverse reactions occur at exactly the same rate. 

The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction. 

A closed system is a system where no substances are added or taken away from the reaction mixture. 

  

The rate of the forward reaction slows down as reactants are used up 

If the system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change. 

The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle. 

  

The effect of changing concentration 

⦁If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the…