H070 Topic Title H470 Topic Title Delivery Guide (Learner Resource) Equilibria H070 Topic Title H470 Topic Title

Most chemical reactions can only go one way Most chemical reactions can only go one way. The reaction of methane and oxygen for example: CH4(g) + O2(g)   CO2(g) + 2H2O(l) It is almost impossible to return the carbon dioxide and water to the original methane and oxygen.

CoCl2·6H2O(s) CoCl2(s) + 6H2O(l) Some chemical reactions, however, will go backwards and forwards depending on the conditions. CoCl 2.6H2O(s)  CoCl 2(s) + 6H2O(l) pink blue This is the symbol for used for reversible reactions. CoCl2·6H2O(s) CoCl2(s) + 6H2O(l)

Equilibrium Reversible reactions reach a balance point, where the amount of reactants and the amount of products formed remains constant. Most common misconception is that they are equal. This is not the case. They remain constant.

Position of the equilibrium Equilibrium can “lie” to the left or right. This is “in favour of products or the forward reaction” or “in favour of reactants or the reverse reaction” Meaning that once equilibrium has been reached there could be more products or more reactants in the reaction vessel. LEFT (REACTANTS) RIGHT (PRODUCTS)

Le Chatelier’s principle involving AMOUNT or CONCENTRATION If you increase A or B you will make more C or D so it will FAVOUR the forward reaction and the equilibrium will LIE to the right. If you increase C or D you will make more A or B so it will FAVOUR the reverse reaction and the equilibrium will LIE to the left. C D A B

Le Chatelier’s principle involving TEMPERATURE If you increase the temperature of an EXOTHERMIC REACTION the reaction will try to cool it down so will FAVOUR the reverse reaction and equilibrium will LIE to the left If you cool down an EXOTHERMIC REACTION it will try to increase the heat so will FAVOUR the forward reaction and equilibrium will LIE to the right. NB: If it is exothermic forward, it will be endothermic in reverse. A B C D HEAT

Le Chatelier’s principle involving PRESSURE If you increase the pressure it will favour the reaction with the smallest number of moles. In this case it would FAVOUR the reverse reaction and equilibrium will LIE to the left. If you lower the pressure it will favour the reaction with the most number of moles. In this case it would FAVOUR the forward reaction and equilibrium would LIE to the right. A B C D E

Strong and weak acids A strong acid is one which is FULLY IONISED in water. It will have a high hydrogen ion concentration HCl  H+ + Cl- in this case ALL of the H+ is made so strong acid pH1 A weak acid is one which is NOT fully ionised and is in equilibrium. It has a low hydrogen ion concentration CH3COOH CH3COO- + H+ In this case there are SOME H+ but not ALL so this makes the acid weaker (ie pH3) . EQUILIBRIUM DOES NOT LIE TO THE RIGHT.

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