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INTERACTING EQUILIBRIA In some cases two or more equilibria interact. The common species must satisfy both (or all) equilibria simultaneously. For example, consider the effect of acid (H+)and base (OH)on the solubility equilibrium of magnesium hydroxide. Here, the acid-base equilibrium with water is also involved. Aqueous magnesium and hydroxide ions will react and establish an equilibrium with solid magnesium hydroxide. Mg2+(aq)+2OH(aq)Mg(OH)2(s) As you will find out in a later chapter, in all aqueous systems, water is in equilibrium with hydrogen ions and hydroxide ions. H+(aq)+OH(aq)H2O(l) The two equilibria may be combined since OH(aq) is common. Mg2+(aq)+2OH(aq)Mg(OH)2(s)2H(aq)22H2O(l) An addition of acid (increase in [H+]) causes the following processes to occur: 1. a shift in the water equilibrium towards water (consuming some of the increase in [H+]), causing. 2. a decrease in the [OH]causing, 3. a shift in the Mg(OH)2 equilibrium towards the ions, replacing some of the decrease in [OH] The result is that some, or all, of the Mg(OH)2(s) dissolves. The effects of adding OH-(aq) would produce the opposite shifts and more Mg(OH)2(s) would form. Write the net ionic equation for the reversible reaction of Ba(NO3)2(aq) with K2CrO4(aq). (1.5 marks) a) There are three interacting (competing) equilibria at work here: i. the barium chromate precipitate equilibrium (written for question 4) ii. the chromate-dichromate equilibrium 2CrO42(aq)+2H+(aq)Cr2O2(aq)+H2O(l) iii. the acid-base equilibrium H+(aq)+OH(aq)H2O(l) Write these three chemical reactions (joined together) showing how the equilibria interact. For an example see the "Interacting Equilibria" section in the introduction. Number the equations 1, 2 and 3 so that you can refer to them when answering the next question. (3 marks) The BaCrO4 precipitate reformed when NaOH (aq) was added. Using your answer to Question 5a, describe, in detail, why this happened and show (using arrows next to the reactions that you wrote above) how the three equilibria would shift