HCI 2015_H1_CHEM_P2_answers Prelims

1 HWA CHONG INSTITUTION Preliminary Examination Higher 1 CANDIDATE NAME CT GROUP 14S CHEMISTRY Paper 2 Candidates answer Section A on the Question Paper. Additional Materials: Data Booklet Writing paper 8872/02 2 Sept 2015 2 hours READ THESE INSTRUCTIONS FIRST Write your name and CT group on all the work you hand in. Write in dark blue or black pen. You may use a pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue, correction fluid or tapes. Section A Answer all questions. Section B Answer two questions on separate answer paper. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. FOR EXAMINERS’ USE ONLY Paper 1 Paper 2 TOTAL Multiple Choice Section A (Structured) Section B (Free Response) Q1 /18 Q4 / 20 Q2 /10 Q5 / 20 Q3 /12 Q6 / 20 / 30 Subtotal / 40 Subtotal / 40 This question booklet consists of 14 printed pages. 110

2 Section A Answer all the questions in this section in the spaces provided. 1 There is concern over the way the oceans are becoming more acidic as more carbon dioxide dissolves in them (a) (i) Draw a “dot-and –cross” diagram for carbon dioxide [1] (ii) Use your diagram to state and explain the shape and bond angle of a carbon dioxide molecule. Linear and 180 o  Two areas of electron density ( bonding pairs) around central C atom  Bonding pairs / electron pairs repel each other; get as far away from each other as possible to minimize repulsion.  ………………………………………………………………………………… [3] (iii) Carbon dioxide forms hydrogen bonds with water. Draw a diagram to illustrate this. Partial charges ; lone pairs ; [2] (iv) Carbon dioxide is not very soluble in water. Suggest an explanation for this in terms of hydrogen bonding. Hydrogen bonds in water Fewer hydrogen bonds between CO 2 and water.  ……………………………………………………………………………… [2] Hydrogen bond

3 (b) When carbon dioxide dissolves in water of the ocean, the following reactions occur. CO2(g)  CO2(aq) equation 1 CO2(aq) + H2O(l)  HCO3 r (aq) + H+(aq) equation 2 HCO3 r (aq)  H+(aq) + CO3 2r (aq) equation 3 (i) The reaction in equation 3 can reach a state of dynamic equilibrium. Explain what is meant by the term dynamic equilibrium. Rate of forward = rate of backward reaction Concentrations of reactants and products remain constant ( formed at same rate) [2] (ii) Suggest why the balance o