AQA GCSE Chemistry My Revision Notes - Hodder Plus Home

AQA GCSE Chemistry My Revision Notes - Hodder Plus Home Chemistry 2 Question 8 2.1 Structure and bonding When sodium reacts with chlorine, sodium ions and chloride ions are formed. (a) Explain how a sodium atom changes into a sodium ion. (2 marks) (b) Explain how a chlorine atom changes into a chloride ion. (2 marks) Rubidium (Rb) is in the same group of the periodic table as sodium. Rubidium reacts with chlorine to make rubidium chloride. (c) Predict the formula of rubidium chloride. (1 mark) (d) Explain why the reaction of rubidium with chlorine is similar to the reaction of sodium with chlorine. (2 marks) (e) What holds the sodium and chloride ions together in a crystal of sodium chloride? (2 marks) (f) Sodium and rubidium are metals. Describe the structure and bonding in metals. A diagram may be used if you wish. (4 marks) (g) Explain how the structure and bonding of rubidium allow it to conduct electricity. (2 marks) (h) Nitrogen and hydrogen react together to form ammonia, NH. What type of 3 bonding occurs in this compound? (1 mark) (i) Use a dot-and-cross diagram to show the bonding between nitrogen and hydrogen in ammonia. (2 marks) ? Philip Dobson 2011 1 Philip Allan Updates Question 9 2.2 How structure influences the properties and uses of substances The diagram shows the structure of diamond. (a) Explain why the bonding in diamond gives it a high melting point. (4 marks) (b) Complete the following diagram to show the electron structure of a hydrogen chloride molecule. Use dots and crosses to represent electrons. (2 marks) (c) Explain why hydrogen chloride is a gas at room temperature. (2 marks) The diagram represents the structure of graphite. (d) Graphite is soft, slippery and conducts electricity. Explain why the structure and bonding in graphite give it these properties. (5 marks) ? Philip Dobson 2011 2 Philip Allan Updates The diagram shows the giant structure of sodium chloride, which has a melting point of 801?C. (e) Explain why the bonding in sodium chloride gives it a high melting point. (2 marks) (f) Explain why solid sodium chloride will not conduct electricity but when molten it will. (3 marks) ? Philip Dobson 2011 3 Philip Allan Updates Question 10 2.3 Atomic structure, analysis and quantitative chemistry A 69 g sample of a carbohydrate was found to contain 42 g of carbon and 3.0 g of hydrogen. (a) Calculate its empirical formula. (4 marks) Salicylic acid (CHO) can be used to produce aspirin according to the equation: 763 CHO + CHO ? CHO + CHCOOH 7634639843 salicylic acid aspirin In an experiment a student used 5.0 g of salicylic acid to prepare a sample of aspirin. (b) What is the maximum mass of aspirin the student could have made? (4 marks) (c) The student actually produced 5.2 g of dry, purified aspirin. What was the yield? (2 marks) (d) Give three reasons that could have reduced the yield in this reaction. (3 marks) ? Philip Dobson 2011 4 Philip Allan Updates Question 11 2.4 Rates of reaction Some students did a series of experiments measuring the rates of the reaction between magnesium and nitric acid. (a) In terms of particles, when will a reaction occur? (2 marks) (b) Write the equation for the reaction between magnesium and nitric acid, using the appropriate state symbols. (3 marks) (c) After some preliminary experiments the students decided that temperature would be a difficult factor to control. Suggest why this might be. (2 marks) (d) Why does a steep line on these rate graphs show the reaction rate is fast? (2 marks) (e) Explain why the concentrated solution of acid reacted faster and produced more product than the dilute solution. (3 marks) (f) Why does increasing the temperature of a reaction increase the rate?(3 marks) The students had used manganese(IV) oxide as a catalyst in a different experiment and did some trials with it in this reaction (between magnesium and nitric acid). They found it had no effect on the reaction. (g) What effect were the students looking for? (1 mark) (h) Why did manganese(IV) oxide not work as a catalyst in this reaction? (1 mark) (i) Why are catalysts important in industrial processes? (1 mark) ? Philip Dobson 2011 5 Philip Allan Updates Question 12 2.5 Exothermic and endothermic reactions Some hydrated magnesium sulfate (MgSO.7HO) was heated in a crucible until 42 there was no further change in mass. The following reaction had occurred: MgSO.7HO(s) , MgSO(s) + 7HO(l) 4242 (a) Why was it heated until there was no further change in mass? (1 mark) (b) When some drops of water were put onto the cooled anhydrous product, the solid became hot and some water turned into steam. What does this tell you about this reaction? (2 marks) (c) The reaction is better written like this: MgSO.7HO(s)ÉMgSO(s)，7HO(l) 4242 What can you now say about the forward reaction? (2 marks) ? Philip Dobson 2011 6 Philip Allan Updates Question 13 2.6 Acids, bases and salts (a) Copper sulfate crystals can be prepared by reacting solid copper(II) oxide with an acid. Which acid would be used? (1 mark) (b) Why would excess copper(II) oxide be used? (1 mark) (c) Suggest how you would remove excess copper oxide when the reaction is complete. (1 mark) (d) What process would you use to produce solid copper sulfate crystals from the solution? (1 mark) (e) What is the name given to a reaction in which an acid reacts with a base to make a salt? (1 mark) (f) How would the pH change as the copper(II) oxide reacted with the sulfuric acid? (1 mark) (g) Insoluble salts can be made by precipitation. How would the addition of 2+limewater to industrial effluent remove dissolved lead (Pb(aq))? (2 marks) (h) Write a balanced chemical equation for the limewater reaction in part (g). (4 marks) ? Philip Dobson 2011 7 Philip Allan Updates Question 14 2.7 Electrolysis In an industrial electrolytic process electricity is passed through a solution of sodium chloride in water. (a) Chlorine gas is produced during the electrolysis. Complete the half-equation that describes its formation. (2 marks) –Cl _______ , + _______e 2 (b) The solution left in the cell is alkaline. Which ion makes the solution alkaline? (1 mark) (c) What is the substance (labelled A) that is formed in solution in the cell? (1 mark) (d) Hydrogen is also produced in the cell. Hydrogen ions gain electrons in this process. What is the name of this type of reaction? (1 mark) Another industrial application of electrolysis is the production of aluminium from molten aluminium oxide. (e) Write the half-equation, including state symbols, to show the formation of aluminium at the cathode. (2 marks) (f) Explain why the carbon anodes have to be replaced. (2 marks) (g) Why is aluminium oxide dissolved in cryolite? (2 marks) ? Philip Dobson 2011 8 Philip Allan Updates Answer 8 (a) It loses its one , outer electron ,. (b) It gains one , outer electron ,. (c) RbCl , Make sure your symbols are clear and unambiguous, e.g. the ‘l’ in Cl is a straight line and not even hinting at being a ‘C’ or a capital ‘L’. (d) Rubidium and sodium both have one electron , in their outer shells/highest energy level ,. You can use ‘electron shell’ or ‘energy level’ — whichever you have been taught and use. (e) An electrostatic force , of attraction , (between oppositely charged ions). An ionic bond is ‘an omnidirectional force of attraction between oppositely charged ions’. If you use this description properly you will score maximum marks. There are no marks for ‘omnidirectional’ but it will impress your examiner. (f) Metals consist of a giant structure , of positive ions , with electrons between the ions , holding them together by strong electrostatic attractions ,. A diagram such as this with the ‘sea’ of delocalised electrons and the positive ions correctly labelled will score two of the marks. (g) The delocalised electrons , are free to move ,. (h) Covalent bonding , (i) ? Philip Dobson 2011 9 Philip Allan Updates 1 mark is for a pair of electrons between each N and H, and 1 mark for the non- bonding pair of electrons. You can draw these diagrams with or without the circles to represent the outer electron shells of the atoms. Answer 9 (a) There are a large number of strong , covalent bonds , between the atoms ,, which require much energy , to break. (b) 1 mark is for a pair of electrons between the H and Cl, 1 mark is for the other three pairs of electrons. It is not essential to pair up electrons in diagrams, but it represents how electrons behave (AS chemistry). It also helps you to get the correct number in each shell and make the diagram clearer for the examiner to mark. (c) The forces between the molecules/intermolecular forces , are weak , (and require little energy to be overcome). You must be careful to clearly distinguish between chemical bonds (within simple molecules or giant structures) and intermolecular forces of attraction. It helps to refer to chemical bonds being broken but intermolecular forces being overcome. (d) Any five of the following ideas will score 5 marks: , The atoms are in layers. , There are weak forces of attraction between the layers. , The layers can slide over each other (so graphite is slippery). , Each carbon atom forms three bonds. , Each carbon atom has one electron not used in bonding. , This electron is delocalised/free to move. , Electrons carry current (so graphite conducts electricity). Any reference here to ionic bonding will reduce your score to a maximum of 4 marks. When answering a question with a number of ideas in it (soft and slippery, conducts electricity, structure, bonding) you must be careful to make a point against each one: , soft and slippery – layers slide , conducts electricity – electrons are delocalised , structure — layers , bonding — three bonds per atom ? Philip Dobson 2011 10 Philip Allan Updates (e) Strong forces between the ions , require much energy , to break them. Any mention of atoms in an ionic question will lose at least 1 mark, if not all the marks! (f) In solid sodium chloride the ions are vibrating about fixed positions ,; they are not able to move towards the electrodes. , In molten sodium chloride the ions are free to move/mobile. , Conduction of electricity can only happen when charged particles (electrons or ions) can move towards electrodes. Be careful: ionic compounds conduct electricity because the ions can move, it is not to do with electrons, so do not mention them! ? Philip Dobson 2011 11 Philip Allan Updates Answer 10 (a) Element symbol C H O Mass 42 3 24 Divide by relative mass 42/12 3/1 24/16 Ratio 3.5 3.0 1.5 Smallest whole ratio 7 6 3 Formula CHO 763 Use a table here and you will get full marks without much thought. 1 mark is for working out the mass of oxygen (it is the difference between the sum of carbon and hydrogen and the sample mass). 1 mark is for dividing by the relative masses. 1 mark is for finding the smallest whole ratio; here by multiplying by 2 (often you need to divide by the smallest number, but that does not work every time). The final mark is for writing the formula — forget this and you drop the mark. (b) formula mass of salicylic acid = 138 , formula mass of aspirin = 180 , ,15 g，180 g mol , maximum mass of aspirin,,1138 g mol = 6.5 g , Alternatively: 138 g ? 180 g 180 1,138 180 5,，5 g138 = 6.5 g , 5.2 gyield,，100% (c) , 6.5 g = 80% , Make sure that your answers to numerical questions are clear and you let the examiner know what you are working out at each stage. It is very difficult to award marks to a disordered jumble of numbers and symbols! Try to make your answer look like this one. (d) 1 The reaction may be reversible/not go to completion. , 2 Some produce can be lost when it is separated from the reaction mixture, for example when filtered. , 3 Some of the reactants may react in ways different from the expected reaction. , ? Philip Dobson 2011 12 Philip Allan Updates It is useful to you and the examiner to have such answers divided into sections. The question asks for ‘three reasons’ so number your answers 1, 2 and 3 and you will know when you have finished! ? Philip Dobson 2011 13 Philip Allan Updates Answer 11 (a) Chemical reactions can only occur when reacting particles collide , with each other and with sufficient energy ,. (b) Mg(s) + 2HNO(aq) ? Mg(NO)(aq) + H(g) ,,, 3322 The 3 marks are allocated as follows: , 1 for the correct formulae , 1 for correctly balancing the equation , 1 for using correct state symbols (c) The reaction is exothermic , so it will generate heat and increase temperature as the reaction proceeds ,. amount of product formedrate of reaction,(d) , time This is the gradient of the line on such graphs. , You may be tempted to use lots of words to explain this straightforward relationship but it is best to keep it simple — waffle hides and sometimes loses marks. (e) Increasing the concentration of reactants in solution increases the frequency of collision , hence the rate increases ,. More product was formed because a concentrated solution has more chemical/a greater amount of chemical in the same volume available to react. , (f) Increasing the temperature increases the speed of the reacting particles , so that they collide more frequently , and more energetically ,. (g) An increase in the rate of reaction/faster bubbling. , (h) Different reactions need different catalysts. , (i) They reduce costs. , Rate questions require very simple answers — you can quote the specification and be assured of maximum marks. ? Philip Dobson 2011 14 Philip Allan Updates Answer 12 (a) To ensure the reaction had finished/all the water was driven off. , (b) The reaction gives out heat ,; it is exothermic ,. (c) The forward reaction is endothermic ,; it absorbs/takes in heat from the surroundings ,. This is quite a short question but worth paying attention to. It is asking you about reversible reactions as well as energy transfer during reactions. Stay alert to follow where the question takes you! ? Philip Dobson 2011 15 Philip Allan Updates Answer 13 (a) Sulfuric acid , Give the name, not the formula. You can incorrectly spell a chemical name and possibly still get the mark, but if you make just one small slip with a formula you will not score at all. (b) To ensure all the acid had reacted. , (c) Filter it. , (d) Crystallisation , Not evaporation. You have to evaporate the solution for a time and stop before any crystals are formed. (e) Neutralisation , (f) The pH would increase. , You must answer the question. It is not the same to say that it would become less acidic. The examiner wants to know that you understand the pH scale. (g) By precipitating , a compound formed from lead ions/lead hydroxide ,. 2+2+(h) Pb(aq) + Ca(OH)(aq) ? Pb(OH)(s) + Ca(aq) ,,,, 22 The 4 marks will be allocated as follows: , 2 for the correct formulae (1 mark per side) , 1 for correctly balancing the equation , 1 for using correct state symbols ? Philip Dobson 2011 16 Philip Allan Updates Answer 14 ––(a) 2Cl ? Cl + 2e ,, 2 – 1 mark is for Cl, and 1 for balancing. –(b) The hydroxide (OH) ion , makes the solution alkaline. (c) Sodium hydroxide/NaOH is formed in solution in the cell. , (d) Reduction , If you use OIL RIG to help remember the difference between oxidation and reduction, you must always include the electrons: thus, oxidation is loss of electrons. If you omit these you could be implying the loss of any other species, for example hydrogen. 3+–(e) Al(l) + 3e ? Al(l) ,, 1 mark is for balancing and the other is for the liquid state symbol. The electrolysis is done in the liquid state; aluminium oxide is insoluble in water. (f) Oxygen is formed at the anodes , and reacts with the carbon , (to form carbon dioxide). (g) Aluminium oxide is dissolved in cryolite to reduce the melting point , and thus reduce energy/electricity consumption ,. ? Philip Dobson 2011 17 Philip Allan Updates