St Robert of Newminster Catholic School and Sixth Form College

Transcription

1 St Robert of Newminster Catholic School and Sixth Form College Year 12 Pre-Course Tasks: CHEMISTRY Exercise Mark Grade Atomic structure Chemical bonding Chemical equations Maths for chemists Moles Name: Page 1

2 Bridging the gap to AS level Chemistry To give yourself the best possible start on your AS Chemistry course you need to ensure that you are confident that you know and understand the following topics from GCSE Science and Maths. (Everyone will have studied different GCSE courses and may know more/less about some areas than others if you are unsure about a certain topic read around the subject there are some useful websites listed at the bottom of the page, and/or ask for help). Complete the exercises in this pack to recap the areas of GCSE that you will build on during your study of AS level Chemistry. Useful Websites for GCSE recap for GCSE recap for A Level chemistry for A Level chemistry Page 2

3 Exercise 1 Atomic Structure What are atoms like? All atoms have a nucleus at the centre containing protons and neutrons Almost all of the mass of the atom is contained in the nucleus which has a positive charge The positive charge arises because each proton in the nucleus has a +1 charge The protons and neutrons each have a relative mass of 1. Neutrons have no charge The electrons orbit the nucleus in shells (or energy levels) The electrons are extremely small compared to the protons and neutrons and have a -1 charge Complete the table: Particle Relative Mass Charge Proton Neutron 0 Electron 1/ marks Page 3

4 Atomic and Mass Numbers The atomic numbers and mass numbers of elements can be found in the in the Periodic table. The atomic number, Z, is also called the proton number as it represents the number of protons in an atom For all atoms the number of protons equals the number of electrons, so the atomic number also tells you the number of electrons an atom has The mass number of an element, A, represents the total number of protons and neutrons in the nucleus. Subtracting the atomic number from the mass number allows you to calculate the number of neutrons in the nucleus. Here is an example; Magnesium appears like this in the Periodic Table Atomic number 24 Mg 12 Mass number Therefore the number of : Protons = 12 Electrons = 12 Complete the table: Page 4

5 Element Symbol Z A Protons Neutrons Electrons Sodium Carbon Fluorine Aluminium Sulfur 30 marks Arranging Electrons Electrons are found in shells (at GCSE) orbiting the nucleus The first shell can only hold a maximum of two electrons The second shell onwards can hold up to eight electrons Concentric circles represent the shells and electrons are drawn on as shown below Remember you should always start filling the innermost levels first Draw the electron arrangement of the following atoms: Oxygen Carbon Sodium Phosphorus Page 5

6 4 marks Forming ions Atoms have no overall charge because they contain equal numbers of protons and electrons If the atom loses or gains electrons it forms a charged particle called an ion If an atom loses electrons it becomes positively charge If an atom gains an electron it becomes negatively charged What charge would an oxygen atom have if it gained two electrons? What charge would a potassium atom have if it lost one electron? What charge would an aluminium atom have if it lost three electrons? marks Exercise 2 Chemical bonds Page 6

7 Ionic Bonding Elements from groups 1, 2, 6 and 7 in the Periodic table form ionic bonds. To form an ionic bond the metal atom donates the electrons in its outer shell to the non-metal. This results in ions forming where each has a full outer shell of electrons. The diagram below shows a typical reaction between elements from group 1 and group 7: Sodium atom gives up outer electron to become Na + Chlorine atom picks up spare electron to become Cl - ion The positively charged Na + ion is attracted to the negatively charged Cl - ion forming an ionic bond Draw a diagram showing how a magnesium atom reacts with an oxygen atom to form an ionic bond. Show clearly the electron transfer process. Page 7

8 4 marks Draw a diagram showing the electron transfer process that results in the formation of CaCl 2 Page 8

9 6 marks Covalent Bonding Ionic bonding only really works between metals with one or two outer electrons and non-metals that are only one or two electrons short. Elements with half filled shells do something different. The diagrams below show two such atoms, carbon and hydrogen Page 9

10 Where dots represent electrons from carbon and crosses represent electrons from hydrogen The dash represents a single covalent bond, this is the most common notation Draw diagrams showing the shared electron pairs in the following molecules: Chlorine Cl 2 2 marks Page 10

11 Ammonia NH 3 3 marks Ethane C 2 H 6 4 marks Exercise 3 Writing and balancing equations Writing word equations In order to write balanced chemical equations you need to be aware of some rules. Many examples you will come across will involve the reactions of acids to form salts. We will look at the rules that apply to these reactions. Page 11

12 Making salts 1. If sulphuric acid is used the salt formed will be a metal sulphate 2. If hydrochloric acid is used the salt formed will be a metal chloride 3. If nitric acid is used the salt formed will be a metal nitrate (Sulphuric acid = H 2SO 4, Hydrochloric acid = HCl, Nitric acid = HNO 3 Reactions involving acids 1. Metal + Acid Salt + Hydrogen 2. Metal oxide + Acid Salt + Water 3. Metal hydroxide + Acid Salt + Water 4. Metal carbonate + Acid Salt + Water + Carbon dioxide Complete the word equations: Aluminium + Nitric Acid Potassium hydroxide + Sulphuric acid Page 12

13 Calcium carbonate + Hydrochloric acid Aluminium oxide + Sulphuric acid Magnesium carbonate and sulphuric acid Sodium hydroxide and hydrochloric acid 6 marks Balancing equations To balance a chemical equation, go through each element in turn making sure the number of atoms on each side of the equations balance Place numbers in front of the chemical formula, if needed to balance the atoms. Example Page 13 Balance the equation:

14 Balance the equations: a) Cu + O 2 CuO b) H 2 + O 2 H 2 O c) N 2 + H 2 NH 3 Page 14

15 d) Na + O 2 Na 2 O Write balanced word and symbol equations for: 4 marks 1. Sodium hydroxide + hydrocholric acid 2. Magnesium oxide + sulphuric acid 6 marks Exercise 4 Maths knowledge Maths Knowledge: A sound understanding of maths is vital when studying AS level Chemistry. You will need to know how to do the following: Use standard form Rearrange mathematical formulae Understand and be able to calculate % Page 15

16 Be able to calculate means/averages from given data. Be able to describe and explain what a graph is showing 1) In standard form = 6.0x10 6. Convert the following into standard form: a) 5800 b) 0.35 c) d) e) 45 f) 8.0 2) Rearrange the following equations to make y the subject: a) 3y = 10 + x b) A = πy 2 c) 85 y = 5 + x d) V = πy marks 4 marks 3) Calculate the following: a) Barry scored 75% in a chemistry test. The test was out of 60 marks, how many marks did he lose? b) A sample of a compound has a total mass of 20g, it contains 6.2g of oxygen. What % of the compound is oxygen? 2 marks Page 16

17 4) A student repeated a chemical reaction to obtain a mass of product. The results are shown below: Experiment Mass obtained (g) Calculate the average mass obtained to 3 significant figures. 3 marks 5) What can you conclude about the relationship between the variables in the following graphs: (Hint mx + c) a) b) Page 17

18 4 marks Exercise 5 MOLES!!!!!! In chemistry measuring numbers of particles that we cannot see with the naked eye is very important. We use MOLES as a measure of the amount of substance. There are huge numbers of atoms even in test-tube reactions. So it is easier to talk about moles than the actual numbers involved. This is one of the most important topics in A Level chemistry and you need to be able to calculate the amount of a substance using MOLE equations. (It s like cashing in 1p coins at the bank. If you take a thousand 1 p coins to your bank, does the cashier count out each coin? The coins are weighed out on scales. The scales know the mass of one hundred 1p coins, and tell the cashier how many pounds ( s) are on the scales). Page 18

19 The number of atoms in 1 gram of hydrogen is called a mole. 1 gram of hydrogen contains approx. 6 X atoms. 600, 000, 000, 000, 000, 000, 000, 000 atoms rather a lot! This number is called Avogadro s constant One mole of any substance always contains the same number of particles. The mass of one mole of a substance is called the Relative Atomic Mass for an atom (A R ) and Relative Molecular Mass for a bonded substance (M R ). At GCSE you were asked to LEARN this equation Example: How many moles of atoms are there in 2.4 g of carbon? = 2.4/12 = 0.2 moles Try these yourself! How many moles are in: a. 2g of hydrogen? b. 36g of carbon? c. 160g of oxygen? Page 19

20 d. 1.4g of nitrogen? e. 0.19g of flourine? f. 1.1g of Carbon dioxide? g. 10g of aluminium sulphate Al2 (SO4)3? (use a periodic table to calculate AR and MR for each of the above) Hint: not all of these are atoms, think about how they exist in elemental form!!!!! Now try these: including units 1. Calculate the mass of carbon in 0.12 moles? 7 marks 2. Calculate the mass of hydrochloric acid in 2 moles? 3. Calculate the mass of magnesium oxide in 0.05 moles? Page 20

21 4. Calculate the mass of chlorine in 3.55 moles? 8 marks Working out the formula of Magnesium Oxide A student heated some Mg as shown. When Mg burns in air it combines with oxygen to make magnesium oxide. Step 1 Results Mass of crucible + lid + Mg before heating = 25.24g Mass of crucible + lid = 25.00g Therefore, mass of magnesium = 0.24g Step 2 - Results Mass of crucible + lid + magnesium oxide after heating = 25.40g Mass of crucible + lid + Mg before heating = 25.24g Page 21

22 Therefore, mass of oxygen in magnesium oxide = 0.16g Step 3 Change the masses into moles Magnesium Oxygen = 0.24 / 24 (A R) = 0.01 mole = 0.16 / 16 (A R) = 0.01 mole Step 4 Work out the ratio of moles Mg : O 0.01 : : 1 Therefore the formula of magnesium oxide is MgO A compound of nitrogen and hydrogen was broken down into its atoms. It was found that 1.4g of nitrogen had combined with 0.3g of hydrogen in the compound. What was the formula of the compound? Page 22

23 5 marks Page 23