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Index:
Page 3: Covalent Bonding Knowledge Card
Page 4: Separation Techniques Knowledge Card
Page 5: Exam Descriptors/Sample Questions
Page 6: Covalent Bonding & How to draw them
Page 7: Simple & Giant Covalent Compounds
Page 8: Miscible and Immiscible
Page 9: Chromatography
Page 10: Practice exam questions
Page 15: Mark Scheme
Page 18: Sample Question Answers
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Covalent Compounds and Separation
Revision Notes
Describe what a covalent bond is Describe the difference between an ionic and covalent bond, in terms of electrons (3)
Describe what a molecule is Out of ionic and covalent compounds, which form lattices and which form molecules? (2)
Draw dot and cross diagrams for hydrogen, hydrochloric acid, water and methane
Draw dot and cross diagrams for hydrogen, hydrochloric acid, water and methane (4)
Draw dot and cross diagrams for oxygen and carbon dioxide
Draw dot and cross diagrams for oxygen and carbon dioxide (4)
Explain how to distinguish between an ionic and covalent compound based on properties
Describe a test to distinguish between an ionic compound and a covalent compound (4)
Describe the properties of simple covalent compounds
Describe the properties of simple covalent compounds (2)
Explain the properties of simple covalent compounds
Explain why simple covalent compounds have low melting points and don't conduct (4)
Describe the properties of giant covalent compounds
Describe the properties of giant covalent compounds (2)
Explain the properties of giant covalent compounds
Explain why giant covalent compounds don't conduct electricity and have a high melting point (4)
Explain, in terms of structure, the difference in uses between graphite and diamond
Explain why diamond is used in tool cutters, whilst graphite is used in pencils and as electrodes (6)
Explain the difference in properties of ionic and covalent compounds
Explain why ionic compounds can conduct only when molten, but covalent compounds cannot conduct at all (4)
Describe how to separate two immiscible liquids
How can you separate the immiscible oil and water? (2)
Explain how to separate two miscible liquids, such as the gases in air
Describe, step by step, how you would separate a miscible mixture of air (nitrogen, oxygen and carbon dioxide) (4)
Describe how to carry out paper chromatography to separate inks
Describe how to prepare a chromatogram for an unknown ink (3)
Explain the results of chromatograms by determining the Rf
In a sample chromatogram, blue ink travels 2.4cm, red ink travels 4.5cm and the water travelled 10cm. Calculate the Rf values for both inks. (3)
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Section 1: Covalent Compounds
Covalent bonding is the sharing of pairs of electrons between
two non-metals
Non-metals need extra electrons to get a full outer shell. Neither can give their electrons up, so
they share.
For example, both hydrogen and chlorine need one electron to get a full outer shell. So one
electron from each element overlaps to ensure that hydrogen has 2 electrons in its shell, and
chlorine has 8 electrons in its shell.
How to draw covalent bonds:
Example: Methane, CH4.
Step 1: Look at which group they are in to find out how many electrons they need to fill their
outer energy shell. Carbon needs 4 and hydrogen needs 1.
Step 2: The number of electrons needed is the number of bonds that
each element can have. Hydrogen can only bond once, and carbon can
bond four times. Think of it like this:
Step 3: Draw the central atom – in this case carbon.
Spread the electrons evenly.
Step 4: For each bond needed, add one of your
hydrogen’s, overlapping to ensure that every electron is
paired up.
Step 5: Check! Make sure each atom has a full outer shell! If not, go back and try again!
Section 2: Alkali Metals
All of the alkali metals, in group 1, are soft and highly reactive – which is unusual because
most metals are strong and quite unreactive.
Their reactivity increases as you go down the group.
They react with water to make hydroxides, and oxygen to make oxides – these are alkaline
substances with a pH of 8 and above. They usually turn universal indicator purple.
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Section 2: Types of Covalent Compounds
Simple Covalent Structures:
Simple covalent molecules have strong covalent bonds, but only have low boiling points
because there are weak intermolecular forces, which don’t require much energy to break:
Giant Covalent structures:
Giant covalent structures also have covalent bonds, which are strong. They do not have
intermolecular forces here, and therefore a lot of energy is needed to break the bonds.
Therefore they have high melting points.
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Section 3: Separation Techniques
If you wanted to separate air – which is miscible – you would cool it down to -200oC and carry
out distillation like above.
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Section 4: Chromatography
Chromatography can be used to separate mixtures of coloured compounds. Mixtures that
are suitable for separation by chromatography include inks, dyes and colouring agents in food.
Setting up a chromatogram:
Simple chromatography is carried out on paper. A spot of the mixture is placed near the
bottom of a piece of chromatography paper and the paper is then placed upright in a suitable
solvent, eg water.
The solvent is the liquid that will dissolve the dye/ink.
As the solvent soaks up the paper, it carries the mixtures with it. Different components of the
mixture will move at different rates. This separates the mixture out.
Retention Factor, Rf:
Different chromatograms and the separated components of the mixtures can be identified by
calculating the Rf value using the equation:
Rf = distance moved by the compound ÷ distance moved by the solvent
The Rf value of a particular compound is always the same - if the chromatography has been
carried out in the same way. This allows industry to use chromatography to identify
compounds in mixtures.
In this example, the unknown sample is an impure mixture of two inks – blue and yellow.
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Q1. Covalent Compounds The diagram shows part of the structure of diamond. Complete the sentences by putting a cross ( ) in the box next to your answer. (i) The bonds between carbon atoms in diamond contain
(1) A protons B neutrons C electrons D ions
(ii) The structure of diamond is
(1) A simple molecular, covalent B giant molecular, covalent C ionic D metallic
(iii) Describe what is meant by a covalent bond.
(2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q2. The drink contained water.
Complete the dot and cross diagram for water by adding the electrons of the hydrogen atoms. (1)
Q3. The atomic number of carbon is 6 and of fluorine is 9. Carbon and fluorine atoms are combined in a tetrafluoromethane molecule, CF4. Draw a dot and cross diagram of a tetrafluoromethane molecule. Show outer electrons only.
(2)
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Q4. Draw a dot and cross diagram of a molecule of hydrogen chloride. Show outer electrons only.
(2)
Q5. Oxygen is a simple molecular, covalent substance. (i) The electronic configuration of oxygen is 2.6. Draw a dot and cross diagram for a molecule of oxygen, O2. Show the outer electrons only.
(3)
Q6. Properties of Covalent Compounds
(a) Complete the sentence by putting a cross ( ) in the box next to your answer. A property of hydrogen chloride is that it
(1) A conducts electricity B has a high melting point C has a low boiling point D exists as a giant structure
(b) The diagram shows part of the structure of diamond.
How many bonds does a typical carbon atom in diamond form?
(1) ........................................................................................................ (c) Nitrogen and oxygen are present in the air. Oxygen has a low boiling point because there are
(1) A weak covalent bonds between the oxygen atoms B weak covalent bonds between the oxygen molecules C weak forces of attraction between the oxygen atoms D weak forces of attraction between the oxygen molecules
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(d) Carbon dioxide is a gas at room temperature. A carbon dioxide molecule is a
(1) A giant molecule that has covalent bonds B giant molecule that has ionic bonds C simple molecule that has covalent bonds D simple molecule that has ionic bonds
Q7. Structures
The table shows some properties of diamond and graphite.
(i) Suggest why diamond and graphite might be expected to have similar properties. (1) ..................................................................................
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(ii) By referring to its structure, explain why diamond is very hard.
(3)
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(iii) By referring to its structure, explain why graphite flakes easily.
(2)
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Q8. (a) The apparatus shown can be used to test the electrical conductivity of liquids. Describe how you would use this apparatus to show that the electrical conductivity of hexane is that of a typical simple molecular, covalent compound.
(2)
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(b) Methane is a gas at room temperature. It exists as molecules, CH4. Methane has a low boiling point. It does not conduct electricity. Explain, in terms of the nature of its molecules and the forces between its molecules, why methane has a low boiling point and does not conduct electricity.
(6)
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Q9. Separating Techniques
(a) Complete the sentence by putting a cross ( ) in the box next to your answer. A mixture of two immiscible liquids can be separated by using
(1) A fractional distillation B a separating funnel C evaporation D filtration
(b) Describe how you can use the apparatus named in part (a) to separate an immiscible mixture of oil and water.
(1)
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(c) Describe how oxygen and nitrogen are obtained from liquid air by fractional distillation.
(3)
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Q10. Chromatography (a) Some food colourings are a mixture of coloured substances. Charles carried out a chromatography experiment to test which food colouring was present in a coloured drink. He used samples of three food colourings, X, Y and Z. He also tested a sample of the colouring in the drink. (i) Charles looked at the chromatogram above to find out which food colourings contained more than one coloured substance. State all the food colourings that contain more than one coloured substance.
(1)
.............................................................................................................................................. (ii) Food colouring Y is banned. Explain how Charles can tell that the drink that he tested did not contain the banned food colouring. (2)
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(iii) Calculate the Rf value for the coloured substance in food colouring X. (2)
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Rf = . . . . . . . . . . . . . . . . . . . . .
(b) Describe in detail how the experiment should be carried out to produce the chromatogram shown in (a). (6)
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Q1. Answer Acceptable answers Mark
(i) C (1)
(ii) B (1)
(iii) An description including
shared (electrons) (1) pair(s) of electrons (between atoms) (1)
Ignore reference to complete/full shells Ignore reference to between two metals Ignore reference to between metal and non-metal Ignore reference to between molecules Any reference to between ions scores 0 (2)
Q2. Answer Acceptable answers Mark
2 electrons to be drawn in, one between each hydrogen and oxygen atom in the overlap region or on the overlapping circles Ignore an inner shell on the oxygen if it has 2 electrons Do not award the mark if additional atoms or electrons added to the diagram
dots/crosses/circles/ e/e- for electrons
(1)
Q3. Answer Acceptable answers Mark
• four bonding pairs shown (1) • six non bonded electrons on each fluorine atom (1)
(2)
Q4.
Answer Acceptable answers Mark
shared pair in molecule (1) rest of molecule consequent on first mark (1)
Allow a diagram without labels for 2 marks any symbols shown must be correct for the 2nd mark allow any combination of dots and crosses for electrons wrong compound = zero marks
(2)
Q5.
Answer Acceptable answers Mark
(i)
electrons shared / between atoms (1)
2 pairs of/four electrons shared / between two atoms (1)
4 additional electrons on both oxygen atoms (1)
ignore any inner electrons shown 3rd Mark is dependent on 2nd
(3)
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Q6. Answer Acceptable answers Mark
(a) C has a low boiling point (1)
(b) four/4 (1)
(c) D weak forces of attraction between the oxygen molecules
(1)
(d) C (1)
Q7. Answer Acceptable Mark
(i) both (pure forms of) carbon / both giant molecular (1)
(ii) An explanation linking three of the following points • (every) carbon atom forms four bonds (1) • strong bonds / hard to separate atoms from lattice (1) • covalent bonds (1) • no weaknesses in molecule (1)
(3)
(iii) An explanation linking the following • (in) layers (1) • weak forces between layers (1)
(2)
Q8. Answer Acceptable Mark
(a) A description including two of the following points • test circuit first to ensure bulb lights • pour hexane / liquid into beaker(1) • bulb does not light(1) • (liquid)does not conduct (electricity)(1)
(2)
(b) A description including some of the following points Molecules: simple / small molecule separate / discrete molecules covalent bonds (between atoms in molecule) displayed structure for CH4 weak forces between molecules Properties: to boil need to separate molecules little energy needed (as weak forces between molecules) therefore low boiling point to be able to conduct must have charged particles which must be free to move no charged particles present no delocalised /free electrons / no ions present all electrons are in covalent bonds therefore does not conduct electricity / cannot carry current (6)
Level No rewardable content
1 1 - 2 a limited description e.g. methane is a simple / small molecule e.g. weak forces between molecules
2 3 - 4 a simple description e.g. methane is a simple / small molecule with weak forces between molecules (so low boiling point) e.g. it is covalent / there are no charged particles (ions or free electrons) to move and carry the current
3 5 - 6 a detailed description e.g. methane is a simple / small molecule with weak forces between molecules (so low boiling point) AND any mention of lack of charged particles e.g. does not conduct electricity because it is covalent /there are no charged particles (ions or free electrons) to move and carry the current AND any mention of separate molecules or weak forces between them
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Q9 Answer Acceptable answers Mark
(a) B (1)
(b) A description linking use separating funnel (1) run off lower layer / liquid / OWTTE (1)
alternative description of separating funnel eg funnel with a tap at the bottom suitable labelled diagram burette allow layers / liquids to separate ignore fractional distillation
(2)
A description including three from
(liquid air enters) (fractionating) column (1)
(liquid air) warms/heats/boils (1)
(gaseous) nitrogen/lower boiling point from top of column (1)
(liquid) oxygen/higher boiling point from bottom of column (1)
ignore references to cooling air etc. can be separated because they have different boiling points (1) alternative to last two marking points
(3)
Q10 Answer Acceptable answers Mark
(a)(i)
Y and Z both must be given with no additional substances
(1)
(a)(ii)
An explanation linking two of only one coloured substance in
drink/ only one spot (at 4) in drink (1) this is not present in Y/ no spot at
4 in Y/ no corresponding spot in Y (1)
the spots would rise to the same point if they were the same substance / the drink is X (1)
two coloured substances in Y/ Y has 2 spots (at 2.5 and 7) (1)
drink does not have spot(s) corresponding to spot(s) in Y (1)
allow values ± 0.5 of those given here Y has more than 1 coloured substance/spot do not allow a specified number greater than 2
(2)
(a)(iii)
(spot moved) 4 ± 0.5 / solvent (moved) 8 (1) Rf = 4/8 (2) (=0.5) consequential on their value for spot moved
4/8 OR 0.5 on its own (2) 8/4 (1)
(2)
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Question: Model Answer: Describe the difference between an ionic and covalent bond, in terms of electrons (3)
Ionic bonding is where metals donate/give electrons (1) to non metals, which receive/accept them (1). Covalent bonding is a shared pair of electrons (1)
Out of ionic and covalent compounds, which form lattices and which form molecules? (2)
Ionic = lattice (1). Covalent = molecules (1)
Draw dot and cross diagrams for hydrogen, hydrochloric acid, water and methane (4)
Check the revision guide for each, or the covalent bonding board
Draw dot and cross diagrams for oxygen and carbon dioxide (4)
Check the revision guide for each, or the covalent bonding board
Describe a test to distinguish between an ionic compound and a covalent compound (4)
Dissolve them in water (1). Add electrodes connected to a circuit (1). It if lights up, it is ionic (1), if not it is covalent (1)
Describe the properties of simple covalent compounds (2)
Low melting points (1) and don't conduct (1)
Explain why simple covalent compounds have low melting points and don't conduct (4)
Low melting points: Weak intermolecular forces / weak forces between the molecules (1) which don't need much energy to break (1) Don’t conduct: No spare electrons (1) to carry on a charge (1)
Describe the properties of giant covalent compounds (2)
High melting points (1) and don't conduct (1)
Explain why giant covalent compounds don't conduct electricity and have a high melting point (4)
High melting points: lots of strong covalent bonds (1) which need a lot of energy to break (1) Don’t conduct: No spare electrons (1) to carry on a charge (1)
Explain why diamond is used in tool cutters, whilst graphite is used in pencils and as electrodes (6)
Diamond: Forms four (1) strong (1) covalent bonds (1), making it hard (1) Graphite: Forms three strong covalent bonds (1). One spare electron (1) which can move (1) and carry a charge ← electrode. Has layers (1) which can slide (1) ← lubricant
Explain why ionic compounds can conduct only when molten, but covalent compounds cannot conduct at all (4)
Ionic: When molten, the ions (1) are free to move (1). Covalent: No spare electrons (1) that are free to move (1)
How can you separate the immiscible oil and water? (2)
Separating funnel (1) turn the tap (1)
Describe, step by step, how you would separate a miscible mixture of air (nitrogen, oxygen and carbon dioxide) (4)
Cool down to -200 degrees (1). Remove carbon dioxide and water (1). Start to warm back up. Nitrogen will become a gas first and can be collected at the top (1). Oxygen stays a liquid (1)
Describe how to prepare a chromatogram for an unknown ink (3)
Draw a line and add a cross in pencil at the bottom of the chromatography paper (1). Add a dot of the ink and add to water (1). The water will move up the paper, moving the ink up the paper (1)
In a sample chromatogram, blue ink travels 2.4cm, red ink travels 4.5cm and the water travelled 10cm. Calculate the Rf values for both inks. (3)
Blue Rf = 2.4 / 10 = 0.24 (1). Red Rf = 4.5/10 = 0.45 (1)
