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GCSE · Chemistry

C4a Reactivity, Acids and Salts

AQA 8464 · Calculator allowed · about 70 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

From the Reactivity Series to a Beaker of Crystals

Original text written for Revision Library.

Long before chemists understood electrons, they noticed that some metals behaved very differently from others. Drop a piece of potassium into water and it skates across the surface, fizzing and sometimes bursting into a lilac flame; drop in a piece of gold and nothing happens at all, even after years underwater. This simple observation, repeated for dozens of metals, produced the reactivity series: an order that predicts how a metal will behave with water, with dilute acid, and even how it must be dragged out of the ground. Metals below carbon in the series, such as iron and zinc, can be prised from their oxide ores simply by heating them with carbon, which takes the oxygen for itself. Metals above carbon, including sodium and calcium, cling to their oxygen far too tightly for that trick to work. The same underlying idea, how easily a metal gives up electrons to form a positive ion, also explains why acids sting, why alkalis feel soapy, and why a chemist preparing a jar of blue or green crystals in the laboratory must choose an acid and a base that react together to leave nothing behind but a pure, soluble salt. This topic covers the reactivity series, reactions of metals with water and dilute acids, extraction of metals by reduction, the pH scale and neutralisation, the required practical for preparing a soluble salt, and, at Higher tier, the distinction between strong and weak acids.

Many household substances contain acids, and water itself has a specific pH.

(a)Name the acid found in vinegar.(1)

(b)Name the acid found in the human stomach.(1)

(c)State whether pure water is acidic, neutral or alkaline, and give its pH value.(2)

(Total for Question 1 is 4 marks)

A student added small, equal-sized pieces of four metals to separate test tubes of dilute sulfuric acid and recorded these observations:
Magnesium: rapid fizzing, and the test tube felt warm
Zinc: a steady stream of bubbles
Iron: slow, occasional bubbles
Copper: no bubbles seen

(a)Use the observations to identify the most reactive of the four metals.(1)

(b)Write a word equation for the reaction between magnesium and dilute sulfuric acid.(2)

(c)Describe a test for hydrogen gas and state the result that confirms it is present.(2)

(d)Explain why no bubbles were seen in the test tube containing copper.(1)

(Total for Question 2 is 6 marks)

Different metals react differently with cold water and with steam.

(a)Describe what you would observe when a small piece of potassium is added to cold water.(2)

(b)Write a balanced symbol equation, including state symbols, for the reaction between calcium and water.(2)

(c)Iron does not react with cold water. Name the two products formed when iron reacts with steam.(1)

(Total for Question 3 is 5 marks)

The position of a metal in the reactivity series determines how it is extracted from its ore.

(a)Explain why gold is found in the Earth's crust as the uncombined element, rather than as a compound.(2)

(b)Iron oxide is reduced by carbon in a blast furnace. Write a word equation for this reaction and state which substance is reduced.(2)

(Total for Question 4 is 4 marks)

The table below lists four metals and their position relative to carbon in the reactivity series:
Sodium: more reactive than carbon
Iron: less reactive than carbon
Zinc: less reactive than carbon
Gold: far less reactive than carbon (very unreactive)

(a)Complete the table to give the method used to extract each metal (reduction with carbon, electrolysis, or found native/uncombined).(3)

(b)Explain, in terms of reactivity, why zinc can be extracted using carbon but sodium cannot.(3)

(Total for Question 5 is 6 marks)

The pH scale runs from 0 to 14 and can be measured using universal indicator or a pH meter. The table shows the approximate pH of some substances:
Lemon juice: pH 2
Pure water: pH 7
Milk: pH 6.5
Oven cleaner: pH 13

(a)State the pH range of an acidic solution.(1)

(b)State the pH of a neutral solution.(1)

(c)Using the table, classify each substance as acidic, neutral or alkaline.(2)

(Total for Question 6 is 4 marks)

Acids and alkalis can be defined in terms of the ions they produce in aqueous solution.

(a)Define an acid in terms of the ions it produces in aqueous solution.(1)

(b)Define an alkali in terms of the ions it produces in aqueous solution.(1)

(c)Using your answers to (a) and (b), write the overall ionic equation that occurs in any neutralisation reaction.(1)

(Total for Question 7 is 3 marks)

Neutralisation reactions occur between acids and bases, including alkalis.

(a)Write the general word equation for the reaction between an acid and an alkali.(2)

(b)Hydrochloric acid reacts with sodium hydroxide solution. Name the salt and the other product formed.(2)

(c)Give one everyday use of a neutralisation reaction.(1)

(Total for Question 8 is 5 marks)

Required practical: a student prepared pure, dry magnesium sulfate crystals from magnesium oxide (an insoluble base) and dilute sulfuric acid using this method:
1. Measure about 25 cm³ of dilute sulfuric acid into a beaker and warm it gently.
2. Add magnesium oxide powder to the acid a little at a time, stirring after each addition, until no more of the white powder dissolves and a little excess remains at the bottom.
3. Filter the mixture to remove the unreacted magnesium oxide.
4. Gently heat the filtrate in an evaporating basin until a hot, saturated solution forms, then leave it to cool undisturbed.
5. Filter off the crystals that form and dry them between sheets of filter paper.

(a)Explain why the dilute sulfuric acid is warmed in step 1.(1)

(b)Explain why magnesium oxide is added until a little excess remains undissolved.(1)

(c)Name the technique used in step 3 to remove the excess, unreacted magnesium oxide.(1)

(d)Explain why the solution in step 4 is only heated to the point of forming a hot saturated solution, rather than heated until it is completely dry.(2)

(e)Write a balanced symbol equation, with state symbols, for the reaction between magnesium oxide and sulfuric acid.(2)

(f)4.0 g of magnesium oxide is reacted with excess dilute sulfuric acid. Calculate the maximum mass of hydrated magnesium sulfate crystals, MgSO4.7H2O, that could be obtained. [Relative atomic masses: Mg = 24, S = 32, O = 16, H = 1](3)

(Total for Question 9 is 10 marks)

Soluble salts can also be prepared from a soluble base (an alkali, such as potassium hydroxide) and an acid. Because both reactants are soluble, this method requires titration.

(a)Explain why an indicator cannot be used when preparing the final, pure batch of salt crystals from potassium hydroxide and dilute nitric acid.(2)

(b)Name the piece of apparatus used to add the acid accurately during the titration, and the apparatus used to measure a fixed volume of the alkali.(2)

(Total for Question 10 is 4 marks)

Balance each of the following equations by writing the correct number in front of each formula (leave a blank if the number is 1).

(a)__Zn + __HCl -> __ZnCl2 + __H2(1)

(b)__CuO + __C -> __Cu + __CO2(2)

(c)__Na + __H2O -> __NaOH + __H2(1)

(Total for Question 11 is 4 marks)

Oxidation and reduction can be defined in terms of oxygen or, at a more advanced level, in terms of electrons.

(a)Define reduction in terms of oxygen.(1)

(b)Define reduction in terms of electrons.(1)

(c)Zinc metal is added to a solution of silver nitrate: Zn + 2AgNO3 -> Zn(NO3)2 + 2Ag. In this reaction, silver ions gain electrons. Write a half equation for this process.(2)

(Total for Question 12 is 4 marks)

Zinc metal is added to a solution of silver nitrate.

(a)Predict whether a reaction occurs.(1)

(b)Explain your answer in terms of reactivity, and name the products formed.(2)

(Total for Question 13 is 3 marks)

Higher tier only. Hydrochloric acid and ethanoic acid are both monoprotic acids. Equal concentrations (0.1 mol/dm³) of each were tested:
Hydrochloric acid: pH 1
Ethanoic acid: pH 3

(a)Identify which of the two acids is the strong acid, and explain your choice in terms of ionisation.(2)

(b)Calculate how many times greater the concentration of H+ ions is in the hydrochloric acid than in the ethanoic acid.(1)

(c)Excess magnesium ribbon is added separately to equal volumes of the two acids. Suggest one difference you would observe during the reactions, and one similarity in the final result, explaining each.(2)

(Total for Question 14 is 5 marks)

Higher tier only. 20.0 cm³ of 0.15 mol/dm³ potassium hydroxide solution is exactly neutralised by 24.0 cm³ of dilute nitric acid.
KOH(aq) + HNO3(aq) -> KNO3(aq) + H2O(l)

(a)Calculate the number of moles of potassium hydroxide used.(1)

(b)State the mole ratio of KOH to HNO3 in this reaction.(1)

(c)Calculate the concentration, in mol/dm³, of the nitric acid.(2)

(Total for Question 15 is 4 marks)

The reactivity series can be used to explain both how metals react with water and acids, and how they are extracted from their ores.

Explain how the position of a metal in the reactivity series (relative to carbon and to hydrogen) determines: (i) how vigorously it reacts with water or dilute acid, and (ii) the method needed to extract it from its ore. Use ideas about electron transfer in your answer.

(Total for Question 16 is 6 marks)

Mark scheme · C4a Reactivity, Acids and Salts

Question 1

(a) B1 ethanoic acid (oe: acetic acid)
(a) Answer: Ethanoic acid
(b) B1 hydrochloric acid
(b) Answer: Hydrochloric acid
(c) B1 neutral
(c) B1 pH 7
(c) Answer: Neutral, pH 7

Question 2

(a) B1 magnesium
(a) Answer: Magnesium
(b) M1 correct reactants: magnesium + sulfuric acid
(b) A1 correct products: magnesium sulfate + hydrogen (oe order)
(b) Answer: magnesium + sulfuric acid -> magnesium sulfate + hydrogen
(c) B1 hold a lit/burning splint at the mouth of the test tube (oe)
(c) B1 a squeaky pop is heard (cao)
(c) Answer: A lit splint held at the mouth of the tube gives a squeaky pop.
(d) B1 copper is less reactive than hydrogen, so it does not react with (cannot displace hydrogen from) dilute acids (oe)
(d) Answer: Copper does not react with dilute acids because it is less reactive than hydrogen.

Question 3

(a) B1 potassium moves rapidly across the surface of the water and fizzes vigorously (oe)
(a) B1 the metal melts into a ball and/or ignites, burning with a lilac flame (any one, oe)
(a) Answer: Potassium moves rapidly across the surface, fizzing vigorously; it may melt into a ball and ignite with a lilac flame.
(b) M1 correct formulae shown (balanced or not): Ca + H2O -> Ca(OH)2 + H2
(b) A1 correctly balanced with state symbols: Ca(s) + 2H2O(l) -> Ca(OH)2(aq) + H2(g) (cao)
(b) Answer: Ca(s) + 2H2O(l) -> Ca(OH)2(aq) + H2(g)
(c) B1 iron oxide and hydrogen (both needed, oe)
(c) Answer: Iron oxide and hydrogen.

Question 4

(a) B1 gold is very unreactive / very low down the reactivity series
(a) B1 it does not readily react with oxygen or other elements to form compounds, so it stays as the metal (oe)
(a) Answer: Gold is very unreactive, so it does not react with other elements to form compounds and is found naturally as the metal itself.
(b) M1 correct word equation: iron oxide + carbon -> iron + carbon dioxide
(b) A1 iron oxide is reduced (cao)
(b) Answer: iron oxide + carbon -> iron + carbon dioxide; iron oxide is reduced.

Question 5

(a) B1 sodium: electrolysis
(a) B1 iron and zinc: reduction with carbon (both needed for this mark)
(a) B1 gold: found native / as the element (no extraction needed)
(a) Answer: Sodium: electrolysis. Iron: reduction with carbon. Zinc: reduction with carbon. Gold: found native.
(b) B1 zinc is less reactive than carbon, so carbon can remove the oxygen from (reduce) zinc oxide (oe)
(b) B1 sodium is more reactive than carbon, so carbon cannot remove the oxygen from sodium oxide (oe)
(b) B1 therefore sodium must be extracted by a different method (electrolysis) that does not rely on carbon (oe)
(b) Answer: Zinc is less reactive than carbon so carbon can reduce zinc oxide; sodium is more reactive than carbon so carbon cannot reduce sodium oxide, meaning electrolysis must be used instead.

Question 6

(a) B1 pH less than 7 (0-6) (oe)
(a) Answer: pH 0-6 (less than 7).
(b) B1 pH 7
(b) Answer: pH 7
(c) B1 lemon juice and milk: acidic (both needed)
(c) B1 pure water: neutral; oven cleaner: alkaline (both needed)
(c) Answer: Lemon juice: acidic. Milk: acidic. Pure water: neutral. Oven cleaner: alkaline.

Question 7

(a) B1 produces H+ (hydrogen) ions in aqueous solution (cao)
(a) Answer: An acid produces H+ ions in aqueous solution.
(b) B1 produces OH- (hydroxide) ions in aqueous solution (cao)
(b) Answer: An alkali produces OH- ions in aqueous solution.
(c) B1 H+(aq) + OH-(aq) -> H2O(l) (cao, state symbols not essential)
(c) Answer: H+(aq) + OH-(aq) -> H2O(l)

Question 8

(a) B1 acid + alkali -> salt (oe)
(a) B1 + water (cao)
(a) Answer: acid + alkali -> salt + water
(b) B1 sodium chloride
(b) B1 water
(b) Answer: Sodium chloride and water.
(c) B1 any valid example, e.g. indigestion remedies neutralising excess stomach acid, or farmers adding lime to neutralise acidic soil (oe, any one)
(c) Answer: Indigestion remedies contain a base (e.g. magnesium hydroxide) that neutralises excess hydrochloric acid in the stomach.

Question 9

(a) B1 warming increases the rate of reaction, so the magnesium oxide reacts/dissolves faster (oe)
(a) Answer: Warming increases the rate of reaction between the acid and magnesium oxide.
(b) B1 to make sure all the acid has reacted, so the final salt solution is not contaminated with unreacted (excess) acid (oe)
(b) Answer: Adding excess magnesium oxide ensures all the sulfuric acid has reacted, leaving no acid in the final product.
(c) B1 filtration
(c) Answer: Filtration.
(d) B1 heating to complete dryness could decompose the crystals or drive off the water of crystallisation, leaving a powder instead of crystals (oe)
(d) B1 cooling a hot saturated solution slowly allows larger, well-formed pure crystals to grow (oe)
(d) Answer: Heating to dryness could decompose the salt or drive off its water of crystallisation; cooling a saturated solution slowly instead allows large, well-formed crystals to grow.
(e) M1 correct formulae shown: MgO + H2SO4 -> MgSO4 + H2O
(e) A1 correctly balanced with state symbols: MgO(s) + H2SO4(aq) -> MgSO4(aq) + H2O(l) (cao)
(e) Answer: MgO(s) + H2SO4(aq) -> MgSO4(aq) + H2O(l)
(f) M1 Mr(MgO) = 24 + 16 = 40, so moles MgO = 4.0/40 = 0.10 mol
(f) M1 mole ratio MgO : MgSO4.7H2O is 1:1, so moles MgSO4.7H2O = 0.10 mol (ecf); Mr(MgSO4.7H2O) = 24+32+(4x16)+7x(2x1+16) = 246
(f) A1 mass = 0.10 x 246 = 24.6 g (awrt 24.6 g, ecf, cao)
(f) Answer: 24.6 g

Question 10

(a) B1 an indicator is a coloured substance that would contaminate the salt / remain mixed in with the crystals (oe)
(a) B1 a titration is first carried out using an indicator to find the exact volume of acid needed, then the reaction is repeated without indicator using this measured volume (oe)
(a) Answer: An indicator would contaminate the crystals, so the exact volumes are first found using an indicator, then the reaction is repeated without indicator.
(b) B1 burette (for the acid)
(b) B1 pipette (for the alkali)
(b) Answer: Burette (for the acid) and pipette (for the alkali).

Question 11

(a) B1 Zn + 2HCl -> ZnCl2 + H2 (cao)
(a) Answer: Zn + 2HCl -> ZnCl2 + H2
(b) M1 correct ratio of Cu identified (2 CuO to 2 Cu) or correct use of 1 C to 1 CO2
(b) A1 2CuO + C -> 2Cu + CO2 (fully balanced, cao)
(b) Answer: 2CuO + C -> 2Cu + CO2
(c) B1 2Na + 2H2O -> 2NaOH + H2 (cao)
(c) Answer: 2Na + 2H2O -> 2NaOH + H2

Question 12

(a) B1 loss of oxygen (cao)
(a) Answer: Reduction is the loss of oxygen.
(b) B1 gain of electrons (cao)
(b) Answer: Reduction is the gain of electrons.
(c) M1 correct species and charge shown: Ag+ and e- and Ag
(c) A1 Ag+ + e- -> Ag correctly balanced (cao)
(c) Answer: Ag+ + e- -> Ag

Question 13

(a) B1 yes, a reaction occurs
(a) Answer: Yes, a reaction occurs.
(b) B1 zinc is more reactive than silver, so zinc displaces silver from the solution (oe)
(b) B1 products: zinc nitrate and silver (both needed)
(b) Answer: Zinc is more reactive than silver, so it displaces silver from silver nitrate solution, forming zinc nitrate and silver metal.

Question 14

(a) B1 hydrochloric acid is the strong acid, because it fully ionises/dissociates in water, releasing a high concentration of H+ ions (oe)
(a) B1 ethanoic acid is a weak acid; it only partially ionises/dissociates (an equilibrium is set up), releasing a much lower concentration of H+ ions (oe)
(a) Answer: Hydrochloric acid is the strong acid: it fully ionises, producing a high H+ concentration and low pH. Ethanoic acid only partially ionises (weak acid), producing a lower H+ concentration and higher pH.
(b) B1 100 times (cao)
(b) Answer: 100 times
(c) B1 difference: the reaction with hydrochloric acid is faster/more vigorous (more rapid fizzing), because its higher initial H+ ion concentration increases the rate of reaction (oe)
(c) B1 similarity: since magnesium is in excess and both solutions contain the same number of moles of acid, the same total volume of hydrogen gas is eventually produced from each (allow: the weak acid continues to ionise/replenish H+ as it reacts, oe)
(c) Answer: The reaction is faster with hydrochloric acid because of its higher initial H+ concentration; however, with excess magnesium, both reactions eventually release the same total volume of hydrogen, since both contain the same number of moles of acid.

Question 15

(a) B1 moles = 0.15 x (20.0/1000) = 0.0030 mol (awrt 3.0 x 10^-3 mol)
(a) Answer: 0.0030 mol
(b) B1 1:1
(b) Answer: 1:1
(c) M1 moles HNO3 = moles KOH = 0.0030 mol (ecf from (a) and the 1:1 ratio in (b))
(c) A1 concentration = 0.0030 / (24.0/1000) = 0.125 mol/dm³ (awrt 0.125, ecf, cao)
(c) Answer: 0.125 mol/dm³

Question 16

Level 3 (5-6): A clear, logically linked explanation connects reactivity (in terms of how readily atoms lose electrons to form positive ions), the vigour of reactions with water/acid, and the extraction method (reduction with carbon versus other methods), for metals both above and below carbon/hydrogen in the series.
Level 2 (3-4): Relevant points about reactivity, reactions with water/acid, and extraction method are given, with some logical linking between at least two of these ideas. Some reference to electron transfer or to carbon's position may be included but not fully integrated.
Level 1 (1-2): Simple, isolated statements are made about the reactivity series (e.g. metal order, or that reactions with acid/water differ), with little or no linkage between reactivity, reactions and extraction. The answer may be largely descriptive.
Level 0 (0): No relevant content.
Indicative content:
- The reactivity series orders metals (and carbon, hydrogen) by how readily their atoms lose electrons to form positive ions.
- More reactive metals (e.g. potassium, sodium, calcium) lose electrons more easily and so react more vigorously with cold water and dilute acid, releasing hydrogen faster.
- Metals below hydrogen (e.g. copper, silver, gold) do not react with dilute acids because they cannot lose electrons readily enough to displace hydrogen.
- Oxidation and reduction can be defined in terms of oxygen (or, at Higher tier, electron transfer): the metal is oxidised (loses electrons) while H+ ions are reduced (gain electrons) to form hydrogen gas.
- Metals less reactive than carbon (e.g. zinc, iron, copper) can be extracted from their oxides by reduction with carbon, because carbon can remove the oxygen from these metal compounds.
- Metals more reactive than carbon (e.g. sodium, calcium) cannot be extracted by reduction with carbon, since carbon cannot remove oxygen from their more stable compounds; other methods, such as electrolysis, are needed instead.
- Very unreactive metals such as gold have such a low tendency to lose electrons or form compounds that they are found in the Earth's crust as the uncombined element, requiring no chemical extraction.
- A well-linked answer connects all three ideas: the same underlying tendency to lose electrons explains both the vigour of a metal's reactions with water/acid and the method (or absence of need) for its extraction.

Reactivity, Acids and Salts - Worksheets, Questions and Revision