Reactions of acids

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Reactions of Acids with Metals

Acids react with some metals to produce a salt and hydrogen gas.
General equation: metal + acid → salt + hydrogen.
Only metals above hydrogen in the reactivity series react with dilute acids; copper does not react.
The more reactive the metal, the more vigorous the reaction; potassium and sodium react explosively.
The salt produced depends on the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates.
Example: Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g).
Test for hydrogen: a lit splint gives a squeaky pop.

Metal-Acid Reactions as Redox (Higher Tier)

Metal-acid reactions are redox reactions: reduction and oxidation occur together.
Oxidation is loss of electrons; reduction is gain of electrons.
In the reaction Zn + 2HCl → ZnCl₂ + H₂, the ionic equation is Zn + 2H⁺ → Zn²⁺ + H₂.
Oxidation half-equation: Zn → Zn²⁺ + 2e⁻ (zinc is oxidised).
Reduction half-equation: 2H⁺ + 2e⁻ → H₂ (hydrogen ions are reduced).
Metal atoms tend to lose electrons and are usually the species oxidised.

Neutralisation and Salt Production

A base neutralises an acid; examples include metal oxides, hydroxides and carbonates.
An alkali is a soluble base; all alkalis are bases, but not all bases are alkalis.
General neutralisation: acid + base → salt + water.
With a metal carbonate: acid + carbonate → salt + water + carbon dioxide.
The salt name has two parts: the metal (from the base) and the acid ending (chloride, sulfate, nitrate).
Hydrochloric acid → chlorides; sulfuric acid → sulfates; nitric acid → nitrates.
Example: 2HCl + CuO → CuCl₂ + H₂O.
Test for carbon dioxide: bubble through limewater; it turns milky/cloudy.

Soluble Salts and Their Preparation

Soluble salts can be made by reacting an acid with an insoluble base (metal, oxide, hydroxide or carbonate).
The solid is added in excess to ensure all the acid reacts; otherwise unreacted acid would become dangerously concentrated during evaporation.
The excess solid is removed by filtration, leaving a solution of salt and water.
The solution is heated to evaporate water until small crystals begin to appear (about half the water removed).
Slow evaporation over days gives larger crystals.
Example: CuO (s) + H₂SO₄ (aq) → CuSO₄ (aq) + H₂O (l).
Acids can also react with metals if the metal is above hydrogen but not too reactive.

Required Practical: Preparation of a Soluble Salt

Aim: prepare a pure, dry sample of a soluble salt from an insoluble oxide or carbonate.
Add dilute acid to a beaker and warm gently.
Add the insoluble oxide slowly until it is in excess (no more dissolves).
Filter to remove excess solid, collecting the filtrate in an evaporating basin.
Heat gently (water bath or electric heater) to evaporate water and make the solution saturated.
Test saturation by dipping a cold glass rod; crystals form on it.
Leave to crystallise, then decant excess solution and dry the crystals.
Expected result: bright blue, regularly shaped hydrated copper(II) sulfate crystals.

The pH Scale and Neutralisation

Acids produce hydrogen ions (H⁺) in aqueous solution; alkalis produce hydroxide ions (OH⁻).
The pH scale runs from 0 to 14; pH 7 is neutral.
Acids have pH below 7; alkalis have pH above 7.
pH 1–3 = strong acid; pH 4–6 = weak acid; pH 8–11 = weak alkali; pH 12–14 = strong alkali.
pH can be measured with universal indicator (approximate) or a pH probe (more accurate).
Neutralisation: H⁺ (aq) + OH⁻ (aq) → H₂O (l).
Not all acid reactions are neutralisations: metal + acid produces a salt but no water.

The pH scale

Strong and Weak Acids

Strong acids are completely ionised in water, giving a high concentration of H⁺ and low pH (1–3).
Examples of strong acids: hydrochloric, nitric and sulfuric acids.
Weak acids are only partially ionised in water, giving a lower concentration of H⁺ and pH 4–6.
Examples of weak acids: ethanoic, citric and carbonic acids.
Weak acids set up an equilibrium, e.g. CH₃CH₂COOH ⇌ H⁺ + CH₃CH₂COO⁻; the equilibrium lies to the left.
Concentration (dilute/concentrated) is different from strength (strong/weak).
A dilute strong acid can have a lower pH than a concentrated weak acid.
pH is logarithmic: a change of 1 unit means a tenfold change in H⁺ concentration.

Titrations

Titrations measure the volume of one solution that reacts exactly with a known volume of another.
Acid-base titrations can determine the concentration of an acid or alkali.
A pipette measures a fixed volume (e.g. 25.0 cm³) of one solution into a conical flask.
A burette delivers the other solution; the volume used is the titre.
A suitable indicator (e.g. phenolphthalein or methyl orange) shows the end-point by a sharp colour change.
Universal indicator is not suitable for titrations because its colour change is not sharp.
Concordant results are within 0.1 cm³ of each other; only these are used to calculate the mean titre.

Titration apparatus set-up

Required Practical: Strong Acid & Strong Alkali Titration

Aim: determine the reacting volumes of a strong acid and a strong alkali by titration.
Use a pipette to place exactly 25.0 cm³ of alkali into a conical flask.
Add a few drops of a suitable indicator (e.g. phenolphthalein).
Perform a rough titration, then repeat accurately, adding dropwise near the end-point.
Record the volume to the nearest 0.05 cm³, reading the bottom of the meniscus at eye level.
Repeat until two concordant results are obtained.
Calculate the mean titre, ignoring anomalous results.

Titration Calculations (Higher Tier)

Use the relationship: moles = concentration × volume (in dm³).
Convert cm³ to dm³ by dividing by 1000.
Use the balanced equation to find the mole ratio between acid and alkali.
Calculate moles of the known solution, then use the ratio to find moles of the unknown.
Concentration = moles ÷ volume (in dm³).
Example: 25.00 cm³ of 0.15 mol/dm³ Ba(OH)₂ neutralises 12.80 cm³ HNO₃; moles Ba(OH)₂ = 0.15 × 0.025 = 3.75 × 10⁻³ mol; moles HNO₃ = 7.5 × 10⁻³ mol; concentration = 7.5 × 10⁻³ ÷ 0.0128 = 0.59 mol/dm³.

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1.Which ion is present in aqueous solutions of all acids?
Easy
- AHydroxide ion, OH⁻
- BHydrogen ion, H⁺
- CChloride ion, Cl⁻
- DHydroxide ion, OH⁺
2.Which ions make a solution alkaline?
Easy
- AHydrogen ions, H⁺
- BHydroxide ions, OH⁻
- COxide ions, O²⁻
- DHydroxide ions, OH⁺
3.All alkalis are bases, but not all bases are alkalis.
Easy
True or false?
4.Which acid should be used to produce magnesium sulfate from magnesium?
Easy
- AHydrochloric acid
- BNitric acid
- CSulfuric acid
- DEthanoic acid
5.Which salt is formed when zinc carbonate reacts with nitric acid?
Easy
- AZinc
- BZinc chloride
- CZinc nitrate
- DZinc oxide
6.Which of the following statements about neutralisation reactions are correct? (select all that apply)
Medium
- ANeutralisation always produces water.
- BThe reaction between an acid and a metal is a neutralisation reaction.
- CAcid + base → salt + water is the general equation for neutralisation.
- DWhen the base is a metal carbonate, carbon dioxide is also produced.
- ENeutralisation always produces hydrogen gas.
7.Match each acid to the type of salt it produces.
Medium
- Hydrochloric acid
- Sulfuric acid
- Nitric acid
- Chlorides
- Sulfates
- Nitrates
8.Which statement correctly describes a strong acid?
Medium
- AIt is completely ionised in aqueous solution.
- BIt is partially ionised in aqueous solution.
- CIt always has a high concentration.
- DIt always contains a high number of acid molecules per dm³.

Thêm câu hỏi

9.A weak acid is partially ionised in aqueous solution.
Easy
True or false?
10.Which of the following is an example of a weak acid?
Medium
- AHydrochloric acid
- BNitric acid
- CSulfuric acid
- DEthanoic acid
11.Which indicator is most suitable for finding the end-point of a strong acid–strong alkali titration?
Medium
- AUniversal indicator
- BLitmus solution
- CPhenolphthalein
- DRed litmus paper
12.Universal indicator is a suitable indicator to use for a titration because it gives a sharp colour change at the end-point.
Easy
True or false?
13.A student prepares copper(II) sulfate crystals by reacting copper(II) oxide with dilute sulfuric acid. Why is the copper(II) oxide added in excess?
Medium
- ATo ensure all the acid reacts.
- BTo increase the rate of reaction.
- CTo make the crystals a deeper blue.
- DTo neutralise the copper(II) sulfate formed.
14.After reacting copper(II) oxide with sulfuric acid, how is the excess copper(II) oxide removed from the mixture?
Medium
- ABy filtration
- BBy crystallisation
- CBy evaporation
- DBy distillation
15.Which statement about the pH scale is correct?
Medium
- AA solution with pH 7 is neutral.
- BAcids have pH values greater than 7.
- CAlkalis have pH values less than 7.
- DThe pH scale goes from 1 to 14.
16.The pH scale is a measure of the concentration of hydrogen ions in a solution.
Easy
True or false?
17.The pH scale is logarithmic. By what factor does the hydrogen ion concentration increase when the pH decreases by one unit?
Medium
- A2
- B10
- C100
- D1000
18.In a titration, the funnel should be left in the burette while taking readings.
Easy
True or false?
19.A student carries out a titration and obtains the following titres: 21.1 cm³, 21.9 cm³, 21.0 cm³, 21.1 cm³, 21.7 cm³. Which three results should be used to calculate the mean titre?
Medium
- A21.1, 21.9, 21.0
- B21.1, 21.0, 21.1
- C21.9, 21.0, 21.7
- D21.1, 21.9, 21.7
20.Which of the following is the correct ionic equation for neutralisation?
Hard
- AH⁺(aq) + OH⁻(aq) → H₂O(l)
- BH⁺(aq) + Cl⁻(aq) → HCl(aq)
- CNa⁺(aq) + Cl⁻(aq) → NaCl(aq)
- DH₂O(l) → H⁺(aq) + OH⁻(aq)
21.Which of the following statements about strong and weak acids are correct? (select all that apply)
Medium
- AA strong acid is completely ionised in water.
- BA weak acid is partially ionised in water.
- CFor the same concentration, a strong acid has a lower pH than a weak acid.
- DA weak acid always has a higher concentration than a strong acid.
- EAll acids are completely ionised in water.
22.When an acid reacts with a metal, the reaction is a neutralisation reaction.
Easy
True or false?
23.Which of the following is a base but NOT an alkali?
Medium
- ASodium hydroxide
- BPotassium hydroxide
- CCopper(II) oxide
- DCalcium hydroxide
24.In the reaction Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g), which species is oxidised?
Medium
- AZn
- BH⁺
- CCl⁻
- DZn²⁺
25.In a metal-acid reaction, the metal atoms gain electrons and are reduced.
Medium
True or false?
26.Arrange the steps for preparing pure, dry copper(II) sulfate crystals in the correct order.
Medium
- Add copper(II) oxide to warm dilute sulfuric acid until it is in excess.
- Filter the mixture to remove the excess copper(II) oxide.
- Heat the filtrate to evaporate some water until the solution is saturated.
- Leave the solution to crystallise, then dry the crystals.
27.Which of the following is a strong acid?
Medium
- AEthanoic acid
- BCitric acid
- CCarbonic acid
- DHydrochloric acid
28.A dilute solution of a strong acid can have a lower pH than a concentrated solution of a weak acid.
Medium
True or false?
29.Which observation shows that a gas produced in an acid-carbonate reaction is carbon dioxide?
Medium
- AA lighted splint gives a squeaky pop.
- BLimewater turns milky.
- CA glowing splint relights.
- DThe gas bleaches damp litmus paper.
30.A student adds universal indicator to an unknown solution. The indicator turns red. What is the approximate pH of the solution?
Medium
- ApH 1–3
- BpH 4–6
- CpH 7
- DpH 8–11
31.Phenolphthalein is colourless in acidic solutions and pink in alkaline solutions.
Easy
True or false?
32.Which of the following is the correct formula for magnesium sulfate?
Medium
- AMgSO₄
- BMg₂SO₄
- CMg(SO₄)₂
- DMgSO₃
33.In a neutralisation reaction between an acid and an alkali, hydrogen ions react with hydroxide ions to produce water.
Medium
True or false?
34.Which of the following is NOT a base?
Medium
- ACopper(II) oxide
- BSodium hydroxide
- CCalcium carbonate
- DHydrochloric acid
35.A student adds a few drops of methyl orange to separate samples of dilute hydrochloric acid and dilute sodium hydroxide. Which of the following statements are correct? (select all that apply)
Medium
- AMethyl orange is red in the acid
- BMethyl orange is yellow in the alkali
- CMethyl orange is colourless in the acid
- DMethyl orange is pink in the alkali
- EMethyl orange is orange in neutral solution
36.Match each indicator to its colour in an alkaline solution.
Medium
- Litmus solution
- Phenolphthalein
- Methyl orange
- Blue
- Pink
- Yellow
37.Arrange the steps of an acid–base titration in the correct order.
Medium
- Rinse the burette with the solution it will contain
- Fill the burette and remove the funnel before taking the initial reading
- Pipette 25 cm³ of alkali into a conical flask and add a few drops of indicator
- Run the acid into the flask, swirling, until the indicator changes colour sharply
- Record the final burette reading and repeat until concordant results are obtained
38.Which piece of apparatus should be used to measure the volume of acid added from the burette during a titration?
Medium
- AA 25 cm³ volumetric pipette
- BA 50 cm³ burette
- CA 100 cm³ measuring cylinder
- DA 250 cm³ conical flask
39.A titration is carried out between dilute sulfuric acid and sodium hydroxide solution using phenolphthalein. Which of the following statements are correct? (select all that apply)
Hard
- AThe solution is colourless before the end-point is reached
- BThe solution turns pink at the end-point
- CThe solution turns yellow at the end-point
- DPhenolphthalein gives a sharp colour change at the end-point
- EUniversal indicator would give a sharper colour change than phenolphthalein
40.A concentrated solution of a weak acid can have a higher pH than a dilute solution of a strong acid.
Easy
True or false?
41.Complete the general word equation for the reaction of a metal with an acid.
Easy
Metal + acid → ____ + hydrogen
42.Name the gas produced when a metal reacts with an acid.
Easy
43.25.0 cm³ of hydrochloric acid was titrated against sodium hydroxide solution of concentration 0.100 mol/dm³. The average titre of sodium hydroxide was 17.3 cm³. Calculate the number of moles of NaOH used. Give your answer in mol.
Medium
44.Name the technique used to find the exact volume of alkali that reacts with a given volume of acid.
Medium
45.What term describes a base that is soluble in water?
Medium
46.Complete the sentences about acids and alkalis using the words provided.
Easy
The ____ ions in nitric acid make it acidic. The ____ ions in sodium hydroxide make it alkaline.
47.In a titration, 25.0 cm³ of 0.100 mol/dm³ hydrochloric acid reacted exactly with 20.0 cm³ of sodium hydroxide solution. Calculate the concentration of the sodium hydroxide in mol/dm³.
Hard
48.Name the salt formed when copper(II) oxide reacts with hydrochloric acid.
Medium
49.Complete the sentences about the ions responsible for acidity and alkalinity.
Medium
In aqueous solution, acids produce ____ ions and alkalis produce ____ ions.
50.Name the ion present in aqueous solutions of all acids.
Medium
51.25.0 cm³ of 0.100 mol/dm³ hydrochloric acid is neutralised by 20.0 cm³ of sodium hydroxide solution. Calculate the concentration of the sodium hydroxide in mol/dm³.
Hard
52.Complete the general equation for the reaction of an acid with a metal carbonate.
Easy
acid + metal carbonate → ____ + ____ + ____
53.Describe how you would obtain pure, dry crystals of copper(II) sulfate from the solution formed after reacting copper(II) oxide with dilute sulfuric acid.
Hard
54.Explain why the pH of a weak acid is higher than the pH of a strong acid of the same concentration.
Hard
55.25.0 cm³ of hydrochloric acid was titrated against sodium hydroxide solution of concentration 0.100 mol/dm³. The average titre of sodium hydroxide was 17.3 cm³. Calculate the number of moles of NaOH used. Give your answer in mol.
Medium
- A0.00173 mol
- B-0.9983 mol
- C2.0017 mol
- D1.0017 mol
56.In a titration, 25.0 cm³ of 0.100 mol/dm³ hydrochloric acid reacted exactly with 20.0 cm³ of sodium hydroxide solution. Calculate the concentration of the sodium hydroxide in mol/dm³.
Hard
- A2.125 mol/dm³
- B0.125 mol/dm³
- C1.125 mol/dm³
- D-0.875 mol/dm³
57.25.0 cm³ of 0.100 mol/dm³ hydrochloric acid is neutralised by 20.0 cm³ of sodium hydroxide solution. Calculate the concentration of the sodium hydroxide in mol/dm³.
Hard
- A1.125 mol/dm³
- B-0.875 mol/dm³
- C0.125 mol/dm³
- D2.125 mol/dm³

Thẻ ghi nhớ

What is the general word equation for the reaction of a metal with an acid?

Metal + acid → salt + hydrogen.

Which metals react with dilute acids?

Only metals above hydrogen in the reactivity series react with dilute acids. The more reactive the metal, the more vigorous the reaction.

Why does copper not react with dilute acids?

Copper is below hydrogen in the reactivity series, so it does not react with dilute acids.

What salt is produced when magnesium reacts with hydrochloric acid?

Magnesium chloride (MgCl₂).

What salt is produced when zinc reacts with sulfuric acid?

Zinc sulfate (ZnSO₄).

What salt is produced when iron reacts with hydrochloric acid?

Iron(II) chloride (FeCl₂).

Describe the test for hydrogen gas, including the expected observation.

Hold a lit splint near the gas; a squeaky pop indicates hydrogen.

What is the general equation for the reaction of an acid with a metal carbonate?

Acid + metal carbonate → salt + water + carbon dioxide.

What is the general equation for the reaction of an acid with a metal oxide or hydroxide?

Acid + metal oxide/hydroxide → salt + water.

What is a base?

A base is a substance that neutralises an acid to form a salt and water. Bases include metal oxides, hydroxides and carbonates.

What is an alkali?

An alkali is a soluble base that produces hydroxide ions (OH⁻) in aqueous solution. All alkalis are bases, but not all bases are alkalis.

What is the ionic equation for neutralisation?

H⁺(aq) + OH⁻(aq) → H₂O(l).

What is the pH scale and what does it measure?

The pH scale goes from 0 to 14 and measures the acidity or alkalinity of a solution based on hydrogen ion concentration. Acids have pH < 7, alkalis have pH > 7, and pH 7 is neutral.

What ions are present in all aqueous solutions of acids?

Hydrogen ions, H⁺.

What ions are present in aqueous solutions of alkalis?

Hydroxide ions, OH⁻.

What is the difference between a strong acid and a weak acid?

A strong acid is completely ionised in aqueous solution, while a weak acid is only partially ionised. Strong acids have lower pH values than weak acids of the same concentration.

Give two examples of strong acids and two examples of weak acids.

Strong acids: hydrochloric acid, nitric acid, sulfuric acid. Weak acids: ethanoic acid, citric acid, carbonic acid.

What is the difference between a concentrated acid and a dilute acid?

Concentration refers to the amount of acid molecules per unit volume of solution. A concentrated acid has a high number of acid molecules per dm³; a dilute acid has fewer.

How does the hydrogen ion concentration change when pH decreases by one unit?

The hydrogen ion concentration increases by a factor of 10.

Why is universal indicator not suitable for titrations?

Universal indicator does not give a sharp colour change at the end-point; a sharp change is needed to identify the end-point accurately.

What is the purpose of a titration?

A titration is used to determine the reacting volumes of solutions, such as finding the volume of alkali needed to neutralise a given volume of acid, or to calculate concentrations.

Why is the insoluble base added in excess when preparing a soluble salt?

To ensure that all the acid has reacted. Any unreacted acid would become dangerously concentrated during evaporation and crystallisation.

How is the excess insoluble base removed after preparing a soluble salt?

By filtration, which removes the unreacted solid and leaves a solution of the salt.

How are solid salt crystals obtained from a salt solution?

The solution is heated to evaporate water until saturated, then left to crystallise slowly. The crystals are then dried.

What is the test for carbon dioxide gas?

Bubble the gas through limewater; if it turns milky or cloudy, carbon dioxide is present.

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