Comparing acids (dilute and strong)

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Comparing

Concentrated and Dilute Acids

F.4 Chemistry

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Physical Property

Characteristic: No new substance will be formed in the

testing process

Examples: pH value, melting point, boiling point, density,

colour, magnetic property, conductivity, solubility …..

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Chemical Property

Characteristic: new Substance(s) will be formed in the

testing process.

Examples: Chemical reactions with other reagents., i.e.

reaction of acids with metals to form hydrogen.

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Physical Properties

1. pH Paper pH paper

Dilute HCl(aq) Conc. HCl(aq)

Reason: related to the concentration of H +(aq)

After dipping

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2. Electrical Conductivity

Dilute HCl(aq) Conc. HCl(aq)

dim bright

Reason: related to the concentration of ions present

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Chemical Properties

1. Reaction with carbonates

• Same for all mineral acids, dilute or concentrated

• Observations

•Gas bubbles evolve

•Solids dissolve

Reason: Concentrated acids have ahigher concentration of Hydrogenion so they react faster and bubblesare given out at a faster rate

carbonateCO 3

2-

+ 2H+

H2O + CO 2

CO 2

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Chemical Properties

2. Reaction with oxides and hydroxides e.g. copper(II) oxide

• same for all acids(dilute or concentrated)

• Observationblack powder dissolvesa green/blue solution remains

Reason: black copper(II) oxidedissolves to form Cu 2+(aq),which is blue/green in colour. Before After

Copper(II) oxide

CuO (s) + 2HCl (aq) CuCl 2 (aq) + H 2O (l)

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Chemical Properties

3. Reaction with metals

Dilute Concentrated

HydrochloricAcid Gives HydrogenGas Gives HydrogenGasSulphuric Acid Gives Hydrogen

GasGives Sulphur Dioxide Gas

Nitric Acid Gives NitrogenMonoxide Gas

Gives NitrogenDioxide Gas

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Observations

1. Hydrogen GasPop

• Colourless gas evolves

• Gives a ‘pop’ sound with a burning splint

• The metal dissolves

N.B.: Not for metals below lead in the Reactivity Series

HCl(aq)

Zinc metal

Zn (aq) + 2 HCl (aq) ZnCl 2 (aq) + H 2 (aq)

Video

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Observations

2. Sulphur Dioxide Gas

• Colourless gas evolves

• The metal dissolves

N.B.: The reaction is faster when hot acid is used.

H2SO

4(l)

Zinc metal

K 2Cr 2O7/H+(aq)

Zn (s) + 2 H 2SO 4 (l) ZnSO 4 (aq) + SO 2 (g) + 2H 2O (l)

• It turns orange potassium dichromate paper green

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Observations

2a. Nitrogen Dioxide Gas

• Brown gas (NO 2) evolves just above liquid level

• The metal dissolves

N.B.: The reaction is faster when hot acid is used.

HNO 3(conc.)Zinc metal

Zn (s) + 4 HNO 3 (conc.) Zn(NO 3)2 (aq) + 2NO 2 (g) +2H 2O(l)

Brown gas

a) Conc. Nitric acid

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Observations

2b. Nitrogen Dioxide Gas

• Brown gas (NO 2) appears at the upper part of thetest tube.

• The metal dissolves

N.B.: colourless nitrogen monoxide forms which reacts withair oxygen to form brown nitrogen dioxide gas.

HNO 3 (dilute)Zinc metal

3Zn (s) + 8 HNO 3 (conc.) 3 Zn(NO 3)2 (aq) + 2 NO (g) +4 H 2O (l)

2NO (g) + O 2 (g) 2 NO 2 (g)

Brown gas

b) Dilute Nitric acid

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Other reaction

Dehydrating property of conc. Sulphuric acid

a. Reaction with sugar

• Black spongy solid of carbon forms

C12

H22

O11

(s) 12 C (s) + 11 H2O (l)

Reason: H and O atomsare removed from sugar

molecules in the ratio 2:1.Water is formed andcarbon solid remains