Acids and Bases

Question 1

What is an acid?

Answer 1

An acid is a substance that ionises and produces hydrogen ions , H+, when dissolved in water. This means that any acid would contain hydrogen atoms.

Question 2

What are the physical properties of acids?

Answer 2

Acids taste sour
Acids conduct electricity in aqueous solutions
Acids turn blue litmus paper red.
Acids have a pH below 7 (chemical property)

Question 3

When do acids show their properties?

Answer 3

Only when dissolved in water

Question 4

What are the uses of sulfuric acid?

Answer 4

Manufacturing battery acid in cars, fertilisers and detergents

Question 5

What are the uses of hydrochloric acid

Answer 5

Acid pickling: used to remove rust from iron and steel objects before painting/electroplating

Question 6

What are the uses of ethanoic acid (vinegar), CH3COOH?

Answer 6

Used as a flavouring agent (sour taste) or for preserving fruits and vegetables.

Question 7

What are the uses of nitric acid?

Answer 7

Production of fertilisers
Making plastics

Question 8

What is meant my the ‘basicity’ of an acid?

Answer 8

Refers to the maximum number of hydrogen ions produced per acid molecule dissociated in water. A monobasic acid means one molecule of the acid produces one hydrogen ion.

Question 9

What reactions are there for acids?

Answer 9

MASH, ACSWC, BAWS (neutralisation)
Metal + Acid → Salt + Hydrogen
Dilute Acid + Carbonate → Salt + Water + Carbon dioxide
Base + Acid → Water + Salt

Question 10

What is meant by the strength of acids?

Answer 10

It means the degree of dissociation when an acid is dissolved in water to form hydrogen ions in water.

Question 11

What are strong acids? Which are they?

Answer 11

Substances that dissociate completely when dissolved in water to produce hydrogen ions.
Examples: HCl, H2SO4, HNO3

Question 12

What are weak acids? Which are they?

Answer 12

Substances that dissociate partially when dissolved in water
Examples: H3PO4, CH3COOH

Question 13

How is the pH of acids determined?

Answer 13

Concentration (the higher, the lower the pH)
Basicity (the greater, the lower the pH)
Strength

Question 14

What is meant by concentration of acids?

Answer 14

The number of acid molecules dissolved per unit volume of water, normally measured in units of “M” or “mol/dm3”.
Can be altered by dilution.

Question 15

What is a base?

Answer 15

A base is any metal hydroxide or metal oxide that only produces water and salt when reacting with an acid. (alternative meaning: accepts hydrogen ions when reacting with an acid). Most bases are insoluble in water except for Group I metals and ammonium.

Question 16

What is an alkali?

Answer 16

An alkali is a substance that dissociates and produces hydroxide ions, OH– when dissolved in water. (aka. Bases that are soluble in water)

Question 17

What are the properties of alkalis?

Answer 17

Tastes bitter and feels soapy
Conducts electricity in aqueous state
Turns red litmus paper blue

Question 18

What are oxides?

Answer 18

Oxides are a compound of oxygen and another element.

Question 19

What are the four types of oxides?

Answer 19

Basic, amphoteric, acidic, neutral

Question 20

What are basic oxides?

Answer 20

Basic oxides react with acids to form salt and water.
Most are insoluble in water except for 1A oxides.
Most metal oxides are basic oxides.
They are solid at rtp.

Question 21

What are amphoteric oxides?

Answer 21

Amphoteric oxides are oxides of metals that react with both acids and alkalis to form salt and water.

Examples: Zinc Oxide (ZnO), Aluminium Oxide (Al2O3) and Lead (II) Oxide (PbO)
‘ZAP’ or ‘LAZ’

Question 22

What are acidic oxides?

Answer 22

Most non-metal oxides are acidic oxides.
Usually gases at room temperature and pressure
Forms acids when dissolved in water
React with alkalis to form a salt and water

Examples: Carbon dioxide (CO2), Sulfur dioxide (SO2), Nitrogen dioxide (NO2) and
Phosphorus (V) Oxide (P4O10)

Question 23

What are neutral oxides?

Answer 23

Neutral oxides are non-metal oxides which do not react with acids or alkalis.

Examples: monoxides like water, carbon monoxide, nitrogen monoxide.

Question 24

What reactions are there for alkalis?

Answer 24

Neutralisation, AASSWA (reaction with ammonium salt to produce salt, water, and ammonia)

Question 25

What is meant by the strength of an alkali?

Answer 25

The strength of an alkali refers to the degree of dissociation when dissolved in water or the extent of dissociation of alkali molecules to form hydroxide ions in water

A strong alkali dissociates completely in water (100% dissociation)
A weak alkali dissociates partially in water (<100% dissociation)

Question 26

Why must we control the pH of soil?

Answer 26

It is important to control the pH of soil because it affects the growth and development of plants.

Plants grow best when the soil is neutral or slightly acidic.

Question 27

What do we do when the pH of soil becomes too low?

Answer 27

When the pH of soil becomes too low for the healthy growth of plants, calcium hydroxide (slaked lime), calcium oxide (quicklime), calcium carbonate (limestone/ marble chips) can be added to the soil to react with the acids and raise the pH. This is known as liming the soil.

Question 28

Why should we not add calcium hydroxide together with fertilsers containing ammonium salts to the soil?

Answer 28

Fertilisers contain ammonium salts to increase the nitrogen concentration to help the growth of the plant (plants need N, P and K to help plant growth). When calcium hydroxide (an alkali) is added to an ammonium salt, the displacement of ammonia from the soil will occur as calcium hydroxide will react with the ammonium salt. This will result in the loss of nitrogen content, which negates the use of fertilisers and the slaked lime.

Question 29

Why should we not use calcium carbonate (s) marble chips?

Answer 29

it will react with the acid in the soil and will produce CO2 (g). This CO2 gas may dissolve in the water and form carbonic acid which defeats the purpose of using a carbonate to neutralise the excess acid.

Question 30

What does it mean when the pH is below 7, pH is 7, and pH is above 7?

Answer 30

<7: Concentration of hydrogen ions is higher than the concentration of hydroxide ions.
7: Concentration of hydrogen ions and hydroxide ions are equal.
>7: Concentration of hydroxide ions is higher than the concentration of hydrogen ions.

Question 31

On the UI, from red, orange, blue and purple, cite the strength of the substance and an example.

Answer 31

Red: Strong acid, pH 1, HCl
Orange: Weak acid, pH 4, Ethanoic acid
Blue: Weak alkali, pH 10, aqueous ammonia
Purple: Strong alkali, pH 14, sodium hydroxide

Question 32

Cite other indicators and their colours in acidic/alkali solutions, and the pH range of occurrence.

Answer 32

Litmus: red acidic,, blue alkaline, 5-8
Methyl orange: red acidic, yellow alkaline, 3-5
Screened methyl orange: violet acidic, green alkaline, 3-5
Bromothymol blue: yellow acidic, blue alkaline, 6-8
Phenolphthalein: colourless acidic, pink alkaline, 8-10

Question 33

Explain the situation of pH curve, temperature curve and electrical curve at the point of neutralisation

Answer 33

pH: There is a rapid change of pH in the titration curve.
Temp: The solution has the highest temperature at neutralisation
Electrical conductivity: When complete neutralisation occurs, the solution has the lowest electrical conductivity