Topic 4: Acid and Bases

Question 1

Basicity (Definition)

Answer 1

the number of hydrogen ions produced by complete ionisation of one acid

miss tam’s definition:
the maximum number of hydrogen ions that can be produced by one molecule of acid.

Question 2

Acid (Definition)

Answer 2

A molecule that ionise to give mobile H+ (aq) ion as the only positively charged ion when dissolved in water
- This definition is limited to the substances containing H.
- Water must be present in the acid

Miss Tam’s definition:
- a covalent compound which produces hydrogen ions (mobile H+ ions) as the only positive ions when dissolved in water)

Question 3

Typical acid (Definition)

Answer 3

An acid which uses H+ (aq) as the ONLY agent to react with metals to give H2 (g)

Question 4

Ionisation (Definition)

Answer 4

formation of ions from atoms and molecules

Question 5

Concentrated hydrochloric acid

Answer 5

• volatile solution (give white fume –> acid mist, give strong acidic odour)
• reaction with metals (give hydrogen gas, reaction rate of conc. HCl is faster than that of dilute HCl)
• typical acid

Question 6

Corrosive Labels (Where???)

Answer 6

only concentrated strong acid should have a corrosive hazard warning label.

Question 7

Concentrated nitric acid

Answer 7

• volatile solution (give acid mist, give strong acidic odour)
• should be stored in brown bottle (as will be decomposed under sunlight to form brown nitrogen dioxide gas —> prevent exposure to sunlight )
• conc. HNO3 reacts with some unreactive metals (eg. Cu, Ag)
• nitrogen dioxide is formed.
• NOT a typical acid –> it is an oxidising agent

Question 8

Dilute nitric acid

Answer 8

• volatile solution
• should be stored in brown bottle (it is decomposed under sunlight)
• dilute HNO3 reacts with some unreactive metals, like Cu, Ag.
• nitrogen monoxide is formed.
  NO(g) is unstable in the presence of O2(g). NO(g) is further oxidised to form NO2(g) immediately.
• Dil. HNO3 (aq) it NOT a typical acid, it is an oxidising agent.

Question 9

Concentrated sulphuric acid

Answer 9

• non-volatile solution (very viscous)
• Conc. H2SO4(l) reacts with some unreactive metals, like Cu, Ag.
• sulphuric dioxide is formed.
• NOT a typical acid, it is an oxidising agent

Question 10

the mineral acid is..

Answer 10

phosphoric acid (H3PO4)

Question 11

chemical formula of carbonic acid

Answer 11

H2CO3

Question 12

Strong acid (must-have hazard warning label)

Answer 12

–> IRRITANT

Question 13

properties of dilute acids (acids in lab –> mineral acid)

Answer 13

• have sour taste
• are conductors of electricity
• react with bases
• react with reactive metals
• react with metal carbonates and hydrogencarbonates

Question 14

CH3COOH (weak acid–>ethanoic acid)

Answer 14

ethanoate ion
–> CH3COOH Mg (magnesium ethanoate)

Question 15

physical test for dilute acids

Answer 15

test the electrical conductivity.
The acid must be in aqueous form, cannot be in solid form
place the acid in a beaker. Immerse the carbon electrodes in the acid. The light bulb glows
Reason: dilute acids contain mobile ions for carrying the current

Question 16

1st type of chemical reaction
- Metal + dilute acid –> salt + hydrogen gas

Answer 16

Possible observations:
- the metal dissolves.
- container with the solution turns warm.
- colourless gas bubbles evolve.
- the solution turns from __________ to ___________
- BUT WHEN SOLID IS FORMED (Eg. CaSO4), the reaction instantly stops because CaSO4(s) is formed on the surface of Ca, preventing further reaction with dilute acid

Question 17

Base (definition)

Answer 17

A substance that reacts with an acid to form a salt and water only

Question 18

2nd and 3rd type of chemical reaction
- oxide / hydroxide of metal + dilute acid –> salt + water

Answer 18

Observations:
- container with solution turns warm
- solution turns from ________ to _________
- ____colour____ solid ___CuO___ dissolves.

Question 19

4th and 5th type of chemical reaction
- carbonate / hydrogencarbonate of metal + dilute acid –> salt + water + carbon dioxide

Answer 19

the presence of limewater can be confirmed by bubbling the gas through limewater. The limewater initially turns milky, but then becomes clear again if excess carbon dioxide is passed through the limewater

Question 20

6th type of chemical reaction
- acid + metal sulphite (SO3 2-) –> salt + water + sulphur dioxide

Question 21

7th type of chemical reaction
- acid + metal hydrogensulphite (HSO3 -) –> salt + water + sulphur dioxide

Question 22

Test for SO2 (g)

Answer 22

• a piece of filter paper soaked with acidified potassium dichromate solution
• the paper changes from orange to green
• sulphur dioxide reduces the orange dichromate ions to green chromium (III) ions.

Question 23

when can the properties of the acids be shown?

Answer 23

dissolved in water
–> BECOME AQUEOUS SOLUTION

Question 24

dissociation (definition)

Answer 24

this refers to the ionic compounds splitting into smaller substances (eg. ions / atoms)

Question 25

Tests testing the presence of H+ (aq) ions

Answer 25

Physical Test: - pH (universal indicator/litmus paper/pH paper/pH meter) - electrical conductivity (ammeter reading / light bulb) Chemical Test: - add Mg(s) --> H2(g) is formed - add MgCO3(s)--> CO2 (g) is formed

Question 26

H+

Answer 26

too reactive to exist on its own --> combines with a water molecule via a dative covalent bond, producing a hydroxinium ion

Question 27

Basicity (if 2 or more)

Answer 27

each time, the chemical equation will only ionise one H+ ion out (the first chemical eq. is irreversible while the second one is reversible)

Question 28

Alkalis

Answer 28

bases that are soluble in water

Question 29

Alkali (definition)

Answer 29

a base which releases mobile hydroxide ions (OH-(aq)) when dissolved in water.

Question 30

drying agent

Answer 30

- anhydrous CaCl2

Question 31

Making aqueous ammonia

Answer 31

the inverted funnel increases the area over which the gas can dissolve, and prevents sucking back dur to pressure difference

Question 32

drain cleanser (contains NaOH)

Answer 32

- cannot use conc. sulphuric acid --> metal pipes with react with it, so not widely used - solid NaOH(s) pellets or (kept dry --> because deliquescent --> absorb water to become liquid) - NaOH --> caustic soda, concentrated, corrosive, dissolve to clear blockage

Question 33

glass cleanser (contain ammonia)

Answer 33

- highly water soluble (can be washed away easily) - evaporation - V alkaline: can dissolve grease - X pungent smell (so good ventilation needed) - X tinted glass

Question 34

antacid tablets (contain aluminium hydroxide and magnesium hydroxide)

Answer 34

- MgCO3 used in the past, but not now (the carbon dioxide gas produced will cause discomfort in the stomach) - gastric juice: hydrochloric acid (conc.)

Question 35

reduce the acidity of soil (Ca(OH)2)

Answer 35

not spray the aqueous solution --> otherwise will be absorbed in the soil too quickly

Question 36

alkalis commonly seen in school

Answer 36

dilute potassium hydroxide (corrosive) dilute sodium hydroxide (corrosive) calcium hydroxide dilute aqueous ammonia (irritant)

Question 37

physical properties of alkalis

Answer 37

- feel slippery - have a bitter taste - conductors of electricity - react with solutions containing certain metal ions to form precipitates - react with ammonium compounds