Acids and Bases

Question 1

An acid

Answer 1

A substance which dissolves in water to release aqueous protons , H+ (aq)

• Acidic solutions have a PH between 1-6

Examples:

Hydrochloric acid HCl
Nitric acid HNO3
Sulfuric acid H2SO4

Question 2

pH

Answer 2

Potenz hydrogen ~a measure of how many aqueous protons are present in the solution.

Question 3

Strong acids

Answer 3

• Fully dissociate in solution
• high concentration of protons

Examples:
Hydrochloric acid ~ HCl
Nitric acid ~ HNO3
Sulfuric acid ~ H2SO4
Phosphoric acid ~ H3PO4

Question 4

Weak acids

Answer 4

• Partially dissociate in solution
• LOW concentration of protons
• The forward reaction is incomplete and the weak acid quickly accepts the proton back after releasing it.

Examples:
Ethanoic acid ~ CH3COOH
Methanoic acid ~HCOOH
Citric acid ~ C6H8O7

Question 5

Making an acid

Answer 5

A polar covalent molecule is placed in water and ionises to release an aqueous proton.

Hydrochloric acid:
HCl (g) + aq ——- H+(aq) + Cl- (aq)

Nitric acid:
HNO3(l) + aq ——— H+(aq) + NO3-(aq)

Sulfuric acid:
H2SO4(l) + aq ——- 2H+ (aq) + SO4 2- (aq)

Ethanoic acid :
CH3COOH (l) + aq ——– CH3COO- (aq) + H+ (aq)

Question 6

A base

Answer 6

• a proton ,H+ , acceptor
• neutralise acids
• metal oxides
• metal hydroxides
• metal carbonates
• alkalis

Question 7

Alkalis

Answer 7

A base which dissolves in water to form aqueous hydroxide ion , OH- (aq).

• sodium hydroxide NaOH
  -Potassium hydroxide KOH
• Calcium hydroxide Ca(OH)2
• Ammonia NH3

Question 8

Ammonia

Answer 8

• when ammonia gas is bubbled into water , a SMALL AMOUNT reacts with water to form AQ HYDROXIDE IONS.
• the reaction is REVERSIBLE
• The ammonia ACCEPTS A PROTON in the forward reaction so its behaving as a base.
• The concentration of hydroxide ions is LOW so the solution is WEAKLY ALKALINE.

NH3 + H2O ——— NH4+ +OH-

Question 9

Corrosive

Answer 9

Alkalis are MORE corrosive than acids.

Alkalis ~ corrosive at 0.5 mol dm -3

Acids ~ corrosive at a concentration greater than 6.5 mol dm -3

Question 10

Neutralisation reactions

Answer 10

An ACID reacts with an ALKALI to produce WATER.

• An acid is an aqueous proton
• An alkali is an aqueous hydroxide

Ionic equation :

H+ (aq) + OH- (aq) ——– H2O (l)

Question 11

A salt

Answer 11

A salt is formed when the H+ ion in an acid is replaced by a METAL ION or NH4+.

Question 12

Acid reactions

Answer 12

Metal + acid —— salt + hydrogen

Metal oxide + acid —— salt + water

Metal hydroxide + acid —– salt + water

Metal carbonate + acid —– salt + water + carbon dioxide

Metal hydrogen carbonate + acid —- salt + water + carbon dioxide

Ammonia + acid —— ammonium salt

Question 13

Anhydrous

Answer 13

A salt which does not contain any water molecules.

Has NO water of crystallisation.

Question 14

Hydrated

Answer 14

A crystalline salt compound which contains water.

has water of crystallisation

Question 15

Water of crystallisation

Answer 15

This is the water needed for the crystals to form.

Shown by a DOT in the formula.

Question 16

Working out the formula of a HYDRATED SALT

Answer 16

Calculate MASS of WATER in the hydrated salt:
Mass of hydrated salt - Mass of anhydrous salt.

Calculate MOLES of ANHYDROUS salt:
n =m/M

Calculate MOLES of water:
n=m/M

Calculate the MOLAR RATIO of the anhydrous salt to water:
Divide by the smallest number of moles to find formula.

Question 17

Water of crystallisation METHOD

Answer 17

1. Take an empty CRUCIBLE and weigh it using a BALANCE.
2. Add several SPATULAS of your HYDRATED salt into the crucible and weigh again.
3. Calculate the MASS of hydrated salt added by subtracting the two values given by the balance.
4. HEAT the crucible using a BUNSEN BURNER , driving of the water of crystallisation as STEAM.

5.Weigh the crucible every couple of minutes until the mass STOPS DECREASING.
At this point, we have driven off all of the water and are left with the ANHYDROUS salt which is WHITE.

6.Calculate the MASS of the anhydrous salt by subtracting the FINAL MASS from the mass of the EMPTY CRUCIBLE.

Question 18

Empirical formula to dot formula

Answer 18

Divide the number of hydrogen atoms by 2.

Example:

MgCl2H10O5 (empirical formula)

MgCl2 . 5H2O (Dot formula)

Question 19

Ways to IMPROVE the method for water of crystallisation

Answer 19

• Use a balance to a higher resolution
• INCREASE the mass of the hydrated salt to reduce % uncertainty.
• Use a lid to avoid spitting
• Controlled heating ~ use an oven set below the decomposition temperature to prevent the decomposition of the hydrated salt.

Question 20

Diprotic acid

Answer 20

An acid which has TWO H+ ions that can be replaced by a different positive ion.