4 a) Acids, Alkalis and salts

Question 1

4.1 describe the use of the indicators litmus, phenolphthalein and methyl orange to distinguish between acidic and alkaline solutions

Answer 1

Types of indicators:

• Litmus paper
• Methyl orange
• Phenolphthalein

Litmus paper in acidic solutions turns red and in alkaline solutions it turns blue. Methyl orange turns red in acidic solutions and yellow in alkaline solutions.

Phenolphthalein turns colorless in acidic solutions and bright pink in alkaline solutions.

Question 2

4.2 understand how the pH scale, from 0-14, can be used to classify solutions as strongly acidic, weakly acidic, neutral, weakly alkaline or strongly alkaline

Answer 2

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pH scale contains 14 colors. From 0-6 all are acid. From 8-14 all are alkaline. 7 means neutral. During acid, the lower the number, the higher the strength of acid. During alkaline the higher the number, the higher the strength of the alkaline.

Question 3

describe the use of universal indicator to measure the approximate pH value of a solution

Answer 3

Universal indicator is a mixture of several different indicators. Unlike litmus, universal indicator can show us exactly how strongly acidic or alkaline a solution is. This is measured using the pH scale. The pH scale runs from pH 0 to pH 14.

Universal indicator has many different colour changes, from red for strong acids to dark purple for strong bases. In the middle, neutral pH 7 is indicated by green.

Question 4

4.4 define acids as sources of hydrogen ions, H+, and alkalis as sources of hydroxide ions, OH-

Answer 4

Acid produces hydrogen ions, H+ when it is dissolved in water. Alkalis produce hydroxide ions, OH- when dissolved in water.

Question 5

4.5 predict the products of reactions between dilute hydrochloric, nitric and sulfuric acids; and metals, metal oxides and metal carbonates (excluding the reactions between nitric acid and metals)

Answer 5

Metals:

Hydrochloric acid + metal ==> metal chloride salt + hydrogen

Nitric acid + metal ==> Not needed

Sulphuric acid + metal ==> metal sulphate + hydrogen

Metal oxides:

Hydrochloric acid + metal oxide==> metal chloride salt + water

Nitric acid + metal oxide ==> metal nitrate salt + water

Sulphuric acid + metal oxide ==> metal sulphate + water

Metal carbonates:

Hydrochloric acid + metal carbonate ==> metal chloride salt + water + carbon dioxide

Nitric acid + metal carbonate ==> metal nitrate salt + water + carbon dioxide

Sulphuric acid + metal carbonate ==> metal sulphate + water + carbon dioxide

Question 6

4.6 understand the general rules for predicting the solubility of salts in water:
i)

Answer 6

all common sodium, potassium and ammonium salts are soluble

Question 7

ii)

Answer 7

all nitrates are soluble

Question 8

iii)

Answer 8

common chlorides are soluble, except silver chloride

Question 9

iv)

Answer 9

common sulfates are soluble, except those of barium and calcium

Question 10

v)

Answer 10

common carbonates are insoluble, except those of sodium, potassium and ammonium

Question 11

4.7 describe experiments to prepare soluble salts from acids

Answer 11

These all involve reacting a solid with an acid:

• acid + metal (but only for moderately reactive metals from magnesium to iron in reactivity series)
• acid + metal oxide or hydroxide
• acid + carbonate

Making soluble crystals of magnesium sulphate:

1. Add enough magnesium to dilute sulphuric acid, so that there is no acid left and the solution stops bubbling Mg(s) + H2SO4(aq) ==> MgSO4 + H2O
2. Filter the unused magnesium
3. Heat the solution until it forms crystal when it is cooled.
4. Leave the solution to dry up and soluble colourless crystals of magnesium sulphate is formed

Question 12

4.8 describe experiments to prepare insoluble salts using precipitation reactions

Answer 12

Preparation of an insoluble salt:

1. Add the sodium salt solution of the anion to the nitrate salt solution of the cation until no more precipitate forms.
2. Filter to collect the residue
3. Wash the residue with cold water
4. Leave residue to dry on filter paper dry in a warm oven

–Example: Describe how to prepare a dry solid sample of silver chloride, AgCl, a salt which is insoluble in water–

Sodium chloride + silver nitrate ==> silver chloride + sodium nitrate NaCl (aq) + AgNO3 (aq) ==> AgCl (s) + NaNO 3 (aq)

Silver nitrate solution contains silver ions and nitrate ions in solution. The positive and negative ions are attracted to each other, but the attraction aren’t strong enough for them to stick together. Similarly sodium chloride solution contains sodium ions and chloride ions - again, the attractions, aren’t strong enough for them to stick together.

When you mix the two solutions, the various ions meet each other. When silver ions meet chloride ions, the attractions are so strong that the ions clump together and form a solid. The sodium and nitrate ions remain in the solution.

Ag+(aq) + Cl- (aq) ==> AgCl(s)

Example: Making pure barium sulfate

Ba2+(aq) + SO42-(aq) ==> BaSO4(s)

Barium chloride and dilute sulphuric acid are mixed together. Hydrogen ions and chloride ions are spectator ions and aren’t involved in the reaction at all. Barium and sulphate ions attract together to form white precipitate of barium sulphate.

The mixture is filtered to get the precipitate. The solid barium sulphate is impure because of the presence of the spectator ions and any excess barium chloride solution or sulphuric acid. It is washed with pure water while it is still on the filter paper and then left to dry.

Question 13

4.9 describe experiments to carry out acid-alkali titrations.

Answer 13

Titration:

i. fill the acid up to the mark in the burette
ii. pipette 25.0cm3 sodium hydroxide into a conical flask
iii. add a few drops of methyl orange indicator
iv. add acid from the burette drop wise with swirling of flask
v. stop when colour change is permanent (turns pink red)
vi. note burette readings
vii. repeat until concordant results are obtained (results are within 0.1 of each other)
viii. take average of results

Uses:

If they ask you how to prepare a soluble salt using an acid and an alkali, titration must be used. You first carry out a normal titration, and find out the exact amount of acid needed to neutralise the alkali. Then you repeat it without an indicator so that the salt is not contaminated with its colour. You remove the salt from the neutralised solution by evaporation, then you dry it.