Unit 18: Salts — Long Questions

Key point Arrangement of Ions in Salts

Arrangement of ions in salts

Salts An ionic compound which is formed by the neutralization reaction of an acid and a base, along with water, is called a salt.

Example Sodium chloride, potassium iodide, and barium chloride etc.

When acids neutralize bases or alkalis, salts and water are formed.

Neutralization Sodium hydroxide, an alkali, neutralizes hydrochloric acid to form sodium chloride and water. This reaction is called neutralization reaction.

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Arrangement of positive and negative ions of a salt

• A salt is a chemical combination of positive and negative ions. These oppositely charged ions are then bonded together by the electrostatic force of attraction.

• The strength of this attraction depends on the magnitude of the charges and the distance between the ions.

• These oppositely charged ions then arrange themselves in a regular and repeating pattern to give a three dimensional network structure called a crystal lattice.

• The properties shown by the ionic compounds are due to the presence of this ordered structure.

• Under ordinary conditions of temperature and pressure ionic compounds exist as crystalline solids.

Crystal lattice of sodium chloride

• In NaCl, each sodium ion is surrounded by six chloride ions and each chloride ion, in turn, is surrounded by six sodium ions. Figure 18.1 shows how these ions are arranged in the crystal lattice having the shape of a face centered cube.

• The bigger chloride ions are present at the corners and at the centre of each face of the cube while the smaller sodium ions occupy the edges of the cube.

Key points Melting Points of Ionic Compounds | Factors Affecting Melting Points of Ionic Solids

Melting points of ionic compounds

• As very strong electrostatic forces of attraction are present between the ions within a crystal lattice, a significant amount of energy is needed to break these forces. This is the reason why the melting points of ionic compounds are generally very high.

• During the process of melting, these attractive forces break down and let the ions move freely.

Factors affecting melting points of ionic solids

The melting points of ionic compounds depend upon the charges present on the ions and their sizes. Higher charges and smaller ion sizes lead to stronger attractions and hence the higher melting points.

Key point Conduction of Electricity by Ionic Compounds

Conduction of electricity by ionic compounds

• Ionic compounds do not conduct electricity in solid state, however, they start conducting electricity in molten state or when they are dissolved in water.

• In the solid state ions are held together by strong forces of attractions, they are not free to move and, therefore, cannot conduct electricity.

Ionic compounds in molten state: When ionic compounds are heated to their melting points, their ions become free to move around and hence start conducting electricity.

Aqueous solution of ionic compounds: When ionic compounds are dissolved in water, the forces of attraction between ions break down and the ions are again free to move independently.

Ionic compounds as strong electrolyte: Ionic compounds serve as strong electrolytes because they dissociate completely into ions when dissolved in water

Key point Soluble and Insoluble Salts

Soluble and insoluble salts

Factors effecting solubility of salts: Several factors effect the solubility of a salt including the nature of salt and the temperature.

• Soluble salts: Soluble salts include all sodium, potassium, and ammonium salts. Similarly, all metallic nitrates are soluble in water and the same is true for most chlorides and sulfates.

• Insoluble salts: All carbonates with the exception of carbonates of sodium, potassium and ammonium are insoluble in water. Similarly, chlorides of silver and lead and sulfates of barium and lead are also insoluble in water.

• Hydroxides: Hydroxides of sodium, potassium and ammonium are soluble while calcium hydroxide is only partially soluble in water. All other hydroxides are insoluble in water.

The following table shows the pattern for the solubility of salts in water

Table 18.1 Solubility of salts in water

Salts | Soluble | Insoluble
Salts of sodium, potassium and ammonium and all nitrates. | All | None
Chlorides, bromides and iodides | Mostly soluble | Silver, Lead (II)
Sulphates | Mostly soluble | Barium, Lead (II) Calcium
Carbonates | Sodium, Potassium, Ammonium | Mostly insoluble
Hydroxides | Sodium, Potassium, Ammonium, Calcium (partially soluble) | Mostly insoluble

Key point Preparation of Soluble Salts

Preparation of soluble salts

Salts which are soluble in water can be prepared by adopting the following two methods.

(a) Reaction of water soluble acid with water soluble base

Titration method This method is also called a titration method. It involves the reaction of a water soluble acid and a water soluble base. The acid and base are selected depending on the nature of salt which is to be prepared.

Preparation of potassium nitrate

(i) For the preparation of soluble potassium nitrate, appropriate volumes of potassium hydroxide and nitric acid are made to react together in a conical flask.

KOH(aq) + HNO3(aq) → KNO3(aq) + H2O(l)

(ii) Evaporation: The solution present in the conical flask now contains the soluble salt and water. Transfer the solution to an evaporating dish. If the salt being prepared is stable towards heat, then evaporate the solution to dryness to get the salt. Alternatively, heat the solution gently until a thin film of crystals is formed on the surface of the solution.

(iii) Cooling: Cool this solution slowly which is now saturated with salt. Filter this mixture to get the pure crystals of the salt.

(iv) Drying: Dry the crystals between the folds of the filter papers. For drying of crystals, desiccator or oven can be used.

(b) Reaction of an acid with an insoluble base

Neutralization reaction This is also a neutralization reaction between the soluble acid and an insoluble base. The insoluble base may be an appropriate metal oxide, a metal carbonate or a metal hydroxide. The acid and the insoluble base are selected depending upon the salt to be prepared. A salt is also formed when an acid reacts with a metal.

Procedure

• This method involves the mixing of the selected acid with the selected base or a metal with constant stirring to ensure that the acid has fully reacted with the base or the metal.

• The insoluble base or the metal should always be added in excess to make sure that all the acid present has reacted.

• Filter the mixture to remove the excess amount of the base or the metal.

• Evaporate the filtrate to get a saturated solution.

• Allow the solution to cool down when pure crystals of the salt will start appearing.

• Filter and dry the crystals carefully.

Key point Preparation of Pure Crystals of Sodium Sulphate

Preparation of pure crystals of sodium sulphate

Method

• Add 50 cm³ of 1 mol/dm³ solution of sodium hydroxide in a clean conical flask with the help of a pipet.

• Add a few drops of phenolphthalein indicator to get a pink colour solution.

• Run 1 mol/dm³ solution of sulphuric acid from the burette into the conical flask until the colour of solution changes from light pink to colourless.

• Note down the volume of sulfuric acid used to neutralize sodium hydroxide.

Reaction

2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)

• Repeat the experiment taking the same volumes of sodium hydroxide and sulphuric acid but without using the indicator.

• Transfer this neutralized solution to an evaporating dish and heat it gently until one-third of the solution is left.

• Dip a glass rod into this hot solution and take it out.

• If the immersed end of the glass rod turns cloudy, the solution is concentrated enough for crystals to form.

• Cool down the solution for the crystallization to occur completely.

• Filter and wash the crystals with a little cold distilled water to remove any soluble impurities and the acid attached.

• Dry the crystals within the fold of filter papers or in an oven.

Key point Preparation of Pure Crystals of Zinc Sulphate

Preparation of pure crystals of zinc sulphate:

Method

• Take about 25 cm³ dilute sulphuric acid in a beaker and warm it gently.

• Go on adding small granules of zinc metal slowly with constant stirring till the metal starts settling down at the bottom of the beaker.

• Filter the solution to remove undissolved zinc metal and collect the filtrate.

• Evaporate the filtrate in an evaporating dish by gentle heating to concentrate the solution without overheating or boiling.

• Cool down the solution slowly and allow the crystals to appear.

• Once all the crystals have formed, carefully filter the mixture to get the crystals.

• Allow the crystals to dry on filter paper or use oven to dry the crystals.

Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)