CHE3: ELECTROLYSIS

Terms used in electrolysis

  • Electrolyte;

Is a compound which conducts electricity in the molten from (fused formed or an aqueous solution and is decomposed by the electro current.

Electrolytes are composed of ions in the solid state the ions are immobile.

 Examples of electrolytes are

  1. Ionic substances (salts e.g. sodium chloride, sodium hydroxide.)
  2. Solutions of acid (ethnic acid, hydrochloric acid)

 A VIDEO ABOUT ELECTROLYTES

Non – electrolyte:

Is a solution or molten compound which does not conduct electricity and therefore is not decomposed by the electric current e.g. paraffin, sugar solution, ethanol

Strong and weak electrolyte

  • Strong electrolyte:

Is one which is completely or almost completely ionized (dissociated) in molten form or in aqueous solution? Strong electrolytes have high electrical conductivity.

 Examples of strong electrolytes

  1. Dilute acids e.g. dilute hydrochloric, sulphuric and nitric acids

moltenWeak electrolytes:

Are slightly ionized in aqueous solution.

They contain very few mobile ions, therefore, have no electrical conductivity

 Examples of weak electrolytes

  1. Aqueous solution of weak acids e.g acetic acid

CH3COOH      water      CH3COO + H+

(Molecular)                     (Hydrogen ion)

(Many molecules)          (Ionized acid / few ions)

 

Ammonia (NH3)

NH3 + H2O         water          NH4 (aq) + H2O(aq)

ELECTRODES

Electrodes are two poles of carbon / graphite or metal at which the current as a flow of electrons enters or leaves the electrolyte.

 

  • Anode is the positive electrode.

When the circuit is completed negative ions anions move towards this direction. It is the electrode at which elections leave the electrolyte and enter the circuit.

Cathode:

It is the negatively charged electrode. Positively charged ions cautions migrate to this electrode.

It is the electrode at which electrons enter the electrolyte.

Ionic theory:’

It states that – electrolytes consist of ions which are positively and negatively charged atoms or radicals.

Anions are negatively charged ions that move to the adobe during electrolysis.

Cautions;

Positively charged ions that move to the cathode during electrolysis

taboz

The properties of anion differ from those of its element or radical ions are chemically inert and stable because they have electronic configuration of noble gases.

Metals and hydrogen form positive ions cautions which move to the cathode during electrolysis; therefore, are electro positive.

An electro positive element is one which forms positive ions after losing outer shell electrons.

Non- metals and acid radicals form negative ions which move to the anode during electrolysis, therefore they are electro negative.

Cl + e               Cl

2:8:7                       2:8:8

Electro negative element – is one which forms negative ions by accepting electrons.

The size of charge of the ion by accepting electrons.

The size of charge of the ion is the same as the valency of atom or radical

Monovalent elements or radicals loss or gain one electron to form ions

trivalent

A VIDEO ABOUT ELECTRODES

A solution or molten compound is neutral because the total positive charges due to cautions out total negative charge to the anions.

Note:

Ionic or molten compound is neutral because in the solid state ions are immobile but in the molten state and aqueous solution they conduct electricity because the ions are free to move.

Electrolysis of dilute sulphuric acid.( acidified water)

Electrolyte – dilute sulphiric acid

cathede

Ion in the electrolyte;

They come from two sources

ionization (dissociation of sulphiric acid in water)

Water

ion

formular

ELECTRODES

Platinum electrodes – (graphite electrodes)

Platinum and graphite are chemically inert- they don’t dissolve in the solution and they don’t react with products to form at the electrolytes.

 

Cathode (-ve)

Hydrogen ions migrate to this electrode and they are discharged.

Equation of the cathode reaction

2H+ (aq)  2e        H2 (g)

Observation at cathode

Evolution of colourless gas which gives a pap sound, when a burning splint is applied.

At anode (+ve)

Sulphate and hydroxyle ions migrate to this electrode, hydoroxyle ions are dichanged in preference to the suplhate ions

Reason

– discharging hydorxyle ions requires lower energy expenditure compared to discharging SO4 ions.

Or

Hydroxyle ions are lower than SO4 ion in the electro- chemical series

Anode reaction equation

4OH(aq)      2H2O (aq)  O2 (aq)  4e

Observation of the anode;

Evolution of colourless gas which support burning

Alkalinity decreases at the anode

Acidity decreases at the cathode and alkalinity increases at cathode

Equation of overall change during electrolysis

It is got by combining the equation for the cathode and anode reactions.

 Anode

4OH(aq)     2H2O (aq)  O2 (aq)  4e

Cathode

2(2H+ (aq)  2e    H2 (aq))

Combining equations i. and ii

4OH (aq)  4H+ (aq)  2H2O (l)  2H2 (g)  O2 (g)

 NB.

The volume of hydrogen gas is twice that of oxygen.

  1. hydroxide ions discharged the anode and hydrogen ions discharged at cathode replace by ionization of more H2O molecules; electrolysis of dilute H2SO4 of water.

platinum

hid

 

 

 

 

cathode

Factors which affect preoperational discharge of ions electrolysis

  1. Position of the ion in the electro chemical series

When two ions of similar charge present at the electrode, the ion lower in the electro-chemical series is discharged in preference to that above.

Positive ion (cautions)

K+

Ca+

Mg2+

Al3+

Zn2+

Fe2+

Pb2

H+

Cu2+

Ag+

Negative ions (anions)

SO4m2-    NO32-     Cl– Br–     OH

 

The above table is the electro chemical series

Note:

The order calcium and sodium is reversed if sodium and calcium ions are present as the only positive ions, sodium ions are discharged in preference to calcium ions.

Concentration of ion in solution

When two ions of similar changes are present, the one with a greater concentration may be discharged preferentially even when it is higher in the series.

Example:

In the electrolysis of sodium chloride solution, the concentration of chloride ions is higher than that of hydroxide ions.

Chloride ions are discharged in preference to hydroxide ions.

EXAMPLE (CALCULATIONS)
a. State the concentration, in moles per liter, of each ion in 1.0 mol Al(NO3)3.
b. State the concentration, in moles per liter, of each ion in 0.20 mol K2CrO4.

SOLUTION
Part a.) Dissolving 1 mol of Al(NO3)3 in water dissociates into 1 mol Al3+ and 3 mol NO3- by the reaction:

Al(NO3)3(s) → Al3+(aq) + 3 NO3-(aq)

Therefore:

concentration of Al3+ = 1.0 M
concentration of NO3- = 3.0 M

Part b.) K2CrO4 dissociates in water by the reaction:

K2CrO4 → 2 K+(aq) + CrO42-

One mol of K2CrO4 produces 2 mol of K+ and 1 mol of CrO42-. Therefore, for a 0.20 M solution:

concentration of CrO42- = 0.20 M
concentration of K+ = 2×(0.20 M) = 0.40 M

ANSWER
Part a).
Concentration of Al3+ = 1.0 M
Concentration of NO3- = 3.0 M

Part b.)
Concentration of CrO42- = 0.20 M
Concentration of K+ = 0.40 M

Type of electrode used. (nature of the electrode)

Different electrodes for given electrodes may cause different products to form at the electrodes.

Electrolysis of copper (II) sulphate solution

  1. Using inert electrodes (graphite electrodes)

Ions in the electrolyte – they come from two sources;

Sulphuric acid

CuSO [4s]                                             O2+ [aq] + SO42-[aq]

  1. Water

H2O[S]                                                  H+ [q] + OH[aq]

hydroxile

At the cathode;

Hydrogen ions and copper (II) ions migrate to this copper (II) ions are discharged in preference to hydrogen ions

Cu2+ (aq)  2e  Cu(s)

 Observation at cathode

  • Reddish – brown solid is deposited (copper metal)
  • Weight of cathode increases because of deposition of copper.

The reaction which occurs at the cathode is reduction

Reduction is addition or gain of electrons

Cu2+ (aq)  2e  Cu(s)

 

Reduction is decrease in charge (charge on a neutral element is always)

Reduction

Cu2+(aq)  2e  Cu(s)

At the anode

Hydrogen and sulphate ions migrate to this anode.

Hydroxyle ions are discharged in preference to sulphate ions

Reasons;

Hydrogen ions are high in electro chemical series

Equation of anode

4OH(aq)     2H2O (aq)  O2 (aq)  4e

Observation

Evolution of colourless gas which supports burning

Equation of the overall change;

It is got by combining the equation for the cathode and anode reaction.

Cathode

anode

The blue colour of the electrolyte faded

This is due to discharge of copper (II) ions at the cathode causing the decrease of thee concentration in the electrolyte.

Note:

Irrespective of the nature of the cathode. The cathode reaction is always the same.

 Electrolysis of copper (II) sulphate solution using copper electrodes

Ions in the electrolyte; copper (II), hydrogen ions, sulphate ions and hydroxyle ions.

At cathode;

Hydrogen an copper (II) will migrate to this cathode

Copper (II) is discharged. (Order of ions in chemical series)

sulph 

Observation

The weight and size of the anode decreases.

Gaining weight of the cathode is equivalent to the loss in mass of the anode.

The net charge is transfer of copper from the anode to cathode. The intensity of the blue colour of the electrolyte does not charge.

This is because copper (II) ions discharged at the cathode are replaced by copper dissolving by the anode.

 Application of electrolysis of copper (II) using a copper anode

  1. Purification of copper metal

Most of the copper is used for electrical purposes example; manufacture of copper wires. Impurities in copper increases electrical resistance in the copper wires.

Thus reducing the efficiency of the copper wire. Therefore copper for this purpose must be purified.

In the purification of copper, impure copper is made the anode and a thin sheet of pure copper, a cathode of the electrolytic cell.

The electrolyte most contain copper (II) ions

Ions in the electrolyte

From copper (II) sulphate

CuSO4(s)    water              Cu2+  SO42- (aq)

From water

H2O (l)    H+  OH(aq)

oh

At the cathode;

Copper (II) and hydrogen ions migrate to these electrodes. Copper (II) ions are discharge in reference to hydrogen

 

Equation of cathode reaction

Cu2+ (aq)  2e  Cu(s)

Observation at cathode

– Reddish- brown solid is deposited

-Cathode increases in mass and size

At anode;

Hydroxyle and sulphate ions migrate to these electrode but neither of them is discharged instead copper dissolves.

Equation of anode reaction

Cu(s)  Cu2+ (aq)  2e

 Observation: –

The mass and size of the anode decreases (due to dissolving of copper metal.) impurities drop to the bottom of the container.

They contain precious metals like gold, silver etc.

The net change is transfer of copper from the anode to the cathode.

The intensity of the blue colour of electrolyte does not change

 

Reason

Copper (II) ions discharged at the cathode are replaced by copper dissolving from the anode.

 

  1. Copper electroplating

Electroplating is coating of graphite / metal with another metal by electrytic method

To copper plate an article [, the article is made the cathode and pure copper the anode of electrolytic cell.

The electrolyte must contain copper (II) ions e.g. copper (II) sulphate solution as shown in the diagram below.

battery

electro

Purpose of electroplating

  1. To improve appearance
  2. To prevent rusting of iron metal
  3. To prevent corrosion of metal.

Corrosion of metals

Corrosion of metals is the effect of metal by reaction with substances in air water and soil.

 

Electrolysis of sodium hydroxide solution

Set up the apparatus:

Some as that for electrolysis of dilute sulphric acid.

Electrodes – platinum

Ions in the electrolyte – from sodium hydroxide

  • Sodium ions, Na+
  • Hydroxide ions, OH-aq

 

From water – hydrogen ions, H+aq

Hydroxide ions, OH(aq)

 

At the cathode;

Sodium and hydrogen ions nmigrate to tis electrode

Hydrogen ions are discharged in preference to sodium ions.

charge

4OH- (aq)  4H+ (aq)  2H2O (l) + O2(g)   H2(g)

4OH- (aq)  4H+ (aq)  2H2O (l) + O2(g)  H2(g)

The value of hydrogen is twice that of oxygen

Electrolysis of sodium chloride solution

Electrolyte

– Sodium chloride solution

Setup of the apparatus;

Same as that 4 electrolysis of dilute sulphric acid

 Electrodes

Cathode;

It must be carbon (graphite) or platinum

Anode;

It must be carbon which resists attack by chlorine

Ions in the electrolysis

Sodium and chloride ion

Hydrogen and hydroxyle ions from water

At cathode;

Sodium and hydrogen migrate to this electrode

Hydrogen ions are discharge in preference to sodium ions

Equation

2H+ (aq) + 2e H2 (g)

Cathode product is hydrogen

At anode;

Hydrogen and chloride ions migrate to these electrons

Chlorine ions are is charged in preference to hydroxyle

Reason;

Chloride ions have higher concentration than hydroxyle ions

Equation

2Cl (aq)  Cl2  2e

Equation of overall reaction

It is got by combining equation for the cathode and anode reaction.

Cathode

2H+ (aq) + 2e H2 (g)

 

Anode

2Cl (aq)  Cl2  2e

The volume of chloride gas is equal to the volume of hydrogen gas.

The volume ratio of chlorine is 1:1

Electrolysis leads (II) bromide

The apparatus was set up as shown in a figure below;-

bromide

 

A and B are graphite electrodes

State what is observed and explain your answer

Bulb doesn’t give light because lead (II) bromide is an ionic compound in solid state ions are immobile therefore circuit is incomplete.

 The circuit is heated using a Bunsen burner flame and till it all melted

Observation

Bulb gave light

At (cathode) – silvery grey solid is deposited (lead metal)

At anode (B) – brown gas. (Bromine gas)

 Write equations for cathode and anode reactions.

At B (anode)

Br (l)  Br (g)  2e

At A (cathode)

Pb2+  2e  Pb(s)

 To what conclusion do you draw from your observations in A and B

Bulb doesn’t light because ionic substance are none conductors in solid form (B)

In B, the circuit is complete cause in molten state movement of ion completes the circuit.

Note:

In solid conductors flow of current is movement of electrons

In electrolytes, flow of current is movement of ions

 GENERAL APPLICATIONS OF ELECTROLYSIS

  1. Purification of metals (revisit notes)
  2. Electroplating
  3. Extraction of electropositive metals like sodium, potassium, magnesium, aluminium
  4. Manufacture of chemicals like hydrogen, chlorine, sodium hydroxyle.

 

catho

 

Electrolyte is brine

Electrodes

 

Cathode

 – Mercury – which flows through the bottom at the cell?

 

Anode

– Graphite anodes – because chlorine does not attack graphite

 Equations of anode and cathode

Anode

2Cl (aq)  Cl2  2e

 Cathode

Chlorine is collected and stored in cylinders

 At cathode (mercury)

– Sodium ions are discharged to form sodium metal

Sodium combines with mercury to form a malgam, which drops into water reacting with it to form sodium hydroxide solution and hydrogen gas.

The hydrogen is collected and stored.

The aqueous layer is immiscible with mercury

Sodium hydroxide is recovered from the qeuos layer by hitting it to dryness.

This electrolysis is used in the manufacture of chlorine gas, hydrogen gas and sodium hydroxide

 SULPHUR AND ITS COMPOUNDS

Sulphur is a nonmetallic element of group 6 of the periodic table

Electronic configuration of sulphur

2:8:6

Valency – 2

 Ion formation

Sulphur forms a negative ion with a charge of -2 by gaining 2 electrons to fill the outer most shell.

5 + 2e             S2- (sulphide ion)

2:8:6              2:8:6

 Extraction of sulphur by the Frasch process

Sulphur deposits occur about 200m or more underground the sulphur is melted then forced to the surface by pressure.

A hole is drilled to the grouped up to the sulphur deposits. 3 concentric pipes of diameter 2cm, 8cm and 15cm are laid in the hole up to the sulphur deposits.

These constitute the sulphur pomp.

Super-heated water at1700cand pressure if forced down the outmost 15cm diameter pipe to melt the sulphur.

Hot air under pressure (15 atmospheres) is forced down the inner most 2cm diameter pipe.

The combined pressure of air and water forces the mixture of molten sulphur and water up the middle 8cm diameter pipe.

The mixture is run into large tanks where sulphur solidified in a lemon yellow solid

Uses of sulphur

  1. In the manufacture of sulphric acid
  2. Used in the vulcanization of rubber. (Rubber is heated with sulphur) this [improves the working properties of rubber.
  3. Use in the manufacture of dyes, fireworks and sulphur compounds like carbon disulphide.
  4. Used in manufacture of ointment for treatment of skin diseases such as ring worms.
  5. It is used for string vines and other fruit trees to prevent growth of fungus.

Effects of heat on sulphur

When solid sulphur is heated.

It melts at about 1130C to a dear amber coloured liquid which flows easily like water (not vuscous)

Solid sulphur and molten sulphur, consists of 58 packed ring molecules

s

h0

tangle

Sulphur boiles at 4440C forming a brownish vapour )

 Preparation of plastic sulphur

Boil sulphur and poor it a thin continous stream into a beaker of water.

Black – brown elastic solid forms.

It is called plastic sulphur. It is a supercoiled liquid. Insoluble in carbon disulphide.

On cooling, it changes to hard yellow, opaque rhombus sulphur

plastic sulphur

Allotropy of sulphur

Allottopy:

– It is existence of an element in more than one form in the same physical state

The different forms are called allotropes

Allotropy is due to difference in arrangement of atoms.

The allotropes of sulphur are;-

A – Monon clinic sulphur (or beta sulphur, )

B – arombic sulphur. (alpha sulphur, )

C – amophus sulpur

 

Monoclinic and arombic sulphur are crystal like while amophus sulphur is non-crystalline.

Polymorphism

Is the ability of an element to form more than one type of crystals?

The element is said to be polymorphic and their different crystalline forms are termed polymorphs

Polymorphs (crystalline forms of sulphur are rbrombic and monoclinic sulphur)

Like – sulphur, carbon is allotropic.

It allotropes are;

  • Graphite
  • Diamonds
  • Amorphous carbon

Graphite and diamond are crystalline while amorphous carbon is non-crystalline e.g. wood cahrcaol and animal charcoal

crystal

Rhombic / alapha sulphur

It consists of rhombic crystals (octahedral crystals)

It is stable below 960C. Its crystals are bright yellow and its melting point is 1130C.

Preparation of rhombic sulphur

Powdered roll sulphur dissolved in methyl benzone (organic solvent)

The solvent is volatile. (No boiling point) and inflammable

Filter the mixture through dry filter paper and collect the filtrate in the beaker.

Cover the beacker with paper with a few holes in it so that methyl benzene can evaporate at a slow rate.

Leave the solution in a fume card board

Bright yellow crystal of rhombic sulphur form a6 volatile methyl benzene evaporates

Preparation of monoclinic sulphur

Warm an evaporating basin full of powered roll sulphur with a small flame until it just melts. And more powered sulphur and continue heating until the basin is almost full of molten sulphur. Then allow it to cool.

When a thin solid crust forms on the surface mke two holes in it.

Poor liquid sulphur through one hole as air enters the other one. Cut away the crust and lift it out. Look under the crust and in the basin. Pale yellow crystals which look like needless are observed.

Difference between rhombic and monoclinic sulphur

 

Rhombic           Monoclninc
Stable below 960C           Stable above 960C
Octahedral crystals            Needlne shaped crystals
Bright yellow            Pale yellow
Melting point is 1130C            Melting poiny 1190C
Density higher 2.06g/cm3            Density lower  (1.980g/cm3)

 

Facts to prove that rhombic and monoclinic sulphur are allotropes of the same substance,

  1. 1gm of monoclinic sulphur changes at room temperature into 1gm of rhombic sulphur.
  1. 1gm of either form will burn in oxygen to give the same mass (2gm) of sulphur dioxideand no other substance.

 

Oxides of sulphur

There   are 2 important oxides of sulphur that is

  • Sulphurdioxide, SO2

trioxide

S1 O3 = SO3

Appropriate name is sulphur [v1] oxide

 Sulphur dioxide

Laboratory preparation

Method 1

Sulphur dioxide can be prepared by heating a mixture of sodium sulphide, Na2SO3 and concentrated sulphuric acid or dilute hydrochloric acid.

Sodium hydrogen sulphite can be used instead of sodium sulphite.

ionic

sulphiric

 Method II

By heating a mixture of copper metal and concentrated sulphuric acid

 

Equation

Cu [s] + H2SO4 [conc]                                        CUSO4 [s] + SO2[S] + H2O[g]

The gas is dried by bubbling it through concentrated sulphuric acid and collected by down ward delivery.

conc

Properties of sulphurdioxide

  • Colorless acidic gas with an irritating smell
  • It’s denser than air.
  • Very soluble in water (can be used to perform the fountain experiment )
  • Turns moist blue litmus red

 Results of experiment

 

  1. Turns blue litmus paper red and bleaches it; sulphur dioxide is an acidic gas and has bleaching properties

SO2 (g)  H2O (l)  H2SO3 (aq)   sulphuric acid

 

  1. It reacts with alkalines forming salts

It bleaches flowers and brown paper

N.B:

Sulphur doxide bleaches by removing oxygen from a substance

When a substance is exposed to air for some time. It reacts with atmosphere oxygen and the original colour is restored. This explains why old newspapers turn yellow.

It decolorizes acidified potassium permanate (from purple t colourless)

This is one of the chemical tests for sulphur dioxide.

In reaction sulphur dioxide behaves as a reducing agent. Reducing manganese (VII) ions (purple) to manganese (II) colorless.

SO2

Mn7+ [aq] + 5E                          Mn2+

Purple                                                  colourless

Reaction with acidified potassium dichromate

Is sulphur dioxide gas is bubbled through acidified potassium dichromate ther is a colour change from orange to green.

In the reaction, sulphur dioxide behaves like a reducing agent and is oxidized to sulphuric acid. Potassium dichromate acts as oxidizing agent.

3SO2 (g)  CL2O7 (aq)  3SO42- + 2Cr3+ [aq] + H2O [L]

R.A                 O.A                                         Green

 

Cr 6+ [aq] + 3e                        Cr 3+ [aq]

Arrange                                               green

 

This is a characteristic test for sulphur dioxide.

When sulphurdioxide is bubbled through a solution of iron 3+.  Salt the yellow or brown solution turns pale green. In the reaction, the ion 3+ salt is reduced to ion 2+ salts.

Fe3+ + e                       Fe2+

Yellow                                    green

Brown

 

In the reaction, sulphurdioxide behaves as a reducing agent and iron3+ as an oxidizing agent. In the presence of water sulphur dioxide behaves as a bleaching agent.

The dye is reduced to a colorless compound.

SO4 [g] + 2H4O [L] + dye                      H2SO4 + [dye + hydrogen]

Colourless

Materials bleached with sulphur dioxide finally regain their original color after a long exposure to air. This explains old newspapers turn yellow.

Sulphurdioxide as a powerful oxidizing agent

  • In the presence of water, sulphur dioxide reacts with hydrogen sulphide to form a yellow deposit of sulphur.

SO2 (g)  2H2S (g)   3S(s)  2H2O (l)

O.A                 R.A                 yellow

Deposit

 

Hydrogen sulphur is a stronger reducing – agent compared to sulphurdioxide.

 

  • Burning magnesium continues to burn is sulphur dioxide for some time, forming a white residue of magnesium and yellow deposit of sulphur.

2Mg(s)  SO2 (g)  2MgO(s)  S(s)

Burning magnesium liberates a lot of heat which decomposes sulphur dioxide to give sulphur and oxygen gas.

Oxygen liberated supports continued burning of the metal.

SO2 (g)  S(s)  O2………………. [i]

            2Mg + O2        2MgO …………….. [ii]

 Reaction with concentrated nitric acid

Nitric acid reacts with sulphur dioxide to form a brown nitrogen gas sulphur dioxide acts as a reducing agent while nitric acid acts as the oxidizing agent.

SO2 (g)  2HNO3 (conc)  2NO2 (g)   H2SO4 (aq)

Uses of sulphur dioxide

  1. Used on a large scale for manufacture of sulphuric acid
  2. Used as bleaching agent
  3. Used as preservative in fruit juices where it reacts with oxygen preventing oxidation of the liquid.
  4. Used in fumigation of houses and clothes:-

This is because it is poisonous and kills micro organisms

 Sulphur trioxide, SO3 (SULPHUR (II) oxide)

sulphur trioxide

It is prepared by passing a mixture of sulphur dioxide and oxygen over vanadium (V) oxide catalyst or platinized asbestos at 4500C trioxide seen as dense white fumes which are solidified in a freezing mixture of ice and salt.

SO2 [g] + O2 [g]  2SO3 [g]

 

SULPHRIC ACID H2SO4

It is a di-basic metal

H2SO4      water     2H+  SO42-

 Manufacture of sulphric acid by contact process 

Sulphur of sulphric acid by contact process

S(s)  O2 (g)  SO2 (g)

Sulphur is burned in air to gieve sulphur dioxide gas

The mixture of sulphur dioxide and excess air is purified and then dried. Purification is important to remove impurities which would “poison” the catalyst make it less effective).

The mixture is then passed over platinum or vanadium (V) oxide catalyst at moderate temperature (4500C – 5000C) and a pressure of atmosphere.

Under the above condition, sulphur dioxide is oxidized to sulphur trioxide.

2S(s)  O2 (g)  2SO2 (g)

Sulphur trioxide is not dissolved in water because it reacts with it to form a mist of acid which is difficult to condense.

It is dissolved in concentrated sulphuric acid (98%) to form a liquid called olenm.

SO3 (g)  H2SO4 conc  H2S2O7 (l)

Oleum is diluted with water to get concentrated sulphuric acid.

H2S2O7 (l)  H2O  2H2SO4 conc

With the aid of equations describe the contact process for manufacture of sulphuric acid.

 Uses of sulphric acid

  1. Manufacture of fertilizers like ammonium sulphate.
  2. It is used in car batteries as an electrolyte
  3. Used in manufacture of points.
  4. For drying some gases in laboratory. (Concentrated sulphuric acid cannot be used to dry alkaline gas like ammonia, NH3.

And reducing gas like hydrogen shulphide)

  1. It is used to clean metal surfaced before plting takes place.

Properties of sulphuric acid

  • It is a colourless liquid and corrosive

 Diluting concentrated sulphuric acid

notes

Reaction with carbonates and hydrogen carbonates

Dilute sulphric acid reacts with hydrogen carbonate and carbonates forming salt, water and liberating ocarbondioxide gas.

Examples

            Na2CO3[s] + H2SO4 [aq]                                NaSO4 + CO2 + H2O

Carbonate + acid                                 salt + carbodioxide + water

 

Ionic equation

CO3 2- + 2H+                           CO2 [g] + H2O [L]

2Na HCO3[s] + H2SO4 [aq]                                              Na2SO4[s] + 2CO2 [g] + 2H2O [L]

HCO3 + H+                                      CO2 + H2O

Conditions under which sulphric acid behaves as an acid

  1. Should be dilute
  2. Non heating (room temperature)

Concentrated sulphric acid as a dehydrating agent

Concentrated sulphric acid has a high affinity for water and it removes it form substance including air ire its hygroscopic can be used as a drying agent for some gases.

Experiment to prove that concentrated sulphirc acid is hygroscopic

Put concentrated slphiric acid in a beaker and mark the level of the acid.

Leave it in open air for about 5 days

What do you observe?

Observation

The level of the acid rises (vlume of the acid increases)

Explanation

Concentrated sulphiric acid is hygroscopic. It absorbed water vapor from the air causing a rise in the acid level.

The resulting mixture is a bigger volume of more dilute sulphiric acid.

Dehydrating reactions of sulpric acid

– Add concentrated acid to white sugar crystals in a glass beaker- record your observations.

Observation

White sugar crystals turn yellow, then brown and a black spongy mass is formed almost filling the beaker.

Steam is given off

The mixture becomes very hot

Explain your observation

Explanation

Concentrated sulphric acid dehydrates sugar leaving a black of spongy mass of carbon.

C12 H22O11                          12C[s] + 11h2O[b]

Sucrose                                    black

[Sugar crystals]                       mole

 

  1. reaction with skin is also dehydration reaction
  2. It dehydrates blue copper (II) sulphate crystal dirty white anhydrous copper (II) sulphate.

CuSO4 – 5H2O                        conc.H2SO4                                   CuSO4[s] + 5H2O

Blue crystals                                                                                        anhydrous

[Dirty white solid]

Reaction with ethanol

It dehydrates ethanol to give a colourless gas ethane, which declourises bromine water and burns with a yellow flame.

C2H5-OH                                             C2H4 + H2O

It dehydrates alkaline acid crystals to give carbondioxide gas and a colourless [gas which burns with a quite lame [Carbon monoxide].

conc H2SO4

H2C2O4[S]                                                       C2H4 + H2O

 

  1. reaction with methanoic acid (formic acid  )

Concentrated sulphiric acid dehydrates methanoic acid to give carbon monoxide gas.

conc H2SO4

HCOOH [L]                                                   CO[g] + H2O

Heat

 

Carb monoxide is a colourless gas which turns with a blue flame

The gas is very poisonous

The reaction recquires heating

Concentrated sulphric acid as an oxidizing agent

In the presence of heat, concentrated sulphiric acid behaves an oxidizing agent and reduces to sulphur dioxide gas.

It oxides by accepting electros or by supplying oxygen.

Reaction with metals

It oxidizes copper, zinc and iron producing the metallic sulphate, water and sulphur dioxide gas with copper l metal.

With copper

Cu[s] + H2SO4 [conc]                               CuSO4 + SO2 [g] + H2O [L]

With sulphur

S + H2SO4 [conc]                                H2O [L] SO2 [g]

With carbon

C[s] + 2H2SO4 [conc]                       SO4 + CO2 + 2

It oxidizes hydrogen sulphide to sulphur forming a yellow deposit.

H2S[g] + H2SO4 [conc]                                 S[s] + 2H2O[L] + SO2 [g]

sulphur trioxidei

THIS VIDEO IS ABOUT ELECTROLYSIS

ASSIGNMENT : CHE3: ELECTROLYSIS Assignment MARKS : 10  DURATION : 1 week, 3 days

 

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