Where the products form

During electrolysis, each ion in the electrolyte is attracted to the electrode with the opposite charge.
Positive ions move towards the negative cathode, and negative ions move towards the positive anode.
Metal ions and hydrogen ions are positive, so metals or hydrogen are always formed at the cathode.
Ions of non-metals, such as chloride, bromide and oxide ions, are negative, so non-metals other than hydrogen are always formed at the anode.
When the ions reach the electrodes they are changed into neutral atoms or molecules, and these are the products that you see.
Using inert electrodes made of platinum or graphite means the electrodes take no part in the reaction, so every product comes from the electrolyte.
In a molten compound, the only ions present are the ions of that compound.
In an aqueous solution, water also provides a small number of hydrogen ions, $H+\text{H}^{+}$ , and hydroxide ions, $OH−\text{OH}^{-}$ , so hydrogen or oxygen can be formed instead of the elements in the dissolved compound.

Molten lead(II) bromide

Lead(II) bromide, $PbBr2\text{PbBr}_2$ , is a white ionic solid made of lead(II) ions, $Pb2+\text{Pb}^{2+}$ , and bromide ions, $Br−\text{Br}^{-}$ .
The solid is placed in a crucible with two graphite electrodes and heated strongly until it melts at about $∘C373\ ^{\circ}\text{C}$ .
A lamp in the circuit lights only once the solid has melted, because only then are the ions free to move.
The positive $Pb2+\text{Pb}^{2+}$ ions move to the cathode, where lead is formed.
The lead is produced as a silvery-grey liquid, because the electrolyte is hotter than the melting point of lead, $∘C327\ ^{\circ}\text{C}$ , and it collects as a shiny bead beneath the cathode.
The negative $Br−\text{Br}^{-}$ ions move to the anode, where bromine is formed.
The bromine escapes from the anode as red-brown fumes, because the temperature is far above the boiling point of bromine, $∘C59\ ^{\circ}\text{C}$ .
Overall, the compound is split into its two elements: $PbBr2(l)→Pb(l)+Br2(g)\text{PbBr}_2(\text{l}) \rightarrow \text{Pb}(\text{l}) + \text{Br}_2(\text{g})$ .
This electrolysis is carried out in a fume cupboard, because bromine vapour is toxic and corrosive and lead compounds are also toxic.

Concentrated aqueous sodium chloride

Concentrated aqueous sodium chloride, also called brine, contains sodium ions, $Na+\text{Na}^{+}$ , and chloride ions, $Cl−\text{Cl}^{-}$ , together with hydrogen ions and hydroxide ions from the water.
At the cathode, hydrogen is formed rather than sodium, because sodium is far too reactive to exist in contact with water.
You see bubbles of a colourless gas at the cathode, and the gas pops with a lighted splint, which shows that it is hydrogen.
At the anode, chlorine is formed from the chloride ions.
You see bubbles of a pale yellow-green gas at the anode, and the gas bleaches damp litmus paper, which shows that it is chlorine.
Chlorine is slightly soluble in water, so at first less gas collects at the anode than at the cathode.
The sodium ions and hydroxide ions left behind form sodium hydroxide, so the solution around the cathode becomes alkaline.
Industry uses this process to make three useful products from salt: hydrogen, chlorine and sodium hydroxide.

Dilute sulfuric acid

Dilute sulfuric acid, $H2SO4(aq)\text{H}_2\text{SO}_4(\text{aq})$ , contains hydrogen ions, $H+\text{H}^{+}$ , and sulfate ions, $SO42−\text{SO}_4^{2-}$ , together with a small number of hydroxide ions from the water.
At the cathode, hydrogen is formed, seen as bubbles of a colourless gas that pops with a lighted splint.
At the anode, oxygen is formed, seen as bubbles of a colourless gas that relights a glowing splint.
The sulfate ions are not changed, and the oxygen comes from the water instead.
The volume of hydrogen collected is twice the volume of oxygen, so the two gases form in a volume ratio of $2 : 1$ .
This ratio arises because the hydrogen and oxygen both come from water, which contains two hydrogen atoms for every oxygen atom: $2H2O(l)→2H2(g)+O2(g)2\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{H}_2(\text{g}) + \text{O}_2(\text{g})$ .
As water is used up, the remaining acid slowly becomes more concentrated.
Pure water contains too few ions to conduct well, so the acid is added to provide plenty of mobile ions.

Practical

Electrolysis of aqueous solutions

Aim: to identify the products at each electrode when concentrated aqueous sodium chloride and dilute sulfuric acid are electrolysed using inert electrodes.
Apparatus: a small electrolysis cell with two graphite electrodes fitted through its base, two small test tubes, a low-voltage d.c. power pack, connecting leads, wooden splints, a Bunsen burner, blue litmus paper, universal indicator, concentrated sodium chloride solution, dilute sulfuric acid and eye protection.
Method, sodium chloride:
- Pour concentrated sodium chloride solution into the cell until the electrodes are covered by about $cm3\ \text{cm}$ of liquid, then add a few drops of universal indicator.
- Fill two small test tubes to the brim with the same solution, close each with a thumb, turn it upside down and lower it over one electrode without letting any air in.
- Connect the electrodes to the power pack set at about $V6\ \text{V}$ d.c. and note which electrode is joined to the negative terminal, as this is the cathode.
- Switch on and watch both electrodes, recording the colour of any gas and any change in the colour of the indicator.
- Switch off when one tube is nearly full of gas.
- Lift the cathode tube out still upside down and hold a lighted splint at its mouth.
- Hold a piece of damp blue litmus paper at the mouth of the anode tube.
Method, sulfuric acid:
- Empty and rinse the cell, then repeat the same steps with dilute sulfuric acid, leaving out the universal indicator.
- Compare the volumes of gas in the two tubes before switching off.
- Test the anode gas with a glowing splint instead of litmus paper.
Results:
- With sodium chloride, the colourless cathode gas pops with a lighted splint and the indicator near the cathode turns blue-purple, showing hydrogen and an alkaline solution.
- With sodium chloride, the pale yellow-green anode gas bleaches the damp litmus paper and the indicator near the anode loses its colour, showing chlorine.
- With sulfuric acid, both gases are colourless, the cathode tube holds about twice as much gas, the cathode gas pops and the anode gas relights a glowing splint.
Watch out:
- Air left in the tubes dilutes the gases and can make the splint tests fail.
- Hydrogen is less dense than air, so test it with the tube held mouth downwards or it escapes before the splint arrives.
- Chlorine dissolves in the solution, so the anode tube fills more slowly than the cathode tube at first.
Safety: wear eye protection, keep the volume of chlorine small and work in a well-ventilated room because chlorine is toxic, and rinse off any sulfuric acid that touches your skin.

Predicting products of molten compounds

Definition

Binary compound

A compound made of only two different elements.

A binary compound contains just two elements, such as sodium chloride, $NaCl\text{NaCl}$ , or zinc bromide, $ZnBr2\text{ZnBr}_2$ .
A molten binary ionic compound contains only two kinds of ion: one metal ion and one non-metal ion.
The metal is therefore always formed at the cathode.
The non-metal is always formed at the anode.
The halogens are released as molecules made of two atoms, so chlorine is $Cl2\text{Cl}_2$ , bromine is $Br2\text{Br}_2$ and iodine is $I2\text{I}_2$ .
Always name the anode product as the element, so the product from a chloride is chlorine and never chloride.
Molten sodium chloride gives sodium at the cathode and chlorine at the anode.
Molten potassium iodide gives potassium at the cathode and iodine at the anode.
These predictions apply only to the molten state, because dissolving the same compound in water adds hydrogen ions and hydroxide ions that can change the products.

Example

Molten zinc bromide

Zinc bromide, $ZnBr2\text{ZnBr}_2$ , is a binary ionic compound made of zinc ions, $Zn2+\text{Zn}^{2+}$ , and bromide ions, $Br−\text{Br}^{-}$ .
When it is molten there is no water present, so these are the only ions in the electrolyte.
The positive zinc ions are attracted to the cathode, so zinc forms at the cathode.
The negative bromide ions are attracted to the anode, so bromine forms at the anode and is seen as red-brown fumes.

$ZnBr2→Zn+Br2\text{ZnBr}_2 \rightarrow \text{Zn} + \text{Br}_2$

Example

Molten and aqueous sodium chloride

Molten sodium chloride contains only sodium ions and chloride ions.
It gives sodium at the cathode and chlorine at the anode.
Concentrated aqueous sodium chloride also contains hydrogen ions and hydroxide ions from water.
It gives hydrogen at the cathode and chlorine at the anode, while the sodium ions stay in solution.
The anode product is the same in both cases, but the cathode product changes because water is present.

Common Mistake

Do not name the anode product as bromide or chloride, because the ions are turned into the elements bromine and chlorine.
Do not give sodium as a product of aqueous sodium chloride, because hydrogen forms at the cathode instead.
Do not mix up the splint tests: hydrogen pops with a lighted splint, while oxygen relights a glowing splint.
Do not give the volume ratio for dilute sulfuric acid the wrong way round, because there is twice as much hydrogen as oxygen.

Exam technique

Describing products and observations

An observation is something you can see, so describe colours and states, such as bubbles of colourless gas or red-brown fumes, rather than just naming the product.
When a question asks for products and observations, give both: name the substance, then describe what is seen.
Quote gas tests in full: hydrogen pops with a lighted splint, oxygen relights a glowing splint and chlorine bleaches damp litmus paper.
Check whether the electrolyte is molten or aqueous before predicting anything, because that decides whether water can supply hydrogen or oxygen.

Self review

Which products form at each electrode when molten lead(II) bromide is electrolysed?
What would you see at the anode during the electrolysis of concentrated aqueous sodium chloride?
Why is the volume of hydrogen from dilute sulfuric acid twice the volume of oxygen?
Which type of element is always formed at the cathode?
Which products form at each electrode when molten potassium iodide is electrolysed?

Where the products form

During electrolysis, each ion in the electrolyte is attracted to the electrode with the opposite charge.
Positive ions move towards the negative cathode, and negative ions move towards the positive anode.
Metal ions and hydrogen ions are positive, so metals or hydrogen are always formed at the cathode.
Ions of non-metals, such as chloride, bromide and oxide ions, are negative, so non-metals other than hydrogen are always formed at the anode.
When the ions reach the electrodes they are changed into neutral atoms or molecules, and these are the products that you see.
Using inert electrodes made of platinum or graphite means the electrodes take no part in the reaction, so every product comes from the electrolyte.
In a molten compound, the only ions present are the ions of that compound.
In an aqueous solution, water also provides a small number of hydrogen ions, $H+\text{H}^{+}$ , and hydroxide ions, $OH−\text{OH}^{-}$ , so hydrogen or oxygen can be formed instead of the elements in the dissolved compound.

Molten lead(II) bromide

Lead(II) bromide, $PbBr2\text{PbBr}_2$ , is a white ionic solid made of lead(II) ions, $Pb2+\text{Pb}^{2+}$ , and bromide ions, $Br−\text{Br}^{-}$ .
The solid is placed in a crucible with two graphite electrodes and heated strongly until it melts at about $∘C373\ ^{\circ}\text{C}$ .
A lamp in the circuit lights only once the solid has melted, because only then are the ions free to move.
The positive $Pb2+\text{Pb}^{2+}$ ions move to the cathode, where lead is formed.
The lead is produced as a silvery-grey liquid, because the electrolyte is hotter than the melting point of lead, $∘C327\ ^{\circ}\text{C}$ , and it collects as a shiny bead beneath the cathode.
The negative $Br−\text{Br}^{-}$ ions move to the anode, where bromine is formed.
The bromine escapes from the anode as red-brown fumes, because the temperature is far above the boiling point of bromine, $∘C59\ ^{\circ}\text{C}$ .
Overall, the compound is split into its two elements: $PbBr2(l)→Pb(l)+Br2(g)\text{PbBr}_2(\text{l}) \rightarrow \text{Pb}(\text{l}) + \text{Br}_2(\text{g})$ .
This electrolysis is carried out in a fume cupboard, because bromine vapour is toxic and corrosive and lead compounds are also toxic.

Concentrated aqueous sodium chloride

Concentrated aqueous sodium chloride, also called brine, contains sodium ions, $Na+\text{Na}^{+}$ , and chloride ions, $Cl−\text{Cl}^{-}$ , together with hydrogen ions and hydroxide ions from the water.
At the cathode, hydrogen is formed rather than sodium, because sodium is far too reactive to exist in contact with water.
You see bubbles of a colourless gas at the cathode, and the gas pops with a lighted splint, which shows that it is hydrogen.
At the anode, chlorine is formed from the chloride ions.
You see bubbles of a pale yellow-green gas at the anode, and the gas bleaches damp litmus paper, which shows that it is chlorine.
Chlorine is slightly soluble in water, so at first less gas collects at the anode than at the cathode.
The sodium ions and hydroxide ions left behind form sodium hydroxide, so the solution around the cathode becomes alkaline.
Industry uses this process to make three useful products from salt: hydrogen, chlorine and sodium hydroxide.

Dilute sulfuric acid

Dilute sulfuric acid, $H2SO4(aq)\text{H}_2\text{SO}_4(\text{aq})$ , contains hydrogen ions, $H+\text{H}^{+}$ , and sulfate ions, $SO42−\text{SO}_4^{2-}$ , together with a small number of hydroxide ions from the water.
At the cathode, hydrogen is formed, seen as bubbles of a colourless gas that pops with a lighted splint.
At the anode, oxygen is formed, seen as bubbles of a colourless gas that relights a glowing splint.
The sulfate ions are not changed, and the oxygen comes from the water instead.
The volume of hydrogen collected is twice the volume of oxygen, so the two gases form in a volume ratio of $2 : 1$ .
This ratio arises because the hydrogen and oxygen both come from water, which contains two hydrogen atoms for every oxygen atom: $2H2O(l)→2H2(g)+O2(g)2\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{H}_2(\text{g}) + \text{O}_2(\text{g})$ .
As water is used up, the remaining acid slowly becomes more concentrated.
Pure water contains too few ions to conduct well, so the acid is added to provide plenty of mobile ions.

Practical

Electrolysis of aqueous solutions

Aim: to identify the products at each electrode when concentrated aqueous sodium chloride and dilute sulfuric acid are electrolysed using inert electrodes.
Apparatus: a small electrolysis cell with two graphite electrodes fitted through its base, two small test tubes, a low-voltage d.c. power pack, connecting leads, wooden splints, a Bunsen burner, blue litmus paper, universal indicator, concentrated sodium chloride solution, dilute sulfuric acid and eye protection.
Method, sodium chloride:
- Pour concentrated sodium chloride solution into the cell until the electrodes are covered by about $cm3\ \text{cm}$ of liquid, then add a few drops of universal indicator.
- Fill two small test tubes to the brim with the same solution, close each with a thumb, turn it upside down and lower it over one electrode without letting any air in.
- Connect the electrodes to the power pack set at about $V6\ \text{V}$ d.c. and note which electrode is joined to the negative terminal, as this is the cathode.
- Switch on and watch both electrodes, recording the colour of any gas and any change in the colour of the indicator.
- Switch off when one tube is nearly full of gas.
- Lift the cathode tube out still upside down and hold a lighted splint at its mouth.
- Hold a piece of damp blue litmus paper at the mouth of the anode tube.
Method, sulfuric acid:
- Empty and rinse the cell, then repeat the same steps with dilute sulfuric acid, leaving out the universal indicator.
- Compare the volumes of gas in the two tubes before switching off.
- Test the anode gas with a glowing splint instead of litmus paper.
Results:
- With sodium chloride, the colourless cathode gas pops with a lighted splint and the indicator near the cathode turns blue-purple, showing hydrogen and an alkaline solution.
- With sodium chloride, the pale yellow-green anode gas bleaches the damp litmus paper and the indicator near the anode loses its colour, showing chlorine.
- With sulfuric acid, both gases are colourless, the cathode tube holds about twice as much gas, the cathode gas pops and the anode gas relights a glowing splint.
Watch out:
- Air left in the tubes dilutes the gases and can make the splint tests fail.
- Hydrogen is less dense than air, so test it with the tube held mouth downwards or it escapes before the splint arrives.
- Chlorine dissolves in the solution, so the anode tube fills more slowly than the cathode tube at first.
Safety: wear eye protection, keep the volume of chlorine small and work in a well-ventilated room because chlorine is toxic, and rinse off any sulfuric acid that touches your skin.

Predicting products of molten compounds

Definition

Binary compound

A compound made of only two different elements.

A binary compound contains just two elements, such as sodium chloride, $NaCl\text{NaCl}$ , or zinc bromide, $ZnBr2\text{ZnBr}_2$ .
A molten binary ionic compound contains only two kinds of ion: one metal ion and one non-metal ion.
The metal is therefore always formed at the cathode.
The non-metal is always formed at the anode.
The halogens are released as molecules made of two atoms, so chlorine is $Cl2\text{Cl}_2$ , bromine is $Br2\text{Br}_2$ and iodine is $I2\text{I}_2$ .
Always name the anode product as the element, so the product from a chloride is chlorine and never chloride.
Molten sodium chloride gives sodium at the cathode and chlorine at the anode.
Molten potassium iodide gives potassium at the cathode and iodine at the anode.
These predictions apply only to the molten state, because dissolving the same compound in water adds hydrogen ions and hydroxide ions that can change the products.

Example

Molten zinc bromide

Zinc bromide, $ZnBr2\text{ZnBr}_2$ , is a binary ionic compound made of zinc ions, $Zn2+\text{Zn}^{2+}$ , and bromide ions, $Br−\text{Br}^{-}$ .
When it is molten there is no water present, so these are the only ions in the electrolyte.
The positive zinc ions are attracted to the cathode, so zinc forms at the cathode.
The negative bromide ions are attracted to the anode, so bromine forms at the anode and is seen as red-brown fumes.

$ZnBr2→Zn+Br2\text{ZnBr}_2 \rightarrow \text{Zn} + \text{Br}_2$

Example

Molten and aqueous sodium chloride

Molten sodium chloride contains only sodium ions and chloride ions.
It gives sodium at the cathode and chlorine at the anode.
Concentrated aqueous sodium chloride also contains hydrogen ions and hydroxide ions from water.
It gives hydrogen at the cathode and chlorine at the anode, while the sodium ions stay in solution.
The anode product is the same in both cases, but the cathode product changes because water is present.

Common Mistake

Do not name the anode product as bromide or chloride, because the ions are turned into the elements bromine and chlorine.
Do not give sodium as a product of aqueous sodium chloride, because hydrogen forms at the cathode instead.
Do not mix up the splint tests: hydrogen pops with a lighted splint, while oxygen relights a glowing splint.
Do not give the volume ratio for dilute sulfuric acid the wrong way round, because there is twice as much hydrogen as oxygen.

Exam technique

Describing products and observations

An observation is something you can see, so describe colours and states, such as bubbles of colourless gas or red-brown fumes, rather than just naming the product.
When a question asks for products and observations, give both: name the substance, then describe what is seen.
Quote gas tests in full: hydrogen pops with a lighted splint, oxygen relights a glowing splint and chlorine bleaches damp litmus paper.
Check whether the electrolyte is molten or aqueous before predicting anything, because that decides whether water can supply hydrogen or oxygen.

Self review

Which products form at each electrode when molten lead(II) bromide is electrolysed?
What would you see at the anode during the electrolysis of concentrated aqueous sodium chloride?
Why is the volume of hydrogen from dilute sulfuric acid twice the volume of oxygen?
Which type of element is always formed at the cathode?
Which products form at each electrode when molten potassium iodide is electrolysed?

4.1.2 Products and observations in electrolysis

Where the products form

Molten lead(II) bromide

Concentrated aqueous sodium chloride

Dilute sulfuric acid

Electrolysis of aqueous solutions

Predicting products of molten compounds

Molten zinc bromide

Molten and aqueous sodium chloride

Describing products and observations

Review 4.1.2 Products and observations in electrolysis by teaching Genie

4.1.2 Products and observations in electrolysis

Where the products form

Molten lead(II) bromide

Concentrated aqueous sodium chloride

Dilute sulfuric acid

Electrolysis of aqueous solutions

Predicting products of molten compounds

Molten zinc bromide

Molten and aqueous sodium chloride

Describing products and observations

Review 4.1.2 Products and observations in electrolysis by teaching Genie

4.1.2 Products and observations in electrolysis

Where the products form

Molten lead(II) bromide

Concentrated aqueous sodium chloride

Dilute sulfuric acid

Electrolysis of aqueous solutions

Predicting products of molten compounds

Molten zinc bromide

Molten and aqueous sodium chloride

Describing products and observations

Review 4.1.2 Products and observations in electrolysis by teaching Genie

4.1.2 Products and observations in electrolysis Revision Guide

4.1.2 Products and observations in electrolysis

Where the products form

Molten lead(II) bromide

Concentrated aqueous sodium chloride

Dilute sulfuric acid

Electrolysis of aqueous solutions

Predicting products of molten compounds

Molten zinc bromide

Molten and aqueous sodium chloride

Describing products and observations

Review 4.1.2 Products and observations in electrolysis by teaching Genie

4.1.2 Products and observations in electrolysis Revision Guide