Electrolysis
2. Electrolytes
Learning outcomes
- I can define an electrolyte.
- I can explain why electrolytes conduct electricity.
- I can identify examples of electrolytes and non-electrolytes.
- I can describe the role of ions in electrolysis.
- I can relate electrolyte properties to practical applications.
What Is an Electrolyte?
An electrolyte is a substance that contains or produces mobile ions and can therefore conduct electricity when molten or dissolved in a suitable solvent such as water.
The key idea is:
electrical conduction requires charged particles that can move.
In metals, the moving charged particles are:
electrons.
In electrolytes, the moving charged particles are:
ions.
Electrolytes are extremely important in:
- electrolysis
- batteries
- fuel cells
- industrial chemistry
- biological systems
What Are Ions?
An ion is an atom or group of atoms with an electrical charge.
There are two main types.
Cations have a positive charge.
Examples:
- Na⁺
- K⁺
- Ca²⁺
- Cu²⁺
- H⁺
Anions have a negative charge.
Examples:
- Cl⁻
- OH⁻
- NO₃⁻
- SO₄²⁻
These charged particles allow electrolytes to carry:
electric current.
Why Do Electrolytes Conduct Electricity?
For electricity to flow through a material, charged particles must be able to:
move.
When an ionic substance is dissolved in water, its ions can often separate and move throughout the solution.
For example:
NaCl → Na⁺ + Cl⁻
The Na⁺ and Cl⁻ ions are now free to move.
When a potential difference is applied, the ions move in response to the:
electric field.
Electrical Conduction in an Electrolyte
Suppose two electrodes are placed in an electrolyte and connected to a power supply.
Positive ions move toward the:
negative electrode.
Negative ions move toward the:
positive electrode.
During electrolysis:
Cations (+) → cathode (−)
Anions (−) → anode (+)
This movement of charged ions allows current to pass through the:
electrolyte.
Electron Flow vs Ion Flow
It is important to distinguish between two types of charge movement.
In the wires:
electrons move.
In the electrolyte:
ions move.
Therefore, in an electrolysis circuit:
Wires and electrodes → electron movement
Electrolyte → ion movement
Both are necessary for the complete electrical circuit.
Ionic Compounds
Many electrolytes are:
ionic compounds.
Ionic compounds consist of positively and negatively charged ions.
Examples include:
- sodium chloride, NaCl
- potassium chloride, KCl
- copper(II) sulfate, CuSO₄
- sodium nitrate, NaNO₃
- calcium chloride, CaCl₂
However, whether an ionic compound conducts electricity depends on whether its ions are:
free to move.
Solid Ionic Compounds
Consider solid sodium chloride.
It contains:
Na⁺ and Cl⁻ ions.
However, these ions are held in fixed positions in a:
giant ionic lattice.
They can vibrate but cannot move freely through the solid.
Therefore:
solid NaCl does not conduct electricity through ionic movement.
Molten Ionic Compounds
When an ionic compound melts, its lattice breaks apart.
The ions become:
mobile.
For example, molten sodium chloride contains mobile:
Na⁺ and Cl⁻ ions.
These ions can carry charge.
Therefore:
molten NaCl conducts electricity.
Ionic Compounds Dissolved in Water
Many ionic compounds also separate into ions when dissolved in water.
For example:
NaCl(s) → Na⁺(aq) + Cl⁻(aq)
The symbol:
(aq)
means:
aqueous — dissolved in water.
Because the ions can move through the solution, aqueous sodium chloride conducts:
electricity.
Three States of Sodium Chloride
Consider sodium chloride in three different situations.
Solid NaCl
Ions present: Yes
Ions mobile: No
Conducts through ions: No
Molten NaCl
Ions present: Yes
Ions mobile: Yes
Conducts: Yes
NaCl dissolved in water
Ions present: Yes
Ions mobile: Yes
Conducts: Yes
The important question is not simply:
"Are ions present?"
Instead ask:
"Are the ions free to move?"
Electrolytes in Water
Some substances produce ions when they dissolve in water.
These solutions can conduct electricity.
Examples include solutions of:
- salts
- acids
- bases
For example:
hydrochloric acid
contains ions including H⁺ and Cl⁻ in aqueous solution.
sodium hydroxide
contains Na⁺ and OH⁻ ions.
These mobile ions make the solutions:
electrolytes.
Acids as Electrolytes
Acids produce ions in aqueous solution.
For example, hydrochloric acid can be represented in simplified form as:
HCl → H⁺ + Cl⁻
Because charged particles are present and mobile, hydrochloric acid solution:
conducts electricity.
Other acids, such as sulfuric acid and nitric acid, also form conducting solutions.
Bases and Alkalis as Electrolytes
Soluble bases called alkalis also produce ions in water.
For example:
NaOH → Na⁺ + OH⁻
The Na⁺ and OH⁻ ions can move through the solution.
Therefore sodium hydroxide solution is:
an electrolyte.
What Is a Non-Electrolyte?
A non-electrolyte is a substance that does not produce enough mobile ions to conduct electricity through a solution.
Many molecular substances dissolve as:
neutral molecules
rather than ions.
Examples include:
- sugar
- ethanol
- many other molecular compounds
Sugar Solution
Sugar dissolves readily in water.
However, dissolving does not automatically mean:
forming ions.
Sugar molecules separate from one another and become surrounded by water molecules, but they remain:
neutral molecules.
Therefore a sugar solution is a:
non-electrolyte.
Dissolving vs Dissociating
These two ideas should not be confused.
Dissolving means particles spread throughout a solvent.
Dissociating means particles separate into ions.
Sugar:
dissolves but does not significantly form ions.
Sodium chloride:
dissolves and dissociates into ions.
Therefore:
NaCl solution conducts electricity
while:
sugar solution conducts very poorly.
Pure Water
Pure water contains a very small concentration of ions because a tiny proportion of water molecules ionize.
Therefore extremely pure water is:
a very poor electrical conductor.
However, ordinary water usually contains dissolved ions from:
- minerals
- salts
- gases
- impurities
These ions increase its:
electrical conductivity.
Why Salt Water Conducts Better
Adding sodium chloride to water increases the concentration of:
mobile ions.
More charge carriers are available.
As a result, the electrical conductivity generally:
increases.
This explains why salt water conducts electricity much better than very pure water.
Strong and Weak Electrolytes
Not all electrolytes conduct electricity equally well.
Electrolytes can sometimes be classified as:
strong electrolytes
and:
weak electrolytes.
A strong electrolyte produces a high proportion of ions when dissolved.
A weak electrolyte produces a smaller proportion of ions.
Strong Electrolytes
Strong electrolytes include many:
- soluble ionic salts
- strong acids
- strong bases
Examples include:
- NaCl
- HCl
- NaOH
These substances produce many mobile ions in solution.
As a result, their solutions can conduct electricity:
strongly.
Weak Electrolytes
Weak electrolytes form fewer ions in solution.
For example, a weak acid ionizes only partially.
This means the solution contains a mixture of:
- neutral molecules
- positive ions
- negative ions
Because fewer mobile ions are available, the solution generally has:
lower conductivity than a comparable strong-electrolyte solution.
Concentration and Conductivity
The concentration of ions can affect electrical conductivity.
Consider two sodium chloride solutions.
Solution A:
low NaCl concentration
Solution B:
higher NaCl concentration
Under otherwise similar conditions, Solution B generally contains more mobile ions per unit volume.
Therefore it will usually have:
greater electrical conductivity.
Conductivity Depends on More Than Concentration
Conductivity can also depend on:
- type of ions
- ion charge
- ion mobility
- temperature
- solvent
- concentration
Therefore two solutions with the same concentration do not necessarily have exactly the same:
conductivity.
Temperature and Electrolytes
Temperature can influence the movement of ions.
In many aqueous electrolyte solutions, increasing temperature allows ions to move more easily.
As a result, conductivity often:
increases with temperature.
This is one reason conductivity measurements should be compared under similar:
conditions.
Testing for Electrolytes
A simple conductivity experiment can be used to compare different solutions.
The apparatus might contain:
- power source
- electrodes
- solution
- conductivity sensor or meter
If the solution contains enough mobile ions, an:
electric current
can flow.
Example Conductivity Test
Suppose students test:
- distilled water
- salt solution
- sugar solution
- hydrochloric acid
- sodium hydroxide solution
They would expect significant conductivity from:
salt solution
hydrochloric acid
and:
sodium hydroxide solution.
Sugar solution would show little conductivity because it contains very few:
mobile ions.
The Role of Electrolytes in Electrolysis
Electrolysis requires charge to move through the cell.
Electrons travel through the external circuit.
However, charge must also move through the:
electrolyte.
This is accomplished by:
ions.
Without mobile ions, the electrolysis circuit cannot function normally.
Cations During Electrolysis
A cation has a positive charge.
During electrolysis, cations are attracted toward the:
negative cathode.
At the cathode, they may gain electrons.
For example:
Cu²⁺ + 2e⁻ → Cu
This is:
reduction.
Anions During Electrolysis
An anion has a negative charge.
During electrolysis, anions are attracted toward the:
positive anode.
At the anode, they may lose electrons.
For example:
2Cl⁻ → Cl₂ + 2e⁻
This is:
oxidation.
Electrolytes Make the Circuit Possible
The electrolyte has several important roles.
It:
- contains mobile ions
- allows charge to move between electrodes
- maintains electrical neutrality within regions of the system
- provides ions that may participate in electrode reactions
Without suitable ion movement, current through the electrochemical system would:
stop.
Electrolytes in Galvanic Cells
Electrolytes are also essential in:
A galvanic cell uses a spontaneous redox reaction to produce electricity.
Electrons move through the:
external circuit.
Ions move through:
electrolyte solutions and/or a salt bridge or separator.
This movement maintains charge balance as the reaction proceeds.
Electrolytes in Batteries
All electrochemical batteries require some form of:
electrolyte.
The electrolyte allows ions to move between the electrodes while the battery operates.
Different battery technologies use different electrolytes.
Examples include:
- aqueous electrolytes
- organic liquid electrolytes
- gel electrolytes
- solid electrolytes
Electrolytes in Lithium-Ion Batteries
In a lithium-ion battery, the electrolyte allows:
Li⁺ ions
to move between electrode materials.
Electrons cannot simply travel through the electrolyte from one electrode to the other.
Instead, electrons travel through the:
external circuit.
This separation allows the battery to provide useful electrical energy.
Electrolytes in Fuel Cells
Fuel cells also contain electrolytes.
In a proton exchange membrane fuel cell, the membrane acts as an electrolyte that allows:
H⁺ ions
to move through the cell.
Electrons must travel through the external circuit.
This produces:
electric current.
Electrolytes in Electroplating
Electroplating requires an electrolyte containing ions of the metal being deposited.
For example, copper electroplating may use an electrolyte containing:
Cu²⁺ ions.
At the cathode:
Cu²⁺ + 2e⁻ → Cu
Copper atoms are deposited onto the object's surface.
Electrolytes in Metal Purification
Electrolytes are also important in:
electrorefining.
During copper purification, the electrolyte allows copper ions to move through the solution.
Copper ions are reduced at the cathode:
Cu²⁺ + 2e⁻ → Cu
Pure copper is deposited.
Electrolytes in the Human Body
Electrolytes are not limited to industrial chemistry.
Body fluids contain ions such as:
- Na⁺
- K⁺
- Ca²⁺
- Mg²⁺
- Cl⁻
These ions are involved in important biological processes.
For example, ions contribute to:
- nerve impulses
- muscle contraction
- fluid balance
- cell function
Electrolytes and Nerve Cells
Nerve cells depend on controlled movement of ions across cell membranes.
Important ions include:
Na⁺ and K⁺.
Differences in ion concentrations across membranes contribute to electrical signals called:
nerve impulses.
This is an example of ions playing an important role in biological electrical processes.
Electrolytes and Sports Drinks
Sports drinks often contain dissolved ions such as:
- sodium
- potassium
These ions are referred to as:
electrolytes.
Sweating causes the body to lose both:
water and dissolved ions.
For most everyday activities, normal food and water provide what is needed, while electrolyte-containing drinks can be useful in some situations involving prolonged or substantial fluid loss.
Electrolytes in Industry
Electrolytes are used in many industrial processes.
Applications include:
- electroplating
- metal extraction
- metal purification
- batteries
- fuel cells
- chemical manufacturing
- electrolysis of water
The properties of the electrolyte influence the:
performance of the electrochemical system.
Choosing an Electrolyte
A useful electrolyte may need to:
- contain suitable ions
- conduct ions effectively
- remain stable under operating conditions
- work at the required temperature
- be compatible with the electrodes
- avoid unwanted reactions
- meet safety requirements
Different applications therefore require:
different electrolytes.
Aqueous Electrolytes
An aqueous electrolyte contains ions dissolved in:
water.
Advantages can include:
- good ionic conductivity
- relatively simple preparation
- low cost in many applications
However, water can also participate in electrochemical reactions.
This can limit the voltage range available in some:
electrochemical systems.
Non-Aqueous Electrolytes
Some batteries use electrolytes based on solvents other than water.
Lithium-ion batteries commonly use:
non-aqueous electrolytes.
These allow the battery to operate over voltage ranges that would be difficult with ordinary aqueous electrolytes.
However, some organic electrolytes can be:
flammable.
This creates additional safety considerations.
Solid Electrolytes
An electrolyte does not always have to be a liquid.
Some materials can conduct ions while remaining:
solid.
These are called:
solid electrolytes.
They are important in research and development of:
solid-state batteries.
Why Solid Electrolytes Are Interesting
Solid electrolytes may offer potential advantages such as:
- improved safety in some designs
- reduced leakage
- new battery structures
- compatibility with future battery chemistries
However, they also present challenges involving:
- ion conductivity
- manufacturing
- contact between solid materials
- cost
- durability
Electrolyte vs Electrode
These terms are easy to confuse.
An electrolyte:
allows ions to move through part of an electrochemical system.
An electrode:
is an electrically conducting material where oxidation or reduction occurs.
Therefore:
electrolyte → ion transport
electrode → electron-transfer reactions
Both are essential in many electrochemical cells.
Electrolyte vs Electron
The words sound similar but describe completely different things.
An electron is:
a negatively charged subatomic particle.
An electrolyte is:
a material containing mobile ions that conducts through ionic movement.
Electrons travel through external conductors.
Ions travel through the electrolyte.
Worked Example 1
Does solid sodium chloride conduct electricity through ion movement?
No.
Although it contains Na⁺ and Cl⁻ ions, they are held in fixed positions.
The ions are not:
mobile.
Worked Example 2
Does molten sodium chloride conduct electricity?
Yes.
When NaCl melts, the ionic lattice breaks apart.
The:
Na⁺ and Cl⁻ ions
can move and carry charge.
Worked Example 3
Why does salt water conduct electricity?
Salt dissolves and produces:
mobile ions.
For sodium chloride:
NaCl → Na⁺ + Cl⁻
These ions move through the solution and transport:
electric charge.
Worked Example 4
Why does sugar solution conduct very poorly?
Sugar dissolves as:
neutral molecules.
It does not produce a significant concentration of mobile ions.
Therefore it is a:
non-electrolyte.
Worked Example 5
A solution contains Cu²⁺ and Cl⁻ ions.
Which direction will each ion move during electrolysis?
Cu²⁺ → cathode
because it is positive.
Cl⁻ → anode
because it is negative.
Remember:
cations → cathode
anions → anode.
Worked Example 6
What happens to Cu²⁺ at the cathode?
Copper ions gain electrons:
Cu²⁺ + 2e⁻ → Cu
This is:
reduction.
Worked Example 7
A student says:
"All substances that dissolve in water are electrolytes."
Is this correct?
No.
A substance can dissolve without producing ions.
Sugar is a good example.
Therefore:
dissolving does not necessarily mean ionizing or dissociating.
Worked Example 8
Solution A and Solution B contain the same electrolyte.
Solution A contains many more ions per unit volume.
Which would generally conduct better?
Under otherwise similar conditions:
Solution A.
It has more mobile charge carriers available to transport:
electric charge.
Worked Example 9
Why does an electrolysis experiment stop working properly if ions cannot move through the electrolyte?
Charge would begin to build up in different regions.
Without continued ionic movement, the complete electrical circuit cannot be:
maintained.
Worked Example 10
Why does a lithium-ion battery need both an electrolyte and an external circuit?
The electrolyte allows:
Li⁺ ions to move internally.
The external circuit allows:
electrons to move externally.
Both movements are necessary for the electrochemical reaction to continue.
Comparing Electrolytes and Non-Electrolytes
| Substance | Mobile Ions Present? | Conductivity | Classification |
|---|---|---|---|
| Solid NaCl | No mobile ions | Very low | Does not conduct ionically |
| Molten NaCl | Yes | High | Electrolyte |
| NaCl solution | Yes | High | Electrolyte |
| HCl solution | Yes | High | Electrolyte |
| NaOH solution | Yes | High | Electrolyte |
| Sugar solution | Very few from sugar | Very low | Non-electrolyte |
| Very pure water | Very few | Very low | Very weak conductor |
Common Mistake: Electrolytes Contain Free Electrons
Electrolytes conduct mainly because of:
moving ions.
Metal wires conduct mainly because of:
moving electrons.
These are different mechanisms of electrical conduction.
Common Mistake: All Ionic Compounds Conduct Electricity
Solid ionic compounds generally do not conduct through ionic movement because their ions cannot:
move freely.
They usually conduct when:
molten or dissolved in a suitable solvent.
Common Mistake: Anything Dissolved in Water Conducts
A substance must produce sufficient:
mobile charged particles.
Sugar dissolves in water but remains as neutral molecules.
Therefore sugar solution is a:
non-electrolyte.
Common Mistake: Pure Water Is a Good Conductor
Very pure water contains only a very small concentration of ions.
It is therefore a:
poor conductor.
Everyday water usually conducts better because it contains dissolved ionic substances.
Common Mistake: Cations Move to the Positive Electrode
Cations are positive.
Opposite charges attract.
During electrolysis:
cations → negative cathode
and:
anions → positive anode.
Common Mistake: The Electrolyte and Electrode Are the Same Thing
They have different functions.
The:
electrolyte transports ions.
The:
electrodes provide surfaces for electron-transfer reactions.
Check Your Understanding
- Define an electrolyte.
- What type of charged particle moves through an electrolyte?
- What is a cation?
- What is an anion?
- Give three examples of cations.
- Give three examples of anions.
- Why do electrolytes conduct electricity?
- Why does solid sodium chloride not conduct through ion movement?
- Why does molten sodium chloride conduct electricity?
- Why does sodium chloride solution conduct electricity?
- What happens to NaCl when it dissolves in water?
- What does the symbol (aq) mean?
- What is a non-electrolyte?
- Give two examples of non-electrolytes.
- Why is sugar solution a non-electrolyte?
- Explain the difference between dissolving and dissociating.
- Why is very pure water a poor electrical conductor?
- Why does adding salt increase the conductivity of water?
- What is a strong electrolyte?
- What is a weak electrolyte?
- Give an example of a strong electrolyte.
- How can ion concentration affect conductivity?
- Name two other factors that can affect electrolyte conductivity.
- Where do electrons move in an electrolysis circuit?
- Where do ions move?
- Which electrode attracts cations during electrolysis?
- Which electrode attracts anions?
- What reaction occurs at the cathode?
- What reaction occurs at the anode?
- Write the reduction half-equation for Cu²⁺.
- Write an oxidation half-equation for Cl⁻.
- Why is an electrolyte necessary for electrolysis?
- What would happen if ions could not move through the electrolyte?
- Explain the role of electrolytes in galvanic cells.
- Explain the role of an electrolyte in a lithium-ion battery.
- Explain the role of the membrane electrolyte in a hydrogen fuel cell.
- How are electrolytes used in electroplating?
- How are electrolytes used in metal purification?
- Give four ions found in body fluids.
- Why are ions important in nerve cells?
- Why are sodium and potassium often called electrolytes?
- Compare an electrolyte with an electrode.
- Compare ionic conduction with metallic conduction.
- What is an aqueous electrolyte?
- Why might a battery use a non-aqueous electrolyte?
- What is a solid electrolyte?
- Give one possible advantage of solid electrolytes.
- Explain why "all substances that dissolve in water are electrolytes" is incorrect.
- A student tests salt water, sugar water, and very pure water. Predict which should conduct best and explain why.
- Explain how the movement of electrons and ions together allows an electrochemical cell to operate.
Key Terms
Electrolyte: Substance containing or producing mobile ions that can conduct electricity through ionic movement.
Ion: Atom or group of atoms carrying an electrical charge.
Cation: Positively charged ion.
Anion: Negatively charged ion.
Ionic conduction: Transport of electrical charge through the movement of ions.
Dissociation: Separation of an ionic substance into its ions.
Aqueous: Dissolved in water.
Non-electrolyte: Substance that does not produce enough mobile ions to conduct significantly when dissolved.
Strong electrolyte: Electrolyte that produces a high proportion of ions in solution.
Weak electrolyte: Electrolyte that produces a smaller proportion of ions in solution.
Conductivity: Ability of a material to conduct electric current.
Electrode: Conductor at which oxidation or reduction occurs.
Cathode: Electrode where reduction occurs.
Anode: Electrode where oxidation occurs.
Solid electrolyte: Solid material through which ions can move.
Key Takeaways
- An electrolyte contains or produces mobile ions that allow it to conduct electricity.
- Electrolytes conduct through the movement of ions, not through free-electron movement like metals.
- Positive ions are called cations.
- Negative ions are called anions.
- Solid ionic compounds generally do not conduct ionically because their ions are fixed in a lattice.
- Molten ionic compounds conduct because their ions become mobile.
- Many ionic compounds conduct when dissolved in water because their ions can move through the solution.
- Dissolving does not automatically produce ions; sugar dissolves but remains mainly as neutral molecules.
- Very pure water is a poor conductor because it contains very few ions.
- During electrolysis, cations move toward the cathode and anions move toward the anode.
- Reduction occurs at the cathode and oxidation occurs at the anode.
- Electrons move through the external circuit while ions move through the electrolyte.
- Electrolytes are essential in electrolysis, batteries, galvanic cells, fuel cells, electroplating, and metal purification.
- Different applications require electrolytes with different conductivity, stability, safety, and chemical properties.
- Understanding electrolytes connects the movement of ions with the operation of practical electrochemical technologies.