What Is an Aqueous Solution?

An aqueous solution is a solution in which a substance has been dissolved in:

water.

The symbol used in chemical equations is:

(aq).

For example:

NaCl(aq)

means sodium chloride dissolved in water.

When an ionic compound dissolves, its ions become free to move.

For example:

NaCl(aq) → Na⁺(aq) + Cl⁻(aq)

These mobile ions allow the solution to conduct:

electricity.

---

Electrolysis in an Aqueous Solution

During electrolysis, two electrodes are placed in the electrolyte and connected to a DC power supply.

In an electrolytic cell:

Cathode = negative

Anode = positive

Positive ions move toward the:

cathode.

Negative ions move toward the:

anode.

Remember:

Cations → Cathode

Anions → Anode

---

The Important Difference: Water Is Present

Electrolysis of an aqueous solution is more complicated than electrolysis of a molten compound.

Why?

Because aqueous solutions contain:

water.

Water can participate in the electrode reactions.

This means there may be more than one possible reaction at each electrode.

For example, aqueous sodium chloride contains:

• Na⁺ ions
• Cl⁻ ions
• H₂O molecules
• very small concentrations of H⁺ and OH⁻ arising from water

Therefore we cannot simply assume:

Na⁺ → sodium

and:

Cl⁻ → chlorine

We must determine which species is preferentially:

discharged at each electrode.

What Does "Discharged" Mean?

When an ion is discharged, it loses its ionic charge by gaining or losing electrons at an electrode.

At the cathode:

reduction occurs

and particles gain electrons.

At the anode:

oxidation occurs

and particles lose electrons.

A discharged species forms a product at the:

electrode.

---

The Cathode in Aqueous Electrolysis

The cathode is:

negative.

Reduction occurs there.

Possible cathode reactions often involve:

• metal ions gaining electrons
• water being reduced to hydrogen

The product depends partly on how easily the competing species can be:

reduced.

---

Predicting the Cathode Product

For many introductory aqueous electrolysis problems, a useful simplified rule is:

If the metal is less reactive than hydrogen, the metal may be deposited.

If the metal is more reactive than hydrogen, hydrogen is usually produced instead.

A simplified section of the reactivity series is:

K

Na

Ca

Mg

Al

↑ more reactive

H

↓ less reactive

Cu

Ag

Therefore:

Na⁺ in water → usually H₂, not sodium

Mg²⁺ in water → usually H₂, not magnesium

Cu²⁺ in water → Cu

Ag⁺ in water → Ag

---

Hydrogen at the Cathode

When hydrogen is produced from neutral or alkaline aqueous solution, a useful half-equation is:

2H₂O + 2e⁻ → H₂ + 2OH⁻

Water gains electrons.

Therefore this reaction is:

reduction.

Hydrogen gas appears as bubbles at the:

cathode.

---

Metal Deposition at the Cathode

If a suitable metal ion is preferentially reduced, metal is deposited on the cathode.

For example, copper:

Cu²⁺ + 2e⁻ → Cu

Silver:

Ag⁺ + e⁻ → Ag

The metal electrode may therefore:

increase in mass.

The Anode in Aqueous Electrolysis

The anode is:

positive.

Oxidation occurs there.

Possible reactions include oxidation of:

• halide ions
• water
• hydroxide ions
• a reactive electrode itself

Therefore the anode product also depends on the:

substances present and the electrode material.

---

Halide Ions

The main halide ions are:

• Cl⁻ — chloride
• Br⁻ — bromide
• I⁻ — iodide

Under suitable aqueous conditions, particularly when halide concentration is sufficiently high, halide ions can be oxidized to:

halogens.

For example:

2Cl⁻ → Cl₂ + 2e⁻

2Br⁻ → Br₂ + 2e⁻

2I⁻ → I₂ + 2e⁻

These are oxidation reactions because electrons are:

lost.

---

Oxygen at the Anode

If a halide is not preferentially discharged, oxygen is commonly produced from water or hydroxide ions.

A useful half-equation in alkaline conditions is:

4OH⁻ → O₂ + 2H₂O + 4e⁻

This is:

oxidation.

Oxygen gas forms at the:

anode.

---

A Simplified Prediction Method

For many school-level problems using inert electrodes, you can use the following approach.

At the cathode:

• less-reactive metal ions such as Cu²⁺ or Ag⁺ → metal
• very reactive metal ions such as Na⁺ or Mg²⁺ → usually hydrogen instead

At the anode:

• sufficiently concentrated Cl⁻, Br⁻ or I⁻ → corresponding halogen may form
• otherwise → oxygen commonly forms

These are useful rules, but actual electrode products can also depend on:

concentration, electrode material, and electrochemical conditions.

---

Example 1: Aqueous Sodium Chloride

Consider sodium chloride solution:

NaCl(aq).

The solution contains:

Na⁺

Cl⁻

and water.

What products form depends partly on concentration, especially at the:

anode.

---

At the Cathode

Sodium is much more reactive than hydrogen.

Sodium metal is therefore not normally deposited from aqueous NaCl.

Instead, water is reduced:

2H₂O + 2e⁻ → H₂ + 2OH⁻

The product is:

hydrogen gas.

---

At the Anode

In concentrated sodium chloride solution, chloride ions are oxidized:

2Cl⁻ → Cl₂ + 2e⁻

The product is:

chlorine gas.

Therefore concentrated aqueous sodium chloride can produce:

Cathode → hydrogen

Anode → chlorine

The solution also becomes enriched in:

sodium hydroxide.

The Chlor-Alkali Process

Electrolysis of concentrated sodium chloride solution is used industrially in the:

chlor-alkali process.

Important products include:

• chlorine
• hydrogen
• sodium hydroxide

These chemicals have many industrial applications.

---

Uses of Chlorine

Chlorine is used in the manufacture or treatment of:

• disinfectants
• some plastics
• water-treatment chemicals
• many industrial chemicals

Because chlorine gas is toxic, industrial systems must carefully control its:

production and handling.

---

Uses of Sodium Hydroxide

Sodium hydroxide is used in:

• soap production
• paper manufacture
• chemical processing
• cleaning products
• many industrial reactions

It is a strong alkali and is:

corrosive.

---

Example 2: Aqueous Copper(II) Sulfate with Inert Electrodes

Consider:

CuSO₄(aq)

using inert electrodes such as graphite or platinum.

The solution contains:

• Cu²⁺
• SO₄²⁻
• water

---

Cathode Reaction

Copper is less reactive than hydrogen.

Cu²⁺ ions are reduced:

Cu²⁺ + 2e⁻ → Cu

Copper metal is deposited at the:

cathode.

The cathode may develop a reddish-brown copper coating and:

gain mass.

---

Anode Reaction

Sulfate ions are not normally discharged under these conditions.

Instead, water/hydroxide species are oxidized and:

oxygen gas

is produced.

A useful half-equation is:

4OH⁻ → O₂ + 2H₂O + 4e⁻

Therefore:

Cathode → copper

Anode → oxygen

What Happens to the Blue Colour?

Cu²⁺ ions give copper(II) sulfate solution its characteristic:

blue colour.

As Cu²⁺ ions are removed and deposited as copper at the cathode, their concentration decreases.

With inert electrodes, the blue colour can therefore become:

paler.

This observation provides evidence that Cu²⁺ ions are being removed from the solution.

---

Example 3: Copper(II) Sulfate with Copper Electrodes

Now suppose the same CuSO₄ solution is electrolyzed using:

copper electrodes.

The reactions change because copper is a:

reactive electrode.

At the cathode:

Cu²⁺ + 2e⁻ → Cu

Copper is deposited.

At the anode:

Cu → Cu²⁺ + 2e⁻

Copper from the anode enters the solution.

---

What Happens to the Electrodes?

At the cathode:

copper deposited → mass increases

At the anode:

copper dissolves → mass decreases

Cu²⁺ ions removed at the cathode are approximately replaced by Cu²⁺ ions produced at the anode.

Therefore the concentration of Cu²⁺ may remain relatively:

constant.

This principle is important in:

copper electrorefining.

---

Electrode Material Matters

Compare CuSO₄ electrolysis.

With inert electrodes:

Cathode → Cu

Anode → O₂

With copper electrodes:

Cathode → Cu

Anode → Cu²⁺ enters solution

Therefore the material used for the electrode can change:

the electrode reaction and products.

---

Example 4: Dilute Sulfuric Acid

Dilute sulfuric acid contains ions in water.

During electrolysis with inert electrodes:

At the cathode, hydrogen is produced.

A convenient acidic half-equation is:

2H⁺ + 2e⁻ → H₂

At the anode, water is oxidized:

2H₂O → O₂ + 4H⁺ + 4e⁻

Overall, water is decomposed into:

hydrogen and oxygen.

Hydrogen and Oxygen Volumes

The overall reaction is:

2H₂O → 2H₂ + O₂

Therefore, under comparable conditions, the volume ratio is approximately:

hydrogen : oxygen = 2 : 1

So approximately twice as much hydrogen gas is produced as:

oxygen gas.

---

Example 5: Aqueous Silver Nitrate

Consider:

AgNO₃(aq)

using inert electrodes.

At the cathode, silver ions are reduced:

Ag⁺ + e⁻ → Ag

Silver is deposited.

At the anode, nitrate is not normally discharged under typical introductory conditions, so oxygen is produced from water/hydroxide species.

Therefore:

Cathode → silver

Anode → oxygen

---

Comparing Molten and Aqueous Electrolysis

The major difference is the presence of:

water.

---

Example: Molten vs Aqueous NaCl

This comparison shows why water matters.

Molten NaCl

Ions:

Na⁺ and Cl⁻

Cathode:

Na⁺ + e⁻ → Na

Anode:

2Cl⁻ → Cl₂ + 2e⁻

Products:

sodium + chlorine

Concentrated Aqueous NaCl

Contains Na⁺, Cl⁻ and water.

Cathode:

2H₂O + 2e⁻ → H₂ + 2OH⁻

Anode:

2Cl⁻ → Cl₂ + 2e⁻

Products include:

hydrogen + chlorine

Therefore the cathode product changes from:

sodium → hydrogen.

---

Why Isn't Sodium Produced from Salt Water?

Producing sodium metal from Na⁺ requires:

reduction.

However, in aqueous solution, water is more readily reduced than Na⁺ under ordinary conditions.

Therefore hydrogen is produced instead.

This is why industrial sodium production uses:

molten sodium chloride, not aqueous sodium chloride.

---

Factors Affecting Product Formation

The products of aqueous electrolysis can depend on several factors.

Important factors include:

• ions present
• relative ease of oxidation or reduction
• concentration
• electrode material
• applied potential
• overpotential
• temperature

At introductory level, the most important factors are usually:

ions present, concentration, and electrode material.

---

Factor 1: Ions Present

The possible electrode products depend on which chemical species are present.

For example:

CuSO₄ contains:

Cu²⁺ and SO₄²⁻

NaCl contains:

Na⁺ and Cl⁻

Different ions create different possible:

electrode reactions.

---

Factor 2: Reactivity and Reduction

At the cathode, different species compete to be:

reduced.

A useful school-level rule compares the metal with hydrogen.

Metal less reactive than hydrogen:

metal may be deposited

Metal more reactive than hydrogen:

hydrogen is usually produced

For example:

Cu²⁺ → Cu

but aqueous:

Na⁺ → usually H₂ instead of Na.

---

Factor 3: Concentration

Ion concentration can affect which reaction occurs.

This is particularly important with:

halide ions.

For example, concentrated sodium chloride solution commonly produces:

chlorine at the anode.

In more dilute chloride solutions, oxygen production can become more competitive.

Therefore concentration must sometimes be considered when predicting:

anode products.

---

Factor 4: Electrode Material

An inert electrode mainly provides a surface for:

electron transfer.

A reactive electrode may itself participate in the reaction.

For example, a copper anode can undergo:

Cu → Cu²⁺ + 2e⁻

Therefore electrode material can significantly affect the:

products.

---

Factor 5: Electrode Potentials

More advanced predictions can use:

electrode potentials.

Different reactions have different tendencies to occur.

Standard electrode potentials can help predict which reaction is thermodynamically more favorable.

However, real electrolysis also depends on factors such as:

• concentration
• temperature
• surface properties
• overpotential

Therefore simple reactivity-series rules are useful introductory models but do not explain every:

real electrolysis system.

---

What Is Overpotential?

An electrode reaction may theoretically be possible but still require additional voltage before it proceeds at a useful rate.

This extra requirement is associated with:

overpotential.

Overpotential helps explain why actual electrolysis products do not always follow a prediction based only on standard electrode potentials.

This becomes particularly important in:

industrial electrochemistry.

---

Writing Aqueous Half-Equations

A half-equation must balance:

atoms

and:

charge.

At the cathode, electrons appear on the:

left.

Example:

Cu²⁺ + 2e⁻ → Cu

At the anode, electrons appear on the:

right.

Example:

2Cl⁻ → Cl₂ + 2e⁻

---

Common Cathode Half-Equations

Copper deposition:

Cu²⁺ + 2e⁻ → Cu

Silver deposition:

Ag⁺ + e⁻ → Ag

Hydrogen from acidic solution:

2H⁺ + 2e⁻ → H₂

Hydrogen from water:

2H₂O + 2e⁻ → H₂ + 2OH⁻

All are examples of:

reduction.

---

Common Anode Half-Equations

Chlorine:

2Cl⁻ → Cl₂ + 2e⁻

Bromine:

2Br⁻ → Br₂ + 2e⁻

Iodine:

2I⁻ → I₂ + 2e⁻

Oxygen in alkaline form:

4OH⁻ → O₂ + 2H₂O + 4e⁻

Oxygen in acidic form:

2H₂O → O₂ + 4H⁺ + 4e⁻

All are examples of:

oxidation.

---

Different Half-Equations Can Describe the Same Process

You may see oxygen formation written as:

4OH⁻ → O₂ + 2H₂O + 4e⁻

or, under acidic conditions:

2H₂O → O₂ + 4H⁺ + 4e⁻

These equations describe oxygen formation using different ways of representing the species present.

Which form is most appropriate depends on the:

solution conditions.

---

A Step-by-Step Method

When solving an aqueous electrolysis problem:

Step 1: Identify the electrolyte.

Step 2: Identify the ions present.

Step 3: Remember that water is also present.

Step 4: Identify the cathode and anode.

Step 5: Determine the possible cathode reductions.

Step 6: Determine the possible anode oxidations.

Step 7: Consider metal reactivity.

Step 8: Consider halide concentration.

Step 9: Check whether the electrodes are inert or reactive.

Step 10: Write balanced half-equations.

---

Worked Example 1

Predict the cathode product during electrolysis of aqueous CuSO₄ with inert electrodes.

Possible metal ion:

Cu²⁺

Copper is less reactive than hydrogen.

Therefore Cu²⁺ is reduced:

Cu²⁺ + 2e⁻ → Cu

Product:

copper.

---

Worked Example 2

Predict the anode product during electrolysis of aqueous CuSO₄ with inert electrodes.

Sulfate is not normally discharged under these conditions.

Water/OH⁻ is oxidized.

Product:

oxygen.

Possible half-equation:

4OH⁻ → O₂ + 2H₂O + 4e⁻

---

Worked Example 3

Predict the cathode product from concentrated aqueous NaCl.

Na⁺ is from a highly reactive metal.

Water is reduced instead:

2H₂O + 2e⁻ → H₂ + 2OH⁻

Product:

hydrogen.

---

Worked Example 4

Predict the anode product from concentrated aqueous NaCl.

The high concentration of chloride ions favors chlorine production under typical chlor-alkali conditions:

2Cl⁻ → Cl₂ + 2e⁻

Product:

chlorine.

---

Worked Example 5

Why does molten NaCl produce sodium while aqueous NaCl does not?

Molten NaCl contains:

Na⁺ and Cl⁻ only.

Na⁺ must therefore be reduced at the cathode.

Aqueous NaCl also contains:

water.

Water is reduced preferentially under ordinary conditions, producing:

hydrogen.

---

Worked Example 6

During electrolysis, an electrode becomes coated with copper.

Which electrode is it?

Copper ions have gained electrons:

Cu²⁺ + 2e⁻ → Cu

This is reduction.

Reduction occurs at the:

cathode.

---

Worked Example 7

A gas is produced at the anode of concentrated NaCl solution.

Identify the gas.

Chloride ions are oxidized:

2Cl⁻ → Cl₂ + 2e⁻

The gas is:

chlorine.

---

Worked Example 8

During electrolysis of CuSO₄ with inert electrodes, the blue solution becomes paler.

Why?

Cu²⁺ ions are being removed:

Cu²⁺ + 2e⁻ → Cu

The concentration of blue Cu²⁺ ions decreases.

Therefore the colour becomes:

less intense.

---

Worked Example 9

Copper electrodes replace graphite electrodes in CuSO₄ solution.

What changes at the anode?

With graphite, oxygen can be produced.

With copper:

Cu → Cu²⁺ + 2e⁻

The copper anode itself is:

oxidized.

---

Worked Example 10

A student predicts that Mg²⁺ in aqueous MgCl₂ will produce magnesium metal at the cathode.

Why is this usually incorrect?

Magnesium is much more reactive than hydrogen.

In aqueous solution, water is preferentially reduced.

Therefore the cathode product is usually:

hydrogen gas.

---

Observations During Aqueous Electrolysis

Electrode reactions can often be identified through observations.

Bubbles

may indicate gas production.

Metal coating

may indicate metal deposition.

Electrode mass decreases

may indicate oxidation of a reactive electrode.

Electrode mass increases

may indicate metal deposition.

Solution colour changes

may indicate changes in ion concentration.

These observations provide evidence about what is happening at the:

electrodes.

Testing Electrode Gases

Common gases produced during electrolysis can sometimes be identified using standard chemical tests.

Hydrogen

A lit splint produces a characteristic:

squeaky pop.

Oxygen

A glowing splint:

relights.

Chlorine

Damp litmus paper is typically:

bleached after an initial acidic colour change.

Gas tests should only be performed using appropriate laboratory procedures and quantities.

---

Aqueous Electrolysis in Industry

Aqueous electrolysis has important industrial applications.

These include:

• chlorine production
• hydrogen production
• sodium hydroxide production
• electroplating
• metal purification
• chemical manufacturing

The products depend on careful control of:

electrolyte composition and operating conditions.

---

Electroplating

Electroplating commonly uses an:

aqueous electrolyte.

Suppose an object is being copper plated.

Cu²⁺ ions move toward the cathode.

At the cathode:

Cu²⁺ + 2e⁻ → Cu

Copper is deposited onto the:

object.

---

Copper Purification

Electrorefining also uses an aqueous electrolyte.

Impure copper forms the:

anode.

Pure copper forms the:

cathode.

Anode:

Cu → Cu²⁺ + 2e⁻

Cathode:

Cu²⁺ + 2e⁻ → Cu

Copper is effectively transferred from the impure electrode to the:

pure electrode.

---

Common Mistake: Aqueous Means Molten

These terms mean different things.

Molten = melted

Aqueous = dissolved in water

Water can participate in aqueous electrolysis, so the products may be:

different.

---

Common Mistake: The Metal Always Forms at the Cathode

Reduction always occurs at the cathode.

However, in an aqueous solution the reduced product may be:

hydrogen instead of the metal.

This is particularly common when the metal is highly reactive.

---

Common Mistake: The Negative Ion Always Forms the Anode Product

Not necessarily.

Water or OH⁻ can also be oxidized.

For example, sulfate-containing solutions with inert electrodes commonly produce:

oxygen at the anode.

---

Common Mistake: Concentration Does Not Matter

Concentration can affect which reaction occurs.

This is particularly important for:

halide solutions.

Concentrated chloride solutions commonly produce chlorine at the anode, while oxygen formation can compete more strongly in dilute conditions.

---

Common Mistake: Electrode Material Does Not Matter

Reactive electrodes can participate directly in electrolysis.

For example:

Cu → Cu²⁺ + 2e⁻

can occur at a copper anode.

Changing the electrode can therefore change the:

reaction.

---

Common Mistake: Reactivity-Series Rules Explain Everything

The reactivity series provides a useful introductory prediction tool.

Real electrolysis also depends on:

• electrode potentials
• concentration
• electrode material
• temperature
• overpotential

Therefore the simple rules are:

models for predicting common cases, not universal laws for every electrochemical system.

---

Check Your Understanding

1. Define an aqueous solution.
2. What does the symbol (aq) mean?
3. Why do aqueous ionic solutions conduct electricity?
4. Which particles carry charge through an aqueous electrolyte?
5. Why is aqueous electrolysis more complicated than molten electrolysis?
6. What additional substance is present in every aqueous electrolyte?
7. What is meant by an ion being discharged?
8. Which electrode is the cathode?
9. Which electrode is the anode?
10. What reaction occurs at the cathode?
11. What reaction occurs at the anode?
12. Which ions move toward the cathode?
13. Which ions move toward the anode?
14. Why is sodium not normally deposited from aqueous NaCl?
15. What forms instead at the cathode?
16. Write a half-equation for hydrogen formation from water.
17. Write the half-equation for copper deposition.
18. Write the half-equation for silver deposition.
19. What commonly forms at the anode when chloride ions are sufficiently concentrated?
20. Write the half-equation for chlorine formation.
21. Write the half-equation for bromine formation.
22. Write the half-equation for iodine formation.
23. What commonly forms at the anode when sulfate is present with inert electrodes?
24. Write a half-equation for oxygen formation.
25. Predict the products of concentrated aqueous NaCl.
26. What industrial process electrolyzes concentrated NaCl solution?
27. Name three important products of the chlor-alkali process.
28. Predict the products of aqueous CuSO₄ using inert electrodes.
29. Why does the blue colour of CuSO₄ become paler during electrolysis with inert electrodes?
30. What changes if copper electrodes are used instead?
31. Write the reaction at a copper anode.
32. Explain why a copper cathode gains mass.
33. Explain why a copper anode loses mass.
34. Predict the cathode product from aqueous AgNO₃ using inert electrodes.
35. Predict the anode product.
36. Compare molten and aqueous NaCl electrolysis.
37. Why can water change the products of electrolysis?
38. Explain how metal reactivity can affect the cathode product.
39. Explain how concentration can affect the anode product.
40. Explain how electrode material can affect product formation.
41. What is overpotential?
42. Why are simple reactivity rules not sufficient for every real electrolysis system?
43. What observation suggests that a gas is forming?
44. What observation suggests that a metal is being deposited?
45. What could cause an electrode to lose mass?
46. Describe the test for hydrogen.
47. Describe the test for oxygen.
48. Explain how aqueous electrolysis is used in electroplating.
49. Explain how aqueous electrolysis is used in copper purification.
50. Describe a step-by-step method for predicting the products of an aqueous electrolysis experiment.

---

Key Terms

Aqueous: Dissolved in water.

Aqueous solution: Solution in which water is the solvent.

Electrolysis: Use of electrical energy to drive a non-spontaneous chemical reaction.

Discharge: Gain or loss of electrons by a species at an electrode, producing an electrode product.

Cathode: Electrode where reduction occurs.

Anode: Electrode where oxidation occurs.

Reduction: Gain of electrons.

Oxidation: Loss of electrons.

Half-equation: Equation showing electron transfer during an oxidation or reduction process.

Metal deposition: Formation of solid metal at an electrode by reduction of metal ions.

Halide: Ion formed from a halogen, such as Cl⁻, Br⁻ or I⁻.

Inert electrode: Electrode that mainly provides a conducting surface without significantly participating in the reaction.

Reactive electrode: Electrode whose material participates in the electrochemical reaction.

Overpotential: Additional potential beyond the thermodynamic value that may be required for an electrode reaction to proceed at a significant rate.

Chlor-alkali process: Industrial electrolysis of concentrated sodium chloride solution to produce chlorine, hydrogen and sodium hydroxide.

---

Key Takeaways

• An aqueous electrolyte contains water as well as dissolved ions.
• Water makes aqueous electrolysis more complicated than molten electrolysis because it can participate in electrode reactions.
• Reduction occurs at the cathode and oxidation occurs at the anode.
• Cations move toward the cathode and anions move toward the anode.
• In many introductory cases, metals less reactive than hydrogen can be deposited at the cathode.
• Highly reactive metal ions such as Na⁺ and Mg²⁺ generally do not produce their metals from ordinary aqueous solutions; hydrogen is produced instead.
• Halide ions can produce halogens at the anode under suitable conditions.
• When halides are not preferentially discharged, oxygen commonly forms at the anode.
• Concentrated aqueous NaCl can produce hydrogen, chlorine and sodium hydroxide.
• CuSO₄ with inert electrodes produces copper at the cathode and oxygen at the anode.
• Electrode material can change the reactions: a reactive copper anode can dissolve to form Cu²⁺.
• Molten and aqueous electrolysis can produce different products from the same compound.
• Product formation depends on the species present, their electrochemical behavior, concentration, electrode material and operating conditions.
• Half-equations show exactly how electrons are gained or lost at each electrode.
• A systematic analysis of the ions, water, electrode material and possible reactions allows the products of common aqueous electrolysis systems to be predicted.