Formulae and Equations

How Chemical Symbols Changed Chemistry

Jacob Berzelius

The Chemist Who Organised Chemical Symbols

Jacob Berzelius did not have an easy start. Born in Sweden, he lost both of his parents early and had to develop independence quickly. He initially studied medicine, but it was through this that he became interested in the chemical substances behind biological processes.

At the time, chemistry was disorganised. Different scientists used different naming systems, making communication difficult. Berzelius focused on fixing this. Instead of chasing flashy discoveries, he worked on creating structure and consistency across the subject.

His most important contribution was introducing the system of chemical symbols we still use today, such as H for hydrogen and O for oxygen, along with accurate measurements of relative atomic masses.

Every time you write a chemical formula, you are using a system he built. Berzelius did not just study chemistry. He organised it.

Chemical Symbols

Why Berzelius Introduced Element Symbols

Symbols for the elements were first introduced by Jöns Jacob Berzelius in 1811.

He explained that the purpose of these new symbols was not simply to label bottles in a laboratory. Instead, they were created to simplify the communication of chemical proportions and to show, without long explanations, the relative number of volumes of the different components present in each compound substance.

Berzelius also believed that chemical symbols should be letters, because they were easier to write and avoided making printed text look untidy. Most of the symbols he introduced are still used today.

However, in the nineteenth century, representing compounds using symbols was not straightforward. Even for simple compounds, scientists needed creativity, careful reasoning and a willingness to test different possibilities.

Chemical Formulae

Why Working Out Formulae Was Difficult

Take water as an example. It can be demonstrated by experiment that 1 g of hydrogen reacts exactly with 8 g of oxygen. This might suggest that the formula of water is HO, with an oxygen atom having a mass eight times greater than that of hydrogen.

Alternatively, the formula could be H₂O, with the oxygen atom having a mass 16 times greater than that of hydrogen. It could even be HO₂, with an oxygen atom having a mass four times greater than that of hydrogen.

Although we now know the correct answer, it could not be worked out from these experimental results alone.

Scientists had to make educated guesses and then try to interpret other evidence. Suppose they assumed that the formula of water was HO, as John Dalton did. This would mean oxygen has a mass of 8 compared with hydrogen.

However, when sulphur and oxygen react to form sulphur dioxide, 8 g of oxygen reacts with 8 g of sulphur. Two atoms cannot have identical masses, so a decision must be made.

Atomic Masses

Trial, Error and the Search for Consistency

It might be assumed that the formula of sulphur dioxide is S₂O, with sulphur having a mass four times that of hydrogen. However, when hydrogen reacts with sulphur, 1 g of hydrogen reacts with 16 g of sulphur.

This suggests that the formula of hydrogen sulphide would be HS₄. Scientists would recognise that this is unlikely, as they expected the ratios of atoms in compounds to be simple.

This would lead to reconsidering earlier assumptions. Eventually, through trial, error and elimination of different possibilities, a consistent answer could be reached, or at least one that fitted the data. This was the approach originally used to determine correct formulae.

Over time, a consistent system was developed, based on ideas proposed by Amedeo Avogadro, who carried out significant work on the reacting volumes of gases.

It is interesting, however, that even by the 1860s, there was still uncertainty about the true mass of a carbon atom relative to a hydrogen atom, and therefore some uncertainty about the formulae of carbon compounds.

Practice Question

Firework Chemistry

Copper chloride is manufactured either by reacting chlorine with copper metal or by reacting copper oxide with dilute hydrochloric acid.

Potassium chlorate (KClO₃) decomposes to release oxygen in the firework mixture.

Write equations for:

(i) reaction between chlorine and copper metal
(ii) reaction between copper oxide and dilute hydrochloric acid
(iii) decomposition of potassium chlorate

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Model Answer

Balanced Equations

(i)

Copper reacts with chlorine to form copper(II) chloride:

Cu + Cl₂ → CuCl₂

(ii)

Copper(II) oxide reacts with hydrochloric acid to form copper(II) chloride and water:

CuO + 2HCl → CuCl₂ + H₂O

(iii)

Potassium chlorate decomposes to form potassium chloride and oxygen:

2KClO₃ → 2KCl + 3O₂

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Practice Question

Paris Green Composition

The ethanoate ion has the formula CH₃CO₂⁻ and the arsenite ion has the formula AsO₂⁻.

Write the formula for:

(i) copper ethanoate
(ii) copper arsenite

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Model Answer

Ionic Formulae

(i) Copper ethanoate

The copper ion is Cu²⁺ and the ethanoate ion is CH₃CO₂⁻.

Two ethanoate ions are needed to balance the +2 charge on copper:

Cu(CH₃CO₂)₂

(ii) Copper arsenite

The copper ion is Cu²⁺ and the arsenite ion is AsO₂⁻.

Two arsenite ions are needed to balance the +2 charge:

Cu(AsO₂)₂

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Practice Question

Scheele’s Green Reactions

A mixture of arsenic oxide is boiled with potassium carbonate until no more carbon dioxide is produced. Potassium hydrogenarsenate is formed.

A solution of copper sulphate is then added, forming an impure precipitate of Scheele’s Green.

(a) Deduce the formula of potassium hydrogenarsenate.

(b) Suggest an equation for the reaction in the first step. Water is also a reactant.

(c) Suggest an equation for the reaction in the second step.

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Model Answer

Formulae and Equations

(a)

Hydrogenarsenate ion has a charge of 2−, so its formula is HAsO₄²⁻.

Potassium ions are K⁺, so two potassium ions are needed to balance the charge.

K₂HAsO₄

(b)

Arsenic oxide reacts with potassium carbonate and water to form potassium hydrogenarsenate and carbon dioxide.

As₂O₃ + 2K₂CO₃ + H₂O → 2K₂HAsO₄ + 2CO₂

(c)

Potassium hydrogenarsenate reacts with copper(II) sulphate to form copper hydrogenarsenate precipitate and potassium sulphate.

K₂HAsO₄ + CuSO₄ → CuHAsO₄ + K₂SO₄

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Author

Luke Edwards-Stuart

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