A-level Chemistry/OCR/Foundation/Atoms, Molecules and Stoichiometry
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What is a chemical reaction? YUNGBENZ
Carbon dioxide is produced when carbon burns.
Carbon is a black solid material. When ignited it burns – that is, it reacts with oxygen from the air. The product is a colourless gas - carbon dioxide (i.e. a totally different substance). This is an example of a chemical reaction.
A chemical reaction is one in which a new substance is produced – one that was not there at the start.
So what is meant by a substance?
Water, hydrogen and oxygen are totally different substances. Each has its own characteristic properties. Water is a compound of hydrogen and oxygen; it is not hydrogen or oxygen or a mixture of these elements.
Pre-chemistry Prior to 1000 BC people had learned to produce a large range of materials (as distinct for finding them): bricks, pottery, bronze, iron, glass, distilled alcohol, etc. The quest by alchemists for a way of converting cheap metals into gold proceeded from 300 BC to 1500 AD and maybe later. The alchemists' goal was not achieved but they did succeed in producing hundreds of substances, including what we know as sulphuric acid, nitric acid, hydrochloric acid, potassium nitrate and silver nitrate, to name a few. Despite all the knowledge which had been painstakingly gathered it was believed that all materials were made from four ‘elements’: earth, air, fire and water. In 1661 Robert Boyle proposed that only substances which could not be decomposed into others should be regarded as ‘elements’. Robert Boyle (1627-1691), born in Lismore, Co. Waterford, is regarded as the “Father of chemistry”. Phlogiston theory (1700~1800) The prevailing theory of combustion in the 1700s presumed the existence of ‘phlogiston’ (or ‘fire-stuff’) in things that burned. Needless to say, nobody ever found it; it had to have no weight or even a negative weight! If phlogiston was produced when something burned how could the ‘ash’ be heavier - as it sometimes was?
Important discoveries As a result of Joseph Black’s (1728-1799) work on lime and carbon dioxide it was realised that changes in weight were of fundamental importance.
In 1766 Henry Cavendish discovered hydrogen. Around 1770 Joseph Priestley discovered oxygen, sulphur dioxide, hydrogen chloride and nitric oxide. It was beginning to be recognised that gases were substances.
Antoine Lavoisier (1743-1794) Lavoisier’s investigations showed that air was 1/5th oxygen and another gas, ‘azote’. He concluded that ‘phlogiston’ did not exist. The phlogiston theory had held back the development of chemistry by 100 years. Lavoisier introduced the systematic naming of substances (1787). He was a victim of the guillotine during the ‘Reign of Terror’(1793–1794)
John Dalton (1766-1844) A number of important gases had been discovered towards the end of the 18th century. Dalton studied their reactions in detail. He was intrigued by the way they reacted in simple volume proportions: 1:1, 1:2, 1:3, 2:3 etc. In order to make sense of his findings he proposed that the gases were composed of particles that combined in simple proportions. In 1808 Dalton published his ideas, and this was the first evidence-based atomic theory. In about 400 B. C. Democritus proposed, on philosophical grounds, that all matter was composed of small, discrete, individual atoms. Robert Boyle (1627-1691) and Isaac Newton (1642-1727) had been led to the same conclusion for other reasons.
Dalton’s proposals
1. Elements are composed of small, indivisible, identical particles called atoms. Atoms of different elements are different.
2. Atoms cannot be created, destroyed, or transformed to other kinds of atoms.
3. Compounds are formed when atoms combine with other atoms in small, fixed, whole number ratios.
4. Equal volumes of gases contain the same number of atoms.
(None of these proposals is strictly correct. However, they were 'good enough' for the time and sufficiently in keeping with the evidence to be useful and important.)
Compounds Dalton proposed that two ‘atoms’ of hydrogen combined with one ‘atom’ of oxygen to form one ‘atom’ of water. Similarly one ‘atom’ of hydrogen combined with one ‘atom’ of chlorine to form one ‘atom’ of hydrogen chloride.
Unexpected Results Dalton expected that one volume of hydrogen would combine with one volume of chlorine to produce one volume of hydrogen chloride. In fact he got two volumes of HCl. He could not explain his results satisfactorily in terms of his theory, yet he had good reasons for not abandoning it.
Avogadro (1776-1856) Around 1811 Amadeo Avogadro proposed a solution to the problem. According to him, equal volumes of gases had equal numbers of molecules. Molecules of hydrogen, oxygen etc. consisted of pairs of atoms.
Avogadro's Proposal Avogadro's solution to the problem was simple and elegant. It made sense of all Dalton’s results. Strangely, it was not accepted until at the time nor indeed in his lifetime.
Stanislao Cannizzaro (1826-1910) Despite all the developments, there was no agreement on atomic weights and as a result no agreement on chemical formulae; there were in fact 19 different formulae for acetic acid in the mid 1800s! At the suggestion of Kekulé the first international conference on chemistry was held in 1860 in Karlsruhe. Cannizzaro proposed the adoption of Avogadro’s hypothesis; order ensued. Sadly Avogadro had died just four years earlier.
Atoms of Matter
The concept of atoms is arguably the most important contribution to our understanding of matter. It enables us to understand such things as the shapes of crystals, the fact that different substances can have the same composition, X-ray diffraction patterns, Brownian motion, osmosis, the structure of proteins, and DNA, etc. From such data the structure of molecules can be determined.
Crystals Crystals are pure forms of substances which exhibit characteristic regular shapes. These shapes can be explained in terms of regular arrangements of atoms. Rorylg 22:21, 12 April 2006 (UTC)
