Worked solutions to textbook questions 9
Chapter 1 How did chemistry begin?
Q1.
Section 1.1 described some applications of chemistry that preceded the development of chemistry as a science.
a List other early examples of chemical technology.
b Give examples of ways in which chemistry has affected your life so far.
A1.
a Other early examples of chemical technology include the making of alloys, such as brass and bronze, to enhance the properties of metals; the extraction of chemicals from plants and animals for use as fabric dyes; the chemical treatment of animal hides; the development of different methods of cooking foods; the development of perfumes; and the extraction and use of oils for skin treatment and in lamps.
b Chemistry affects all aspects of our lives every day. Chemical reactions are fundamental to all processes that sustain life, to the production of food, the manufacture of goods such as clothing and plastics, the extraction of metals, the combustion of fuels, the effectiveness of medication, and so on.
Q2.
How would you distinguish between a chemist and an alchemist? List any present-day practices that you consider resemble alchemy rather than chemistry.
A2.
Alchemists were intent on converting one substance into another, particularly lead into gold. In their efforts to perform this conversion they would sometimes observe certain mystical rites. Some unscrupulous members of the profession were prepared to resort to trickery to obtain the support of the wealthy. They were secretive, showing little inclination to share their discoveries, and were not easily diverted by discoveries of new materials and reactions from their main goal of obtaining valuable substances.
The modern chemist, on the other hand, is interested in all aspects of the behaviour of materials. Hypotheses are subjected to testing by experiment, and new theories and discoveries are published in specialised scientific journals. However, today some practices and the claims they make resemble the practices of the alchemists—for example, some of the alternative cures prepared and offered are not scientifically tested and much of the ‘industry’ concerned with the production of illegal drugs is not scientifically controlled, resulting in unpredictable results for the consumer.
Q3.
Gold was used extensively thousands of years ago by the Egyptians. What properties of gold are linked to this early use?
A3.
Gold occurs in nature as the pure element, so does not need to be extracted. It is easy to work and fashion into objects, and it does not corrode.
Q4.
Rewrite in your own words Boyle’s definition of an element.
A4.
The essentials of Boyle’s definition were that an element was a pure substance (i.e. not mixed with anything else). He also explained that elements could combine to form more complex substances and that these complex substances could be separated into their constituent elements.
Q5.
Reactions involving oxygen were common as the early chemists attempted to identify new elements. Suggest an explanation for the importance of oxygen in this stage of chemistry’s development.
A5.
Oxygen was readily available as it comprises 21% of the Earth’s atmosphere. Many elements react readily with oxygen, with or sometimes without heating in either pure oxygen or air. As oxygen is effectively the only gas in air that reacts with this range of elements, the compounds formed (known as oxides) will have a characteristic set of identifiable properties. In particular, the mass ratio of element to oxygen in the product was one useful piece of data in identifying an element.
Q6.
Restate in your own words the law of conservation of mass.
A6.
Your statement should contain the idea that, in a chemical reaction, mass is conserved. Provided you did not ‘lose’ any of your reactants or products, the total mass of substance that you had before the reaction would equal the total mass of substance after the reaction.
Q7.
Priestley converted mercury(II) oxide (HgO) to mercury and oxygen. If these were the only three substances present, apply the law of conservation of mass to calculate:
a the mass of mercury oxide that would produce 2.00 g of liquid mercury and 0.16g of oxygen gas
b the mass of mercury produced when 4.6 g of mercury oxide is completely decomposed to produce 0.37 g of oxygen
A7.
a Mass of mercury oxide = mass of mercury + mass of oxygen = 2.16 g
b Mass of mercury = mass of mercury oxide – mass of oxygen = 4.23 g
Q8.
Why are Robert Boyle and Antoine Lavoisier sometimes called the ‘fathers’ of modern chemistry?
A8.
Boyle and Lavoisier took a systematic approach to understanding some of the experimental results that came from the work of the alchemists. Boyle argued that chemistry was a distinct branch; his definition of an element was an essential piece in establishing the basis for chemistry. Lavoisier brought careful measurement to his experimental work in order to support his theory of combustion. His experimental results were sufficiently accurate to lead to one of the early laws of chemistry — the law of conservation of mass.
Q9.
Of the nine earliest elements (see Appendix 1 (page 385)), which ones appear to have symbols that were derived from Latin names for the elements?
A9.
gold, silver, copper, iron, lead, tin
Q10.
Lithium, sodium, potassium, rubidium and caesium form a group of elements with similar properties. Refer to Appendix 1 (page 385) to write the symbol of each element alongside its name. Which of these elements appear to have symbols derived from Latin names?
A10.
lithium: Li, sodium: Na (Latin, natrium), potassium: K (Latin, kalium), rubidium: Rb, caesium: Cs
Q11.
Helium, neon, argon, krypton, xenon and radon form the last group of similar elements to be discovered. Write down the symbol for each of these elements.
A11.
helium: He, neon: Ne, argon: Ar, krypton: Kr, xenon: Xe, radon: Rn
Q12.
Dalton’s atomic theory was, in part, based on the constant mass ratio in which particular elements combined in chemical reactions. For example, in Lavoisier’s experiments, mercury and oxygen combined in the ratio that was always close to 25 g of mercury to 2g of oxygen.
On the basis of this information, calculate the mass of oxygen that would combine with:
a 5 g mercury
b 4.2 g mercury
c 0.27 g mercury
A12.
a Given information: 25 g of mercury combines with 2 g of oxygen
By ratio, 5 g of mercury combines with m g of oxygen.
5/25 = m/2
i.e. the mass of oxygen is 0.4 g
b Using the same method as in part a, the mass of oxygen that reacts with 4.2 g of mercury = 4.2 × 2/25 = 0.34 g
c Mass of oxygen that reacts with 0.27 g of mercury = 0.27 × 2/25 = 0.022 g
E1.
Suggest why it took until the 1860s for the periodic law to be proposed.
AE1.
There was too small a number of known elements until then to be able to analyse similarities in properties.
E2.
Give two ways in which the Newlands’ table is inferior to that of Mendeleev.
AE2.
His arrangement of elements, although reflecting the properties of the lighter elements, did not fit those of the heavier elements. He did not allow for, as yet, undiscovered elements.
Q13.
There were several forms of a ‘periodic table’ before the one proposed in 1869 by Mendeleev. What motivated chemists to look for a way of organising the elements?
A13.
The large increase in the amount of systematic experimental work in the 18th and 19th centuries resulted in the discovery of many more elements and compounds. Scientists were looking for ways to organise this information as a means of identifying relationships and, thereby, understanding why elements behaved as they did.
Chapter review
Q14.
Before the year 1604 only 12 elements had been discovered. What do you think were the main reasons behind the discovery of over 100 more elements in the early years of the 21st century?
A14.
In the 16th century, the work of the astronomer Nicholas Copernicus, who proposed that the Sun was the centre of the solar system, and of Galileo Galilei, who overturned existing ideas about the motion of falling bodies, led to a revolution in our approach to the natural world. People began to critically examine the materials about them and to perform experiments in order to test hypotheses. Civilisation emerged from a period in which the emphasis had been on spiritual questions and the study of literature and the arts. Although the alchemists discovered some elements, they were secretive and intent on their quest for gold. It was only once the ‘modern’ approach to science took hold that the discipline of chemistry can be said to have begun. Elements were then discovered at a greater rate than ever before.
Q15.
Huxley referred to ‘the great tragedy of science—the slaying of a beautiful hypothesis by an ugly fact’. Give an example from this chapter of a scientist who was not prepared to change his mind when experimental evidence suggested his hypothesis was incorrect.
A15.
Joseph Priestley held to his belief that combustible materials contained a mysterious substance named ‘phlogiston’, even though Lavoisier had demonstrated by repeated experiments and careful measurement that combustion involved a reaction with oxygen in the air.
Q16.
Copy the diagrams of the oxides as given in Figure 1.9. Use Dalton’s key to his symbols to ‘translate’ each of these ‘formulas’ into current chemical language. (Appendix 1 (page 385)) gives the symbols of all elements.) (Note that the fourth of these formulas refers to a compound that we now know doesn’t exist.)
A16.
The five that represent known compounds are FeO, Fe2O3, PbO, PbO2 and ZnO.
Q17.
Consider the eight points of Dalton’s atomic theory as outlined and list:
a those that he based directly on experimental data
b those that are hypotheses consistent with the experimental data available to Dalton
c any that you consider to be correct under certain conditions only
d any that you consider to be incorrect
A17.
a Based on experimental data:
· compounds are formed from the combination of the atoms of two or more elements
· the proportion and kind of atoms is fixed in a given compound
· atoms of each element have a unique mass
· atoms are neither created nor destroyed in reactions
b Those that are hypotheses consistent with the experimental data available to Dalton:
· all matter consists of indivisible atoms
· atoms of a particular element are identical in mass and have identical properties
· atoms combine in simple numerical ratios
· the most stable compounds of two elements contain atoms in a one-to-one ratio
c Any that you consider to be correct under certain conditions only:
· atoms are neither created nor destroyed in reactions. (This is true for chemical reactions but not for reactions involving the nucleus—fission and fusion.)
d Any that you consider to be incorrect:
· all matter consists of indivisible atoms
· atoms of a particular element are identical in mass and have identical properties. (Chemical properties are identical but, for isotopes, physical properties differ.)
· the most stable compounds of two elements contain atoms in a one-to-one ratio
Q18.
In which way did Dalton’s path to his proposed atomic theory:
a differ from the approach taken by the Greek philosophers such as Democritus and Aristotle?
b resemble the approach of the Greek philosophers?
A18.
a Dalton’s path to his atomic theory was based on the experimental results of the early chemists such as Boyle and Lavoisier, whereas the Greek philosophers (as their name suggests) were imagining what matter must be like. They were not involved in experimentation.
b Dalton’s method of arriving at proposals about the properties of atoms that were not based directly on experimental evidence resembled the philosophical thinking of the early Greeks.
Q19.
The concept of periodicity of elements was central to the development of the periodic table. What is meant by the term periodicity?
A19.
Periodicity refers to any event (e.g. the seasons) or, in the case of chemistry, properties of elements, that recur in a systematic way.
Q20.
Advances made by one scientist depend on the earlier work of others. Use a diagram to illustrate how the work of Lavoisier, Dalton and Berzelius contributed to Mendeleev’s proposed periodic table.
A20.
Q21.
The following processes all play a part in developing or refining chemical ideas and knowledge:
· the development of models
· observation—the gathering of evidence and data
· hypothesis formulation
· evaluation
· classification—the ordering of knowledge
a Use a flowchart diagram to order these steps in a logical sequence.
b What factors can cause scientists to review a hypothesis?
c Give an example from this chapter of a reviewed hypothesis.
A21.
a
Observation¯ / The gathering of evidence and data
e.g. observations, including measurements from individual chemical reactions
Classification
¯ / The ordering of knowledge
e.g. organising a collection of observations and/or data from different chemical reactions
Hypothesis formulation
¯ / A proposed explanation for an observation
e.g. the step from observing that two elements always combine in a particular mass ratio to suggesting that these elements must be made up of particles of fixed size or mass
The development of models
¯ / A model is used to represent important features of something scientists are trying to describe or understand
e.g. Dalton’s atomic model
Evaluation / A process in which future observations are tested against the model; this may lead to the modification or rejection of a model
e.g. revision of aspects of Dalton’s model of the atom such as his hypothesis that the most stable compounds of two elements contain atoms in a one-to-one ratio
b A hypothesis may be reviewed if:
· new experimental evidence emerges that is inconsistent with an existing hypothesis
· someone suggests a simpler or better explanation for existing evidence
c Priestley’s hypothesis on the nature of combustion (involving phlogiston) was rejected on the basis of the experimental results of Lavoisier.