Irish Leaving Certificate Chemistry

Chemistry is a way of finding out how matter behaves and then explaining that behaviour with models. The facts in the subject were established by people who measured carefully, checked each other's work and changed their ideas when the evidence required it, so the methods matter as much as the results.

Understanding about chemistry: models and their limits

A model is a simplified representation of a system or phenomenon, built on stated assumptions. A model is useful when it explains observations and predicts what will happen in a new situation. Every model also has limits, and knowing where it stops working is part of knowing the model.

The kinetic theory treats particles as tiny hard spheres in constant motion. That explains pressure, diffusion and changes of state, but it ignores the forces between particles and the space the particles occupy, so it fails for real gases at high pressure and low temperature. The Bohr model of the atom explains the line spectrum of hydrogen very well and fails for atoms with more than one electron. Ball-and-stick models show the shape of a molecule and hide the fact that electrons spread over regions of space rather than sitting on sticks.

Models change when new data arrives. Dalton's solid atom gave way to Thomson's atom with electrons in it, then to Rutherford's nuclear atom, Bohr's energy levels and finally the orbital model. Each step kept what the earlier model explained and added what it could not. Older models remain in use where they are good enough, which is why a chemist still draws Lewis diagrams and Bohr-style energy level pictures alongside orbitals.

Moving between the visible and the particulate

Chemistry works at three levels: what can be observed (a solution turns colour, a gas is given off), the particles that explain it (ions, molecules, electrons) and the symbols that represent it (formulae and equations). A good explanation links them. The brown colour of bromine fading when ethene is bubbled through it is the observation; the addition of bromine across the carbon-carbon double bond is the particle-level explanation; $\ce{C2H4 + Br2 -> C2H4Br2}$ is the symbolic form.

How scientific knowledge is built

Scientific knowledge rests on evidence that other people can test. A result is repeatable when the same person gets it again by the same method, and reproducible when a different person or laboratory gets it with a different apparatus. Findings are published with enough detail for others to check them, and peer review means that experts in the field examine the work before it is accepted. Science is international: its units, symbols, naming rules and standard methods are agreed across countries so that results can be compared.

Scientists can be wrong, and sources can be biased. A source may lack detail, may report only the results that suit a sponsor, or may present opinion as fact. When evaluating a claim in the media or a piece of research, the useful questions are who produced the evidence, how it was collected, how large the sample was, whether the conclusion follows from the data and whether anyone with an interest in the answer paid for it.

Chemistry also has limits. It can say what a substance does and what would happen if it is used, but whether a product should be made, or a process used, depends on economic, social, environmental and ethical factors as well as chemical ones.

Planning an investigation

An investigation starts with a question that can be tested. A hypothesis is a testable statement that follows from a theory, such as the prediction that doubling the concentration of a reactant will roughly double the initial rate of a reaction. The investigator then decides:

• the independent variable, which is changed on purpose;
• the dependent variable, which is measured;
• the control variables, which are kept constant so that the test is fair;
• what equipment gives the precision needed, and how many repeats to do.

The following terms describe how good the measurements are.

A set of readings can be precise without being accurate, for example when a balance has not been zeroed and reads 0.12 g too high every time.

Units and scientific notation

Chemistry uses SI units. The base units used most often are the metre (m), kilogram (kg), second (s), kelvin (K) and mole (mol). Other quantities are derived from these, such as the pascal ($\mathrm{Pa} = \mathrm{N\,m^{-2}}$), the joule (J) and the litre or cubic decimetre for volume. Conversions are made with prefixes and need care.

Very large and very small numbers are written in scientific notation, as a number between 1 and 10 multiplied by a power of ten. The Avogadro constant is $6.02 \times 10^{23}\ \mathrm{mol^{-1}}$ and the mass of an electron is about $9.11 \times 10^{-31}\ \mathrm{kg}$. A quantity's unit travels with it through every step of a calculation, and checking that the final unit is the one expected is a quick test of the method (unit analysis).

The number of significant figures in an answer should match the least precise measurement used. A value from a burette is normally read to 0.05 cm³, so a titre of 22.35 cm³ has four significant figures, and a final concentration quoted to six figures would claim precision the experiment never had.

Error

Every measurement has some uncertainty. There are two kinds of error.

Random errors scatter the readings above and below the true value, for example from judging the end point of a titration by eye or from small fluctuations in a thermometer reading. Repeating the measurement and taking a mean reduces their effect.

Systematic errors push every reading the same way, for example from a balance that is not zeroed, a burette that was not rinsed with the solution it holds, or heat lost to the surroundings in a calorimetry experiment so that the temperature rise is always too small. Repeating does not remove a systematic error; it has to be found and corrected, and a change of method or apparatus often reveals it.

An anomalous result is one that lies well outside the pattern of the others. It is identified by comparing it with the repeats or with the trend of a graph, and then checked for a cause before it is left out. Concordant results, in a titration those within 0.10 cm³ of each other, are the ones averaged.

Handling data

Data can be qualitative (colour, smell, whether a precipitate forms) or quantitative (mass, volume, temperature, time). Results are recorded in a table with headings that carry the quantity and unit, such as time / s and volume of gas / cm³. A graph is drawn with the independent variable on the horizontal axis, a scale that uses most of the paper, labelled axes with units, and a smooth line or line of best fit. The shape of a graph gives the relationship: a straight line through the origin means direct proportion, and the gradient of a curve at a point gives a rate.

Trends in tables, such as boiling points down a homologous series or ionisation energies across a period, are used to make predictions, and predictions are then tested. A conclusion is justified only as far as the data supports it.

Primary data is collected by the investigator; secondary data comes from another source such as a data book, a published paper or a database. Both must be assessed for detail, date and bias.

Safe laboratory practice

Before an experiment, a risk assessment identifies each hazard (corrosive, flammable, toxic, oxidising, irritant), how likely harm is, and what reduces it. Eye protection and a laboratory coat are worn, flammable liquids are kept away from flames, volatile or toxic substances are handled in a fume cupboard, acids and alkalis are kept away from skin and eyes and spills are dealt with straight away. Waste is disposed of in the way appropriate to it: dilute acids and alkalis are diluted and neutralised, and solutions of heavy metals and organic solvents are collected. A portfolio or logbook records the plan, raw data, calculations, evaluation and the sources used, in enough detail for another person to repeat the work.

Chemistry in society

Chemists have shaped medicine, materials, food production and energy, and the same knowledge can be misused or cause harm that was not foreseen. Examples include the discovery of penicillin, the Haber process that made fertiliser for food production, plastics that are very useful and do not break down, and the burning of fossil fuels that has changed the climate. Research is influenced by society as well as influencing it: funding, public concern, regulation and industrial demand all decide which problems get investigated. Scientists communicate their results to other scientists in technical language and to the public in plain language, and both need the evidence to be stated honestly.