Atoms and Reactions

Question 1

What are isotopes?

Answer 1

Atoms of the same element with the same number of protons and electrons, but different number of neutrons and so different masses

Question 2

What are the relative masses of protons, neutrons and electrons?

Answer 2

• Proton = 1.0
• Neutron = 1.0
• Electron = 1/1836

Question 3

What are the relative charges of protons, neutrons and electrons?

Answer 3

• Proton = 1+
• Neutron = 0
• Electron = 1-

Question 4

Molecular ions: radicles

Answer 4

Question 5

What is a mole? (mol)

Answer 5

A mole is a unit used to measure the amount of substance (n). One mole of a substance contains as many units as there atoms in 12g of the carbon-12 isotope

Question 6

What is the relative atomic mass?

Answer 6

The (weighted) mean mass of an atom or element compared with one the twelfth of the mass of an atom of carbon-12

Question 7

What is the relative isotopic mass?

Answer 7

The mass of an atom of an isotope compared with one twelfth of the mass of an atom of carbon-12

Question 8

What is the unit of atomic masses?

Answer 8

u: unified atomic mass unit
- The mass of a carbon-12 atom is defined as 12 u

Question 9

What is a mass spectrometer?

Answer 9

A mass spectrometer is a piece of apparatus that can be used to find out about molecules. It can be used to:
- Indentify an unknown compound
- Find the relative abundance of each isotope of an element
- Determine structural information about molecules

Question 10

What is mass spectrometry?

Answer 10

A mass spectrometer determines the mass of a molecule or isotope by measuring the mass-to-charge ratio of ions. It does this to by causing substances to become positive ions. These positive ions are then passed through the apparatus and separated according to their mass and charge. A computer within the mass spectrometer analyses the data on the ions present and produces a mass spectrum (similar to a complex bar graph)

Question 11

Answer 11

Question 12

Fifth Century BCE (Before the Common Era) - the Greek atom

Answer 12

The Greek philosopher Democritus developed the first idea of the atom. He suggested that you could divide a sample of matter only a certain number of times. Eventually, he believed, you would end up with a partice that could not be split any further. Democritus called this particle ‘átomos’, which is Greek for ‘indivisible’

Question 13

Early 1800s - Dalton’s atomic theory

Answer 13

In the early 1800s, John Dalton developed his atomic theory. This stated that:
- atoms are tiny particles that make up elements
- atoms cannot be divided
- all atoms of a given element are the same
- atoms of one element are different from those of every other element

Dalton used his own symbols to represent atoms of different elements. He also developed the first table of atomic masses. Many of Daltan’s predictions still hold true and can be applied to chemistry today

Question 14

1897-1906 - Joseph John (J.J.) Thomson discovers electrons

Answer 14

Scientists had recently discovered cathode rays, which were emitted from cathode ray tubes. Thomson discovered that cathode rays were a stream of particles with the following properties:
- They had a negative charge
- They could be deflected by both a magnet and an electric field
- They had a very, very small mass

Cathode rays were, in fact, electrons. Thomson concluded that they must have come from within the atoms of the electrodes themselves. The idea that an atom could not be split any further, proposed by the andient Greeks and by Dalton, had been disproved. Thomson proposed that atoms are actually made up of negative electrons moving around in a ‘sea’ of positive charge. This model is commonty called the plum pudding atom. In Thomson’s atom, the overall negative charge is the same as the overall postive charge. This means that the atom is neutral with no overall charge

Question 15

1909 -11 - Ernest Rutherford’s gold-leaf experiment

Answer 15

In 1909, Rutherford and two of his students, Hans Geiger and Ernest Marsden, carried out an experiment where they directed α-particles (alpha particles) towards a sheet of very thin gold foil. They measured any deflection (change in direction) of the particles. Rutherford calculated that a plum-pudding atom would hardly deflect α-particles at all

The results were astonishing:

• Most of the particles, as expected, were not deflected at all
• However, a small percentage of particles were deflected through large angles
• Very few particles were actually deflected back towards the source

In 1911, he proposed the following new model for the atom based on these results:

• The positive charge of an atom and most of its mass are concentrated in a nucleus, at the centre
• Negative electrons orbit this nucleus, just as the planets orbit the Sun
• Most of an atom’s volume would be space between the tiny nucleus and the orbing elections
• The overall positive and negative charges must balance

Rutherford had proposed the nuclear atom and disproved the plum-pudding model

Question 16

1913 - Niels Bohr’s planetary model and Henry Moseley’s work on atomic numbers

Answer 16

In 1913, the Danish physicist Niels Bohr altered Rutherford’s model to allow electrons to follow only certain paths. Otherwise, electrons would spiral into the nucleus. This was the planetary atom, in which electrons orbited a central nucleus ‘sun’ in ‘shells’

Bohr’s model helped to explain some periodic properties, such as:

• spectral lines seen in emission spectra
• the energy of electrons at different distances from the nucleus

In the same year, Henry Moseley discovered a link between X-ray frequencies and an element’s atomic number (i.e. its order in the periodic table). At the time, Moseley couldn’t explain this

Question 17

1918 - Rutherford discovers the proton

Answer 17

Rutherford’s discovery of the proton was able to explain Moseley’s finding that an atom’s atomic number was linked to X-ray frequencies. We now know that the atomic number tells us the number of protons in an element’s atom

Question 18

1923-26 - wave and particle behaviour

Answer 18

In 1923, the French physicist Louis de Broglie suggested that particles could have the nature of both a wave and a particle

In 1926, the Austrian physicist Erwin Schrodinger suggested that an electron had wave-like properties in an atom. He also introduced the idea of atomic orbitals

Question 19

1932 - James Chadwick discovers the neutron

Answer 19

In 1932, an English physicist called James Chadwick observed a new type of radiation emitted from some elements. He showed that this new type of radiation was made up of uncharged particles with approximately the same mass as a proton. These uncharged particles became known as neutrons, because they have no charge

Question 20

Modern day

Answer 20

It is now thought that protons and neutrons themselves are made up of even smaller particles called quarks. Our understanding of the atom is likely to progress with time as science advances further and further

Question 21

What is amount of substance?

Answer 21

It is the quantity that has moles as its unit

Question 22

What is Avogadro’s constant? (N_A)

Answer 22

The Avogadro’s constant is the number of atoms per mole of the carbon-12 isotope
- 6.02 x 10²³ mol^-1

(it’s the number of particles found in one mole)

Question 23

number of particles =

Answer 23

number of moles x Avogadro’s constant

Question 24

What is the Molar mass? (M - or RFM)

Answer 24

The mass per mole of a substance (gmol^-1)

Question 25

1 mole mass rule:

Answer 25

The mass of 1 mole of a substance is the same as the relative molecular mass of the substance in grams (e.g. the RFM of carbon is 12, so 1 mole of carbon is 12g)

Question 26

Formula linking mass, moles and molar mass:

Answer 26

Question 27

What is the empirical formula?

Answer 27

The simplest whole number ratio of atoms of each element in a compound

Question 28

How to calculate the empirical formula:

Answer 28

• Divide the mass/percentage of each element by its molar mass
• Divide each answer by the smallest answer
• This is your ratio - if necessary, multiply the answer by a suitable value to make sure the ratio is in whole numbers

Question 29

What is the molecular formula?

Answer 29

The actual number of atoms of each element in a compound

Question 30

What is the empirical mass?

Answer 30

The total mass of all the atoms in the empirical formula

Question 31

Avogadro’s law:

Answer 31

At RTP (room temperature and pressure):
- 1 mole of gas occupies approximately 24.0 dm³ (24 000 cm³)
- the volume per mole of gas molecules is 24.0 dm³mol^-1

This is because he hypothesised that 1 mole of gas takes up the same space of space as 1 mole of any other gas, and that equal numbers of gas occupy the same volume

Question 32

What is the molar gas volume?

Answer 32

• The molar gas volume is the volume per mole of a gas. The units are dm³mol^-1
• At RTP, the molar volume of a gas is 24.0 dm³mol^-1

Question 33

equation to find the volume of a gas at RTP:

Answer 33

Question 34

What is used to calculate the volume of gases when they are not at RTP?

Answer 34

The ideal gas equation

Question 35

The ideal gas equation:

Answer 35

Question 36

p =
V =
n =
R =
T =

Answer 36

p = pressure (Pa)
V = volume (m³)
n = number of moles (mol)
R = gas constant (8.314 Jmol^-1K^-1)
T = temperature (K)

Question 37

How to convert between units of volume:

Answer 37

Question 38

How to convert between units of temperature:

Answer 38

0K = -273 °C
°C -> °K = +273

Question 39

How to convert between units of pressure:

Answer 39

1 atm = 101325 Pa
1 KPa = 1000 Pa

Question 40

What is the concentration?

Answer 40

The concentration of a solution is the amount of solute, in mol, dissolved per 1 dm³ (1000 cm³) of solution. Concentrations are measured in moles per cubic decimetre (mol dm^-3)

Question 41

How do you find the amount in mol, of a solution with a known concentration?

Answer 41

Question 42

What is a standard solution?

Answer 42

A solution of known concentration. Standard solutions are normally used in titrations to determine unknown information about another substance

Question 43

How do you prepare a standard solution?

Answer 43

• Using the weigh by difference method (mass of the solute and weighing boat - mass of the weighing boat), weigh out the solute on a 2 decimal place scale
• Completely dissolve the solute in solvent in a beaker, stirring with a glass rod. Transfer the solution to a volumetric flask using a funnel and rinse the beaker repeatedly, using more solvent, adding the rinsings to the flask
• Add solvent to the flask, but do not fill it all the way up to the graduation line
• Carefully add solvent drop by drop up to the line on the flask, using a pipette, until the bottom of the meniscus sits exactly on the graduation mark on the flask. If the meniscus goes over the graduation line, you must throw it away and start again
• Finally, you must mix your solution thoroughly, by inverting the flask several times

Question 44

What is a mass concentration?

Answer 44

• The mass concentration of a solute is the mass dissolved in 1 dm³ of solution
• Mass concentrations are measured in g dm³

Question 45

What is a concentrated solution?

Answer 45

A solution with a large amount of solute per dm³

Question 46

What is a dilute solution?

Answer 46

A solution with a small amount of solute per dm³

Question 47

Molar solutions

Answer 47

‘M’ means Molar and refers to a solution with a concentration in moles per cubic decimetre (mol dm³)

Question 48

Why may the percentage yield of products not be 100% in practical work?

Answer 48

• The reaction may be at equilibrium and may not go to completion
• Side reactions may occur, leading to by-products
• The reactants may not be pure
• Some of the reactants or products may be left behind in the apparatus used in the experiment
• Separation and purification may result in the loss of some of the product

Question 49

How do you calculate percentage yield?

Answer 49

Question 50

What is atom economy: by-products of a reaction

Answer 50

• By-products are usually waste and usually have to be disposed of. This is costly, poses potential environmental problems, and wastes valuable resources
• By products may be sold on or used elsewhere in the chemical plant. This practice is likely to increase in the future, as we become increasingly concerned about preserving the Earth’s resources and minimising waste
• Atom economy considers not the only desired product, but also all the by-products of a chemical reaction. It describes the efficiency of the reaction in terms of all the atoms involved. A reaction with high atom economy uses atoms with minimal waste

Question 51

How to find the atom economy:

Answer 51

Question 52

How can atom economy can benefit society?

Answer 52

We are now much more aware of our environment. By using processes with a higher atom economy, chemical companies can reduce the amount of waste produced. This is good news, as it has been suggested that about 5-10% of the total expenditure of a chemical company goes on waste treatment. Reactions with high atom economies make processes much more sustainable as they can be maintained at a productive level without completely depleting resources

Question 53

Atom economy: types of reaction

Answer 53

• Addition reactions have an atom economy of 100%
• Reactions involving substitution or elimination have an atom economy of less than 100%

Question 54

What is a hydrated compound?

Answer 54

A crystalline compound containing water molecules

Question 55

What is an anhydrous compound?

Answer 55

The form of a compound containing no water molecules

Question 56

What is water of crystallisation?

Answer 56

Water of crystallisation refers to water molecules that form an essential part of the crystalline structure of a compound

Question 57

Acid-base titrations

Answer 57

An acid-base titration is a special type of volumetric analysis, in which you react a solution of an acid with a solution of an alkali

You must know the concentration of 1 of the 2 solutions. This is usually a standard solution

Question 58

How to carry out a titration:

Answer 58

• Using a pipette, add a measured volume of a solution of known concentration to a conical flask. Add a suitable indicator
• Place the other solution in a burette
• Add the solution in the burette to the solution in the conical flask until the reaction has just been completed (colour change reached) - this is called the end point of the titration. Measure the volume of the solution added from the burette
• You now know the volume of one solution that exactly reacts with the volume of the other solution
• After a rough titre, repeat until concordant results (within 0.1 cm³ of each other) have been found

Question 59

Common acid-base indicators

Answer 59

Question 60

Oxidation numbers:

Answer 60

Chemists use oxidation numbers to keep track of how electrons are used in bonding

Question 61

Oxidation numbers

Answer 61

Question 62

An exception to the oxidation number rules:

Answer 62

When bonded to fluorine, oxygen has an oxidation number of +2

Question 63

Oxidation numbers in compounds

Answer 63

The sum of the oxidation numbers must equal the overall charge of 0 of the compound

Question 64

Oxidation numbers in molecular ions:

Answer 64

The sum of the oxidation numbers must equal the overall charge of the molecular ion

Question 65

Oxidation numbers: Roman numerals

Answer 65

When an element has its valency number specified in Roman numerals, its oxidation number is the number in brackets after the compound/element it describes - e.g. iron (II) chloride

Question 66

What is oxidation?

Answer 66

• The gain of oxygen
• The loss of electrons
• An increase in oxidation number
• The loss of hydrogen

Question 67

What is reduction?

Answer 67

• The loss of oxygen
• The gain of electrons
• A decrease in oxidation number
• The gain of hydrogen

Question 68

What is a redox reaction?

Answer 68

A reaction where both reduction and oxidation take place

Question 69

How to calculate percentage uncertainty/error?

Answer 69

Question 70

Acids in aqueous solution:

Answer 70

• When an acid is added to water, the acid releases H⁺ ions (also known as protons) into solution
• HCl_(g) —> H⁺_(aq) + Cl^-_(aq)

The H⁺ ion is the active ingredient in acids:

• An H⁺_(aq) ion is responsible for all acid reactions
• One definition of an acid is a proton donor

Question 71

Acids and dissociations:

Answer 71

• Strong acids are very good at giving up H⁺ ions
• They are said to fully dissociate
• Weak acids are not very good at giving their H⁺ ions away
• Once H⁺ ions are released from weak acids, they are quickly taken back again
• Weak acids are very good accepting these H⁺ ions back, whereas strong acids are not
• Weak acids only partially dissociate

Question 72

What are bases:

Answer 72

The opposite of an acid:

• They are proton (H⁺) acceptors
• They neutralise acids

Question 73

Alkalis in solution:

Answer 73

• An alkali is a special type of base that is able to dissolve in water to form aqueous hydroxide ions, OH^-_(aq)
• NaOH_(s) + aq —> Na⁺_(aq) + OH^-_(aq)

Question 74

Acids and alkalis in solution:

Answer 74

In solution, the hydroxide ions from alkalis neutralise the protons from acids, forming water:

H⁺_(aq) + OH^-_(aq) —> H₂O_(l)

Question 75

Ammonia as a weak base:

Answer 75

• Ammonia (NH₃) is a gas that dissolves in water to form a weak alkaline solution
• Ammonia is a weak base because only a small proportion of the dissolved NH₃ reacts with the water

Question 76

Atmospheric substances:

Answer 76

Some substances can behave as acids and bases. These are known as atmospheric substances. An example of this is an amino acid molecule, such as glycine, which contains:
- a carboxyl acid group, COOH, which is able to donate a proton
- an amino basic group, NH₂, which could accept a proton