Motivated Notes

Chemistry Page

Radioactivity is defined as the spontaneous breaking up of certain unstable nuclei, accompanies by the emission of radiation

Radioactivity is detected by a Geiger-Muller tube.

α particles (alpha)

Alpha particles are helium nuclei with a positive charge and little penetrating energy

Alpha decay involves:

• 2 protons lost – atomic number -2
• 2 protons + 2 neutrons lost – mass number -4
• New element formed

β particles (beta)

Beta particles are electrons with a negative charde and greater penetrating ability than alpha particles

Beta decay involves:

• Neutron breaks up into 1 proton and 1 electron
• Proton stays – atomic number +1
• Neutron replaced by proton – mass number no change
• New element formed

γ rays (gamma)

Gamma rays are high energy electromagnetic radiation, with greater penetrating power than beta particles. Neutral charge.

• Gamma rays are a form of energy, not particles – no mass or charge
• No new element formed
• Energy emitted

Radioactive Reactions

Nuclear reactions cause elements to change into other elements – changes in nucleus

Chemical reactions involve changes in distribution of electrons forming compounds – no new elements

Radioisotopes are unstable radiosactive isotopes e.g. carbon-14

Half-life of a radioactive isotope is the time take for half of the atoms in a sample of the isotope to decay

Background radiation is the low level of ionising radiation surrounding us – mainly radon gas from rocks and soil

Uses for radioisotopes

1. Archaeology

• Used to determine age of objects comtaining carbon
• Measure of the changed ration between stable carbon-12 andn unstable carbon-14

2. Medicine

• Cobalt-60 gamma rays used in radiotherapy to treat cancer

3. Food Preservation

• Cobalt-60 preserves food by irradiation

See Scientists for information on Marie Curie and Henry Bequerel

Chemistry Page

It is important to have some knowledge about the major scientists involved in the major discoveries of chemistry. This is often only a short question in Q4 or a 3 marker in a long question, but its an easy way to add marks

Boyle

• An element is a substance that cannot be broken down into simpler materials
• Boyle’s Law

Davy

• Developed electrochemical techniques for breaking down compounds
• Use of electrolysis

Moseley

• Discovery of atomic numbers
• Use of X rays in the discovery of a characteristic positive charge in the nucleus

Dobereiner

• Law of Triads
• Groups of three elements have similar properties

Newlands

• Law of Octaves
• Properties repeat every eight elements

Mendeleev

• Periodic table
• Left gaps for undiscovered elements
• Ordered by atomic weights
• Iodine and tellurium the exception (atomic number later explained this)

Dalton

• First atomic theory based on experimental evidence
• Indirect evidence
• Atoms are indivisible and indestructible
• Atoms of a given element are identical
• Atoms of different elements vary in mass
• Compound contains atoms combined in fixed proportions
• Law of conservation of mass

Crookes

• Cathode rays are particles (moved a small paddle)

Thompson

• Cathode rays have negative charge
• Negatively charged particles are extremely light
• Plum pudding model of the atom – electrons embedded in a positively charged sphere

Stoney

• Suggested name “electron”

Millikan

• Oil drop experiment
• Electrical force used to suspend drops mid air
• Electron charge accurately measured

Rutherford

• Alpha particles fired at gold foil
• Large number not deflected (empty space)
• Very small numberbounced back (concentrated positive charge)
• Discovery of nucleus
• Positive charge confined to nucleus
• Discovery of proton

Chadwick

• Aplha particles fired at beryllium
• Alpha particles >>>knocks out>>> neutrons in Be >>>knocks out>>> protons in parafin >>>detected
• Discovery of neutron

Bequerel

• Study of chemicals which emitted light and X rays, which had been exposed to white light
• Photographic plate in black paper with crystals of uranium – placed in sunlight – image of crystals developed
• Without sunlight, image still developed
• Crystals emitted radiation which caused image
• Radiation also ionised air – air conducts electricity

Curie

• Coined term “radioactivity”
• Discovered that amount of radiation depended on proportion of uranium
• Isolated new elements (polonium and radium) which are more radioactive than uranium
• 1903 Nobel Prize (with Bequerel and her husband) – discovery of radioactivity
• 1911 Nobel Prize – discovery of elements

Chemistry Page

Intramolecular Bonds the bond within the molecule that holds the atoms together

Intermolecular Forces forces between molecules

Types of Intermolecular Forces

1. Van der Waals Forces

• Weak attractive forces caused by the movement of electrons within a molecule
• Randomly moving electrons may at one point be nearer to one atom than another
• Temporary dipole formed
• If two molecules with opoositely charged temporary dipoles are near each other an attractive force will exist E.g. H(-)H(+) — H(-)H(+)
• Sometimes a temporary dipole in one molecule will induce a dipole in another
• Results in greater boiling point
• Greater number of electrons = greater number of temporary dipoles = greater boiling point
• Occurs in non-polar and polar molecules

2. Dipole – Dipole Interactions

• Negative end of one dipole is attracted to the positive end of another
• Permanent forces (due to polarity)

• Stronger than van der Waals forces, weaker than hydrogen bonding
• Results in greater boiling point
• Occurs in polar molecules

3. Hydrogen Bonding

• Occurs when hydrogen is bonded to highly electronegative atoms (O, N, F)
• Strong – Requires more energy to break than regular covalent bonds – Higher boiling point
• Oxygen (-) has an attraction to neighbouring hydrogen (+) molecules, and vice versa

• Covalent bond > Hydrogen bond > Dipole-dipole interactions > van der Waals forces
• H2S should have a higher boiling point (greater molecular mass) but the H-S bond is much less polar than the O-H bond
• Hydrogen bonding only occurs between hydrogen and small atoms (O, N, F) because the charge is much more concentrated and therefore, more effective

Chemistry Page

Linear Shape

• BeH2
• 180°
• 2 bonding pairs

Trigonal Planar

• BF3
• 120°
• 3 bonding pairs

Tetrahedral

• CH4
• 109.5°
• 4 bonding pairs

V-Shaped

• H2O
• 104.5º
• 2 bonding pairs and 2 lone pairs

Pyramidal

• NH3
• 107º
• 3 bonding pairs and 1 lone pair

Electron Pair Repulsion Theory the electron pairs in the outer shell of the central atom repela each other and end up as far apart as is geometrically possible

Since lone pairs are closer to the nucleus of the central atom, they are closer together, so their mutual repulsion is greater than that between bond pairs >>>> distorted shape

Order of Strength of Repulsions

1. lone pair:lone pair
2. lone pair:bond pair
3. bond pair: bond pair

Symmetry and Polarity

Symmetrical Atoms

• Linear
• Tetrahedral
• Trigonal Planar
• Non- polar (even if bonds are polar)

E.g. BF3

Boron (central atom) is slightly positive >>> centre of positive charge is here

Each flourine is slightly negative >>> centre of positive charge is the central point between these atoms

Centre of positive coincides with centre of negative >>> non-polar

Non-Symmetrical Atoms

• V-Shaped
• Pyramidal
• Polar

E.g. NH3

Nitrogen (central atom) is slightly negative >>> centre of negative charge will be at apex of pyramid

Hydrogen atoms areslightly positive >>> centre of positive charge at base of pyramid

Centre of negative does not coincide with centre of positive >>> polar

Chemistry Page

Catalysts work by providing an alternative reaction route with a lower activation energy.

Intermediate Formation Theory of Catalysis

The reactant molecules and the catalyst form and unstable intermediate complex that breaks up to form products and regenerate the catalyst

How does the reaction of aqueous cobalt (II) chloride between H2O2 and potassium sodium tartrate give evidence for the intermediate formation theory of catalysis?

1. Initial solution is pink
2. During reaction, there is a colour change from pink to green [intermediate complex formed]
3. Frothing and bubbling reaction [fast rate of reaction - products formed]
4. Reaction returns to pink [catalyst regenerated]

Surface Adsorption Theory of Catalysis

The reactant molecules adsorb onto solid catalyst where the greater local concentration leads to a quick reaction – bonds formed must be strong enough to adsorb and increase concentration, but weak enough to decompose quickly and form products

How does the oxidation of methanol using a hot platinum catalyst provide evidence for the surface adsorption theory of catalysis?

1. Series of mild explosions and glowing platinum [fast exothermic rate of reaction]
2. H atoms are removed more quickly by reactant adsorbing to surface of catalyst which weakens and breaks bonds.

Catalytic Poisons

When the active sites of the catalyst are blocked by substances bonding to it more strongly than the reactants, the catalyst is poisoned

e.g. Lead, arsenic, sulfur

Chemistry Page

A catalyst is a substance that alters the rate of a chemical reaction but is not consumed in the reaction

Heterogeneous catalysis involves reactants in different physical states i.e. liquid reacting with solid e.g.  MnO2 on H2O2

Homogeneous catalysis involves reactants in same physical states i.e. both in aqueous solution e.g. potassium iodide on H2O2

Enzymes are homogeneous biological catalysts e.g. amylase on starch

Autocatalysis occurs when the product of a reaction increases the reaction rate i.e. reaction makes its own catalysts e.g. reduction of manganate (VII) ions with Fe 2+

Activation Energy

Activation energy is the minimum energy required by particles colliding to cause a reaction

Exothermic reactions give out heat [Energy of: products < reactants ]

Endothermic reactions take in heat [Energy of: products > reactants ]

Average kinetic energy of particles is directly proportional to the temperature – greater the energy, greater the speed, greater the reaction rate. This means:

1. the number of collisions per second increases
2. each collision is more energetic and a higher proportion of collisions has the necessary activation energy

The second factor is more significant

Pollution and Catalytic Converters

Engines produce harmful CO, NO, NO2 and hydrocarbons.

Catalytic converters (e.g. palladium and platinum) speed up reactions to reduce harmful emissions

e.g. 2CO + O2 >>> 2CO2

 2CO + 2NO >>> 2CO2 + N2

This is an example of heterogenous catalysis.