Chemistry - Unit 1

1. Gaseous sample is injected into the mass spectrometer and is first vaporised.

2. The vapour is bombarded with high energy electrons, which collide with the atoms in the sample. This knocks one or more electrons out of the atoms within the sample to create positive ions. This causes no significant difference to their mass.

3. Now they are charged they can be accelerated by an electric field.

4. They pass through a velocity selector which makes them travel at the same velocity, meaning any effect caused by the magnetic field in the next step is because of different charges or masses of the ions and not their different speeds.

5. The ions enter a magnetic field which deflects them. The amount they are deflected depends on the mass and charge of the ion: heavier ions are deflected less than lighter ions and ions with a small positive charge are deflected less than those with a bigger positive charge. The strength of the magnetic field is steadily increased.

6. The detector detects how many ions pass through the machine at each magnetic field setting and each setting of the velocity selector. It shows how many ions of each different mass:charge ratio there are in a sample.

Ionisation Energies

This is the complete removal of an electron from an atom. It is an endothermic process because work must be done on an electron to overcome the attractive force between it and the nucleus.

The first ionisation energy is a measure of how tightly the outer electron is attracted to the nucleus. The less tightly it is bound, the more reactive the element will be. 

Subshells

Quantum mechanics tells you that each shell may contain a number of subshells. They are described by the letters: s, p, d, f.

Shell 1 is closest to the nucleus so it takes the most energy to remove electrons in this shell. Within a shell, the subshells have different energies, with electrons in the lowest energy subshells being closest to the nucleus: s < p < d

Each subshell consists of one or more orbitals, which is the region where the electrons are most likely to be found.

S-block elements

The s block is made up of group 1 and 2. This is because the outermost electrons are in the s subshells (containing either 1 or 2 electrons). These electrons are easily lost to form positive ions. Hydrogen and Helium are also s-block elements but are very different to other s-block elements so they are often treated as a serperate group.

D-block elements

This block lies between group 2 and 3, or the transition metals. They are much less reactive than elements in group 1 or 2 because inner d orbitals are being filled while the outer s subshell is full. These metals all conduct electricity and heat and many are shiny and hard (with mercury being the exception).

P-block elements

This block is made up of group 3, 4, 5, 6, 7 and 8. For these elements electrons are being added to p orbitals in the outer shell. The p block contains all the non-metals and metalloids as well as some metals. Noble gases in group 8 are extremely unreactive as a result of their completely filled shells. 

Moving across the periodic table, elements gain electrons. Moving down a group, elements gain electron shells. This changes the size of the atoms, which affects both their physical and chemical properties.

The atomic radius decreases across a period - as you move across a period, the nuclear charge becomes increasingly positive as the number of protons in the nucleus increases. Although the number of electrons increases, they are all in the same shell which means they are attracted more strongly to the nucleus, thus reducing the atomic radius across a period.

The atomic radius generally increases down a group - the outer electrons enter new energy levels passing down the group so although the nucleus also gains positive protons, the electrons are further away and are being shielded by more electron shells. This means the atomic radius increases as a result of them not being held so tightly.

Ionic bonding

These are usually formed when metals bond with non-metals. Oppositely charges ions are formed, which are held together by a strong electrostatic force of attraction. The ions are held in a n arrangement known as a giant lattice structure. An ion attracts ions with the opposite charge and repels ions with the same charge. The forces exerted by the ions  act equally, holding the ions together tightly.

Covalent bonding

This is a bond formed by electron sharing. This is especially true for elements in the middle of the periodic table, for which loss or gain of three or four electrons would require a great deal of energy. 

Enthalpy change of reaction - an exothermic reaction that releases energy to the surroundings in the form of heat.

Standard enthalpy change of combustion - when one mole of a substance is completely burnt in oxygen under standard conditions.

Standard enthalpy change of formation - when one mole of a compound is formed from it's elements under standard conditions.

Standard enthalpy change of atomisation - when one mole of its atoms in the gaseous state is formed from the element under standard conditions.

Standard enthalpy change of neutralisation - when one mole of acid is neutralised by an alkali in their standard states at 25oC in solutions containing 1 mol dm-3

ΔHf = standard enthalpy change of formation

ΔHat = standard enthalpy change of atomisation (where elements become gas)

ΔEm1 = first ionisation energy (removal of one electron, creating a positive ion)

ΔEm2 = second ionisation energy (removal of a second electron, creating a 2+ charged ion)

ΔEaff = first electron affinity (making a negative ion)

ΔHlat = all the enthalpy changes can be measured, ehich allows you to calculate the lattice energy of the compound.

Moles = mass / molar mass

Molarity = moles of solute / volume of solution

Percentage by Mass = (mass of solute / mass of solution) x 100

Percentage by Volume = (volume of one component / total volume) x 100

Percentage Yield = (actual yield / theoretical yield) x 100

Concentration = (mass of component / mass of solution) x 1,000,000 parts per million

Atom Economy = (mass of atoms in product / mass of atoms in reactants) x 100

Hazard - the hazard presented by a substance or an activity is it's potential to do harm. This potential is absolute.

Risk - the risk associated with a particular hazard is the chance that it will actually cause harm. It is effected by a number of things, including the nature of the hazard involved and the exposure to it. The level of exposure is dependent on factors such as the expertise of the person working with the chemical, the volumes being used and the protective clothing and equipment used. 

Ways of reducing risk

• Working on a smaller scale
• Taking specific precautions
• Careful use of safety measures
• Changing the conditions under which a reaction takes place
• Using alternative methods with less hazardous substances

Hyrdocarbons can be split into the aliphatic hydrocarbons (straight- and branched-chain molecules), the alicyclics (hydrocarbons with closed rings) and arenes (carbon atoms stabilised by delocalised of electrons). All of these groups have certain properties in common:

• They are insoluble in water
• They all burn, and in sufficient oxygen they give carbon dioxide and water as the only combustion products.

Homologous Series        General Formula

Alkanes                           CnH2n+2

Alkenes                           CnH2n

Alkynes                           CnH2n-2

Alcohols                          CnH2n+2O

This is a saturated hydrocarbon. The single most important source of the alkane is the fossil fuels. Over great lengths of time, immense heat and pressure has formed crude oil and petroleum from decomposing material. The fossilised remains of land plants form coal by a similar process. Giant prehistoric 'fern forests' were largely destroyed and the remains of these plants, fossilised after million of years buried underground, form coal.

Crude oil is a mixture of a number of different hydrocarbons and needs to go through primary distillation to be turned into the different fractions. 

Crude oil from different source contains different proportions of each alkane. The lighter fractions are in greater demand as they are used for fuels and as raw chemicals in industry, however some of the heavier fractions are of less use and can often make up almost 50% of the products. The demand for lighter fractions is enormous, and pressure to find heavier fractions more useful is constant.

To address the imbalance, cracking is used. Heating long-chain alkanes causes the chains to split and form shorter-chain molecules and alkenes, particularly ethene. Catalytic cracking uses catalysts while cracking the long chain alkane, so it can be carried out at a lower temperature. This makes the process less expensive.

Combustion occurs when alkanes are heated in a plentiful supply of air. Alkanes are energetically unstable with respect to their oxidatioon products, water and carbon dioxide, so once lit will burn completely. They only burn when in the gaseous state, so they must be vaporised before they will burn.

Combustion of alkanes is important in our every day life as it is used to generate electricity, to fuel fires, to provide central heating, for cooking and for transport. This is an exothermic change because the release of energy results from the fact the energy needed to break the bonds in the chemical reaction of combustion is less than the energy returned when the new bonds are made as the products of cobustino form. 

Incomplete combustion occurs if an alkane is burnt without plenty of oxygen. The products of incomplete combustion can include carbon or carbon monoxide, a potentially fatal gas. Many people who use heaters and boilers may be unaware of the potential hazard of an inadequate supply of oxygen to a heater, or an inadequate venting of the waste gases.

Bond Fission occurs in two ways:

Homolytic Fission - this involves equal sharing out of the electrons in the bond, so each of the participants in the bond receives one electron when thebond splits. The electron gained by each atom is indicated by a dot, and is then known as a free radical. Due to the unpaired electrons, they are extremely reactive which is because the unpaired electron have a very strong tendancy to pair up with an electron from another substance.

Heterolytic Fission - this involved an unequal sharing of the electrons of the covalent bond, so that both electrons go to one atom. This results in tro charged particles - the atom receiving the electrons gaining a negative charge and the other atom gaining a positive charge. This is usually seen when the covalent bond already has a degree of polarity.

Alkanes react with chlorine, but only with an input of sunlight or UV light.  The light provides the input of energy needed to break the hydrocarbon bonds. This is typical of a reaction that takes place by a free-radical mechanism. This is an example of a substitution reaction, with chlorine atoms substituting for the hydrogen atoms in the methane molecule.

Initiation - The Cl-Cl bond is easier to break than the C-H bond and light provides the energy to split the chlorine molecule into atoms. The splitting is an example of homolytic fission, because both atoms receive one electron from the broken bond, meaning that they are free radicals and are very reactive.

Propagation - The free radicals then react with a methane molecule, forming hydrogen chloride, HCl, and a mehtyl free radical, CH3. The methyl free radical then forms with another chlorine molecule to form chloromethane, CH3Cl, and another chlorine free radical. This process is repeated hundreds of times, causing a chain reaction. This carries on until the termination step.

Termination - This is a reaction between two free radicals - a highly exothermic proceses. It happends every few thousand reactions. 

Alkenes are unsaturated hydrocarbons, meaning they contain double bonds. Ethene is particularly important in chemical industry. It is used for the production of polymers, detergents, solvents and many other chemicals.

The σ bond represents the electron cloud, which is symmetrical, in a carbon-carbon single bond. A σ bond allows rotation.

A carbon-carbon double bond has a σ bond as well as a π bond. The π bond does not allow rotation around the axis, which has a big effect on the structure. This arrangement of electron density also explains two properties of molecules containing carbon-carbon double bonds:

• They are reactive, due to the attraction of positively polarised groups to the electron-rich π bond within the double bond. These electron-seeking groups are known as electrophiles
• Many unsaturated compounds show geometric isomerism. Rotation about the double bond is not possible without breaking the π bond.

Geometric isomers are molecules with a lack of free rotation around the double bond. The traditional way of naming isomers is known as cis-trans isomerism.

Cis- = the same side

Trans- = the oppisite side

The cis-isomer has the methyl groups on the same side of the double bond, and both hydrogen atoms are on the other. In the trans-isomer, the two methyl groups are on oppisite sides of the double bond.

The cis-trans system doesn't work for all geometric isomers. Although cis-trans isomers will still be used, the E-Z system is better because it works for all geometric isomers.

The groups around a carbon-carbon double bond are ranked based on their atomic number. The atom with the highest atomic number has the highest priority. You then look at the atom with the highest atomic number on one side of the carbon-carbon double bond and compare it with the higher-priority group at the other end of the double bond.

If the higher priority groups are on the same side, the isomer is a Z-isomer. If they higher priority groups are on the oppisite sides of the rigid bond then the isomer is an E-isomer.

Reactions of Alkenes with Hydrogen

Alkenes do not react with hydrogen under standard conditions, they need a nickel catalyst to be present and a moderately high temperature. They undergo an addition reaction with hydrogen to form the corresponding alkane.

Reactions of the Alkenes with the Halogen Halides

The double bond in alkenes reacts with hydrogen halides, producing corresponding halogenoalkanes. The addition of hydrogen halides to alkenes can lead to two possible products; the minor product and the major product.

Markovnikov's Rule: When HX adds across an asymmetric double bond, the major product formed is the molecule in which hydrogen adds to the carbon atom in the double bond with the great number of hydrogen atoms already attached to it. 

Polymers are made up of monomers and have the following properties:

• The average length of the polymer chain - strength and melting temperature increase with the length of the chain.
• Branching of the chain - branched chains cannot pack together as regularly as straight chains, so polymers with highly branched chains tend to have low strength and low melting temperature and density.
• Presence of intermolecular forces between chains -  if there are strong intermolecular forces between chains the polymer will be strong and tend to have a high melting temperature. The strength of the forces is largely determined by the side groups on a polymer chains.
• Cross links between chains - these chemical bonds holding chains together make a polymer very rigid, hard and brittle, usually with a very high melting temperature.

Problems

• The energy costs - energy is used for both production and manufacturing of the polymers and the products made from them.
• Resources used - polymers are made from alkenes from fossil fuels, so they use up valuable resources. This means there is a limited supply and ever-rising costs of fossil fuels which gives an incentive to develop different polymers from other sources.
• Disposal problems - synthetic polymers are not easy to dispose of, which causes waste problems. When they burn, a variety of toxic gases are produced, including hydrogen chloride and hydrogen cyanide.

Solutions

• Renewable energy sources - if electricity generated from nuclear, solar or wind power is used instead of fossil fuels, the environmental effect is reduced. 
• Reducing the use of polymer products - reusing polymer products or using other materials decreases the energy cost
• Recycling - this reduces the demand for new plastics.