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# A-level Chemistry/OCR (Salters)/The Elements of Life

The Elements of Life is the first unit in the Salters Advanced Chemistry course. It reviews essential topics from GCSE-level chemistry and introduces new content, including:
* the structure, properties, and behaviour of individual atoms
* the interaction of atoms with electromagnetic radiation
* the three main types of chemical bonding
* chemical patterns in the periodic table
* the shapes of molecules

## Chemical Storylines sections

* EL1: What are we made of?
* EL2: Take two elements...
* EL3: Where do the chemical elements come from?
* EL4: Looking for patterns in elements
* EL5: The molecules of life
* EL6: Summary

## Chemical Ideas sections

* 1.1 Amount of substance
* 1.2 Balanced equations
* 2.1 A simple model of the atom
* 2.2 Nuclear reactions
* 2.3 Electronic structure: shells
* 3.1 Chemical bonding
* 3.3 The shapes of molecules
* 6.1 Light and electrons
* 11.1 Periodicity
* 11.2 The s block: Groups 1 and 2

## Activities

* EL1 How do we know the formula of a compound?
* EL2.1 How much iron is in a sample of an iron compound?
* EL2.2 Making the most of your study of chemistry
* EL3.1 Investigating the chemistry of Group 1 and Group 2 elements
* EL3.2 How do the physical properties of elements change across a row in the Periodic Table?
* EL3.3 Check your notes on The Elements of Life : Part 1
* EL4.1 How do we know about atoms?
* EL4.2 Isotopic abundance and relative atomic mass
* EL4.3 Investigating a spectroscopic technique
* EL4.4 Radon in the rocks
* EL5 Balloon molecules
* EL6 Check your notes on The Elements of Life : Part 2

## External links

* EL unit map
* EL Check Your Notes #1
* EL Check Your Notes #2
# A-level Chemistry/OCR (Salters)/Substances

| Substance | Chemical formula | Molar mass / g mol$^{−1}$ |
| aspirin | C$_9$H$_8$O$_4$ | 180.16 |
| benzene | C$_6$H$_6$ | 78.11 |
| ethane | C$_2$H$_6$ | 30.07 |
| methane | CH$_4$ | 16.04 |
| methanol | CH$_4$O | 32.04 |
| ethanol | C$_2$H$_6$O | 46.07 |
| buckminsterfullerene | C$_{60}$ | 720.64 |
| thionyl chloride | SOCl$_2$ | 118.97 |
# A-level Chemistry/OCR (Salters)/The Atmosphere

The Atmosphere is the fourth unit in the Salters Advanced Chemistry course.

## Chemical Storylines sections

* A1: What's in the air?
* A2: Screening the Sun
* A3: Ozone: a vital sunscreen
* A4: The CFC story
* A5: How bad is the ozone crisis
* A6: Trouble in the troposphere
* A7: Keeping the window open
* A8: Focus on carbon dioxide
* A9: Coping with carbon
* A10: Summary

## Chemical Ideas sections

* 6.2 What happens when radiation interacts with matter?
* 6.3 Radiation and radicals
* 7.1 Chemical equilibrium
* 10.1 Factors affecting reaction rates
* 10.2 The effect of temperature on rate
* 10.5 What is a catalyst? (revision)
* 10.6 How do catalysts work?
* 13.1 Halogenoalkanes

## Activities

* A2.1 What substances can act as sunscreens?
* A2.2 Investigating sunscreens
* A2.3 What is the effect of the atmosphere on the Sun’s radiation?
* A3.1 More about ozone
* A3.2 The photodissociation of bromine (Optional teacher demonstration)
* A3.3 Investigating the reaction between bromine and cyclohexane
* A4.1 How do halogenoalkanes differ in reactivity?
* A4.2 Making a halogenoalkane
* A4.3 Designing refrigerants
* A5 Chemistry in the stratosphere
* A6 Which are the greenhouse gases?
* A8.1 The effect of concentration changes on chemical equilibria
* A8.2 Measuring the concentration of carbon dioxide in air samples
* A9 Controlling carbon dioxide
* A10 Check you notes on The Atmosphere

## External links

* A unit map
* A Check Your Notes #1
* A Check Your Notes #2
# A-level Chemistry/OCR (Salters)/Standard conditions

In chemistry, the standard conditions are defined as:
* a specified temperature
* 1 atm pressure
* 1 mol dm$^{−3}$ concentration
To indicate that a quantity was measured under standard conditions, a superscript plimsoll line symbol, o, is added to the quantity's symbol.

## Why do we use standard conditions?
### Measurements depend on conditions

If you measure a physical quantity (like enthalpy change), you will get different values under different conditions. For example, the enthalpy change of a particular reaction will be different at different temperatures, different pressures or different concentrations of reactants.

### Using standard conditions avoids infinitely long data tables

You could imagine having tables of values for every possible combination of physical quantity, temperature, pressure, concentration and so on. In practice, this is unfeasable and unnecessary. A more elegant solution is to define a set of conditions (called standard conditions) and always measure physical quantities under these conditions. You can calculate the value of the physical quantity under non-standard conditions using appropriate mathematical techniques.

### Standard conditions can often be used unmodified

Often, you won't even need to adjust the value of a quantity measured under standard conditions to what it would be if it were measured under specific non-standard set of conditions. For example, if you are looking at trends in the periodic table by comparing, say, ionisation enthalpy, the pattern should be equally clear under standard conditions or not. In fact, having the values for different elements all measured under the same conditions saves a great deal of effort — imagine having to adjust hundreds of pieces of data to correct for the fact that they were each recorded under different conditions.
# A-level Chemistry/OCR (Salters)/Sulfur

Sulfur is the sixteenth element in the periodic table, lying below oxygen and above selenium in Group 16. Sulfur is in Period 3, with phosphorus to its left and chlorine to its right. Sulfur exists as a yellow solid, which contains eight-membered rings stacked together. This is in stark contrast with oxygen, which exists as the molecular gas dioxygen, O$_2$, but similar to phosphorus and selenium.

## Isotopes

95% of naturally occurring sulfur is sulfur-32, $^{32}$S. Most of the rest is sulfur-34, $^{34}$S. Two other stable isotopes exist, $^{33}$S and $^{36}$S. Sulfur's relative atomic mass is 32.065 ± 0.005.

## Electron configuration

The electron configuration of the neutral sulfur atom, S, is
$1{\mbox{s}}^{2}2{\mbox{s}}^{2}2{\mbox{p}}^{6}3{\mbox{s}}^{2}3{\mbox{p}}^{4}\,\!$
or in more compact notation,
$[{\mbox{Ne}}]3{\mbox{s}}^{2}3{\mbox{p}}^{4}\,\!$

## Electronegativity

Sulfur has an electronegativity of about 2.6, which is similar to carbon and selenium. Its electronegativity is greater than that of any metal or metalloid, and greater than that of the nonmetal phosphorus (2.2). The other non-metals (I, Br, Cl, N, O and F) are more electronegative.

## Compounds
### Sulfides

Sulfur forms sulfides by direct reaction with metals:
    Fe(s) + S(s) → FeS(s)
    Zn(s) + S(s) → ZnS(s)
    2Li(s) + S(s) → Li$_2$S(s)
    2Al(s) + 3S(s) → Al$_2$S$_3$(s)
Sulfides formally contain the sulfide ion, S$^{2−}$, but the bonding in sulfides is somewhere in-between ionic and covalent, with some sharing of electrons between metal and sulfur. S$^{2−}$ is almost nonexistent in solution, because it is extremely basic and will remove a proton from almost anything. Consequently, sulfides are insoluble in all solvents, unless they react with the solvent to form something more soluble.

### Organosulfur compounds

* Sulfonic acids, RSO$_3$H and sulfonates, RSO$_3$$^−$, covered in Colour by Design
* Thioethers (sulfides), e.g. dimethyl sulfide, (CH$_3$)$_2$S, mentioned in The Oceans
* Many other organic compounds containing sulfur exist, but they are not discussed in Salters Advanced Chemistry
# A-level Chemistry/OCR (Salters)/The Polymer Revolution

The Polymer Revolution is the fifth unit in the Salters Advanced Chemistry course.

## Chemical Storylines sections

* PR1 The start of the revolution
* PR2 The polythene story
* PR3 Towards high density polymers
* PR4 The Teflon man
* PR5 Dissolving polymers
* PR6 Polymers that outdo nature
* PR7 Summary

## Chemical Ideas sections

* 3.4 E/Z isomerism
* 5.3 Bonds between molecules: temporary and permanent dipoles
* 5.4 Bonds between molecules: hydrogen bonding
* 5.6 The structure and properties of polymers
* 6.4 Infrared Spectroscopy
* 12.2 Alkenes
* 13.2 Alcohols and ethers

## Activities

* PR1 Some important polymers: introductory data
* PR2 Making poly(phenylethene) (Optional extension)
* PR3 Using spaghetti to model polymer structure
* PR5.1 Deflecting jets
* PR5.2 Viscosity in alcohols
* PR5.3 Now you see it...
* PR5.4 Making ‘slime’
* PR5.5 Oxidation of alcohols
* PR6 Poly(pyrrole) - a conducting polymer
* PR7 Check your notes on The Polymer Revolution

## External links

* PR unit map
* PR Check Your Notes
# A-level Physics (Advancing Physics)/Fission

Nuclear fission is the splitting of the nucleus of a massive atom into smaller nuclei. This is used to produce energy in power stations and nuclear bombs.

## Chain Reaction

In order to start nuclear fission, one nucleus must be made to split apart. This is achieved by getting the nucleus to absorb a slow-moving neutron. When the nucleus splits, it releases energy, two components, and possibly some more neutrons. If at least one neutron is released, then a chain reaction occurs. This neutron goes on to make another nucleus unstable, which splits, and produces more neutrons, and so on.
If this chain reaction is uncontrolled, a massive amount of energy is released very fast. This is an atomic explosion, which is used in nuclear bombs. In order to use nuclear fission in a power station, the number of neutrons released must be controlled by inserting a substance such as boron into the reactor, which absorbs the neutrons, preventing them from going on to make more nuclei split.

## Binding Energy

The reason nuclear fission produces energy is that the binding energy of the original nucleus is greater than the binding energy of the products of the fission reaction. This difference in binding energy is the amount of energy released as photons (some of which are infra-red). This energy is used to heat up steam, pressurizing it, and enabling it to turn a turbine, producing electricity.

## Neutron Moderator

Neutrons have to be moving slowly in order to cause a nucleus to become unstable and split. If they are moving too fast, then they simply bounce off. A neutron moderator (such as graphite or heavy water) is used to slow them down.

## Questions

1. A neutron is fired at some Uranium-235. Barium-141 and Krypton-92 are produced:
$_{0}^{1}n+_{92}^{235}U\to _{56}^{141}Ba+_{36}^{92}Kr+N_{0}^{1}n$
How many neutrons are produced (i.e. what is the value of N)?
2. What proportion of the neutrons produced must be absorbed in order to make the reaction stable?
3. What would happen if too many neutrons were absorbed?
4. Alternatively, Uranium-235 can split into Xenon-140, two neutrons and another element. What is this element? (You will need to use a periodic table.)
Worked Solutions
# A-level Physics (Advancing Physics)/Flux

A coil of wire creates magnetic flux. The amount of magnetic flux created depends on three things: the number of coils in the wire, the amount of current flowing through the wire, and the permeance of the object through which the flux is flowing. So:
$\Phi =\Lambda NI$,
where Φ is flux (in webers, denoted Wb), Λ is permeance (in WbA$^{−1}$) and I is current (in A). This is the total flux induced. NI is the number of "current-turns". Permeance is related to permeability (a material property) by the following equation:
$\Lambda ={\frac {\mu A}{L}}$,
where μ is permeability, A is cross-sectional area, and L is length. A permanent magnet is just like a coil, except that a current does not need to be generated to maintain the flux. Over smaller areas, we need to know the flux density B. This is the amount of flux per. unit area:
$B={\frac {\Phi }{A}}$
Therefore:
$\Phi =AB$
The flux around a coil of wire varies - ΛNI only gives the total flux, not the flux across a certain area. To show this, we use lines of flux. These obey the following rules:
1. Lines of flux go from the north pole of a permanent magnet to the south pole.
2. Lines of flux go clockwise about wires carrying current away from you.
3. Lines of flux never touch, intersect, or cross.
The direction of the flux is shown with an arrow. Flux is a bit like electricity in that it must have a complete circuit. The lines of flux always take the route of most permeance. Iron (Fe) has around 800 times as much permeability as air. So, flux goes through the iron, and not the air.

## Questions

1. A circular steel core has a cross-sectional area of 9 cm$^2$, and a length of 0.5m. If the permeability of steel is 875 μNA$^{−2}$, what is the permeance of that core?
2. A coil of insulated wire is wrapped 60 times around the top of the core, and a 9A direct current is put through the coil. How much flux is induced?
3. Assuming that all the flux goes through the core, what is the flux density at any point in the core?
4. Draw a diagram showing the lines of flux within the core.
Worked Solutions
# A-level Physics (Advancing Physics)/Flux/Worked Solutions

1. A circular steel core has a cross-sectional area of 9 cm$^2$, and a length of 0.5m. If the permeability of steel is 875 μNA$^{−2}$., what is the permeance of the core?
$\Lambda ={\frac {\mu A}{L}}={\frac {875\times 10^{-6}\times 9\times 10^{-4}}{0.5}}=1.58{\mbox{ }}\mu {\mbox{WbA}}^{-1}$
2. A coil of insulated wire is wrapped 60 times around the top of the core, and a 9A direct current is put through the coil. How much flux is induced?
$\Phi =\Lambda NI=1.58\times 10^{-6}\times 60\times 9=0.851{\mbox{ mWb}}$
3. Assuming that all the flux goes through the core, what is the flux density at any point in the core?
$B={\frac {\Phi }{A}}={\frac {0.851\times 10^{-3}}{9\times 10^{-4}}}=0.945{\mbox{ Wbm}}^{-2}$
4. Draw a diagram showing the lines of flux within the core.
# A-level Physics (Advancing Physics)/Forces and Impulse in Collisions

You should already know that the force exerted on an object is proportional to its acceleration. The constant of proportionality is known as the mass of the object:
$F=ma$
In the case of a collision, for one of the particles in the collision, the acceleration is simply the difference between its velocity before the collision (u) and its velocity after the collision (v) per unit. time:
$F={\frac {m(v-u)}{\Delta t}}={\frac {mv-mu}{\Delta t}}$
So, force is the rate of change of momentum. The quantity on top is known as the impulse of the collision, measured in Ns; Δ t is the length of time it took for the collision to take place. So, the impulse I is given by:
$I=\Delta p=mv-mu=F\Delta t$
In a collision where a certain change in momentum (impulse) occurs, a force is exerted. If the collision time is small, a larger force is exerted. If the collision time is long, a smaller force is exerted. If you have a graph of force against time, impulse is the area under the graph, since:
$I=\int {F\;dt}$
The impulse on one particle in a simple collision is the negative impulse on the other particle.

## Questions

1. Escape velocity from the Earth is 11.2kms$^{−1}$. How much impulse must be exerted on a 47000 kg payload to get it to travel away from the Earth?
2. Two billiard balls, of mass 10g, collide. One is moving at 5ms$^{−1}$, and the other at 2ms$^{−1}$. After the collision, the first billiard ball is moving backwards at 4ms$^{−1}$. The collision takes 1 ms. What force was exerted on this ball?
3. What impulse and force were exerted on the second ball?
4. A 60 kg spacewalker uses a jet of gas to exert an impulse of 10Ns. How many times would he have to do this to reach a speed of 1 ms$^{−1}$ from stationary?
5. A 5 kg bowling ball collides with a stationary tennis ball of mass 0.1 kg at 3ms$^{−1}$, slowing to 2.5ms$^{−1}$. It exerts a force of 100N on the ball. How long did the collision take?
Worked Solutions
# A-level Physics (Advancing Physics)/Finding the Distance of a Remote Object/Sonar

1. What sort of wave does your system use? What is an approximate wavelength of this wave?
Sound waves travelling in water, with an approximate wavelength of the order of 100μm.
2. What sort of distance is it usually used to measure? What sort of length would you expect the distance to be?
Distances to things under water, so up to 11 km.
3. Why is measuring this distance useful to society?
It enables fishing trawlers to find schools of fish, which can be used to feed people.
4. Draw a labelled diagram of your system.
5. Explain how the system works, and what data are collected.
A sonar pulse of known velocity is transmitted down into the sea bed. It is reflected of an object, and returns to the ship-board receiver. The time taken to travel there and back is recorded.
6. Explain how the distance to the object is calculated using the data collected.
$v={\frac {r}{t}}$
$r=tv\,$,
where r = distance, t = time and v = velocity. In this case, the time taken to travel to the object (e.g. fish) t is half of the total time taken to travel to the object and back T, so:
$r={\frac {Tv}{2}}$
7. What limitations does your system have? (e.g. accuracy, consistency)
All sorts of weird things happen with different shapes of object and different frequencies. Accuracy is inversely proportional to the width of the range of frequencies used.
8. What percentage error would you expect these limitations to cause?
9. How might these problems be solved?
Use a narrower band of sound.
# A-level Physics (Advancing Physics)/Fission/Worked Solutions

1. A neutron is fired at some Uranium-235. Barium-141 and Krypton-92 are produced:
$_{0}^{1}n+_{92}^{235}U\to _{56}^{141}Ba+_{36}^{92}Kr+N_{0}^{1}n$
How many neutrons are produced (i.e. what is the value of N)?
Consider the mass numbers:
$1+235=141+92+N$
$N+233=236$
$N=3$
2. What proportion of the neutrons produced must be absorbed in order to make the reaction stable?
Two thirds, assuming that Uranium-235 always produces three neutrons when split, which it doesn't.
3. What would happen if too many neutrons were absorbed?
The reaction would stop.
4. Alternatively, Uranium-235 can split into Xenon-140, two neutrons and another element. What is this element? (You will need to use a periodic table.)
Consider the mass numbers:
$1+235=140+N+2$
$N+142=236$
$N=94$
Then consider the atomic numbers:
$0+92=54+n+2(0)$
$n=38$
So, the element is strontium-94:
$_{0}^{1}n+_{92}^{235}U\to _{54}^{140}Xe+_{38}^{94}Sr+2_{0}^{1}n$
# A-level Physics (Advancing Physics)/Force/Worked Solutions

1. What force is exerted by a 1T magnetic field on an electron (of charge -1.6 x 10$^{−19}$C) moving at 5% of the speed of light (3 x 10$^8$ ms$^{−1}$)?
$F=Bqv=1\times -1.6\times 10^{-19}\times 0.05\times 3\times 10^{8}=2.4\times 10{-12}{\mbox{ N}}$
This may seem tiny, but remember that the acceleration of the electron will be far greater, as it has such a small mass. The fact that the electron was moving at 0.05c means that we don't have to do any relativistic corrections.
2. What force is exerted by a 5mT magnetic field on a 20 cm wire with resistance 1μΩ attached to a 9V battery?
$V=IR$
$I={\frac {V}{R}}$
$F=BIl={\frac {BVl}{R}}={\frac {5\times 10^{-3}\times 9\times 0.2}{10^{-6}}}=9{\mbox{ kN}}$
3. The following diagram shows a positive charge moving through a magnetic field. Draw an arrow representing the direction of the force on the charge.
Note that, if the charge had been negative, the force would be in the opposite direction (upwards).
4. The following diagram shows a wire in a magnetic field. Draw an arrow representing the direction of the force on the wire.
# A-level Physics (Advancing Physics)/Finding the Distance of a Remote Object/Radar

1. What sort of wave does your system use? What is an approximate wavelength of this wave?
Radio waves, with a wavelength ranging from 2.7mm to 100m.
2. What sort of distance is it usually used to measure? What sort of length would you expect the distance to be?
The distance to an object within the radio horizon. This width is given by the formula:
$\mathrm {horizon} _{\mathrm {km} }=3.569\times {\sqrt {\mathrm {height} _{\mathrm {metres} }}}.$
So, a radar 10m above the Earth's surface has a range of 11.3 km.
3. Why is measuring this distance useful to society?
e.g. Radar is used at airports to locate aeroplanes and co-ordinate them so that they can land safely, avoiding collisions.
4. Draw a labelled diagram of your system.
5. Explain how the system works, and what data are collected.
The 'dish' rotates, and the transmitter on it transmits a pulse of radio waves. The waves, if they hit an aircraft, are reflected by it. The dish reflects the reflected radio pulse onto a receiver (allowing for some variety in incoming angles due to varying distances of aircraft). The time taken for the signal to travel to the aircraft and back is recorded, and the speed of the radio pulse (3 x 10$^8$ms$^{−1}$) is already known.
6. Explain how the distance to the object is calculated using the data collected.
$v={\frac {s}{t}}$
$s=tv\,$,
where s = distance, t = time and v = velocity. In this case, the time taken to travel to the aircraft t is half of the total time taken to travel to the aircraft and back T, so:
$s={\frac {Tv}{2}}$
7. What limitations does your system have? (e.g. accuracy, consistency)
* Random noise e.g. birds, weather
* Deliberate attempts to evade radar detection e.g. radio-wave-absorbent paint
* The aircraft are moving, so, in the time taken for the signal to return to the radar station and be processed, the aircraft will no longer be in the position calculated.
8. What percentage error would you expect these limitations to cause?
If we assume a distance from station to aircraft as 5 km, the speed of EM waves as 3 x 10$^8$ms$^{−1}$ and the aircraft's speed as 500kmh$^{−1}$, the time taken for the signal to return from the aircraft would be 1.67 x 10$^{−5}$s. 500kmh$^{−1}$ = 500000mh$^{−1}$ = 139ms$^{−1}$. Therefore, the aircraft would have moved 2.31mm in the time taken for the signal to return to the radar station. This means that there is a ±0.0000463% potential for error in the distance reading, depending on the direction of the aircraft's travel. This may seem insignificant, but readings are only taken once every rotation of the dish, so this potential error gets a lot larger in reality.
9. How might these problems be solved?
Take multiple readings over a period of time, and use a computer to predict where the aircraft will be next given the aircraft's current trajectory.