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OXIDATION STATES

The oxidation state of an element in a compound, molecule or ion is essentially the number of electrons it is released into bonding compared with the uncombined element. You are expected to learn and use the rules for assigning oxidation states. Using the rules of assignment below you can use them to work out the oxidation state of an element in a compound or ion.

 To calculate the oxidation state of Sulphur in H₂SO₄

H = +1, O= -2 and net charge = 0

So (2 x +1) + S + (4 x -2) = 0

So 2 + S – 8 = 0

So -6 + S = 0

S=+6

To calculate the oxidation state of Nitrogen in N₂O

O= -2 and net charge = 0

So (2 x N) + (1 x -2) = 0

So 2N – 2 = 0

So 2N = 2

N=+1

To calculate the oxidation state of Chlorine in ClO₃^- 

O= -2  and net charge = -1

So Cl + (3 x -2) = -1

So Cl + (– 6) = -1

So Cl = 6 – 1

Cl=+5

1. Go through each paper and list how many marks are available for each main specification topic area
2. Calculate an average of the marks for all the past papers (so if 10, 12, 11 etc for acids in june 2003, jan 2004, june 2005) then you know acids is worth about 11.
3. Create a league table, ranking the topics which average the highest on top.
4. Voila! You now have a list of importance to organise your revision.

This works so well because it stops you spending too much time on one area that’s never worth many marks. For example, in CHEM4 for AQA, acids are woth on average 15 marks compared to 7 for rates, so you’d spend twice as much time revising acids than rates! It also starts gettting you familiar with the formats of the papers.

Perhaps if you do the league tables, you could post them here as a reply so others could share…

• Learn how to box
• go to the gym
• run off
• read up on boxing

Not one of them asked me “what do I have to do?”

And in the above example, I forgot to tell them they would be the ticket person on the door! So they may have done all that preparation for no purpose whatsoever, as they failed to find out what was expected of them.

See the analogy to exam prep here?

The modern A-level student seems obsessed with “learning everything” and then “doing” questions in the hope that what they learned comes up. I think it’s more to do with procrastination and putting off the inevitable.

The bottom line is this. The past papers represent a window into your own boards expectations of you. I don’t mean that you can predict questions etc..that’s plain stupid, but you can easily see the followig patterns and getting familiar with the papers helps you …

• get used to the ways in which the board ask questions
• get familiar with the style and wording to minimise missunderstanding of what they are getting at
• find out which bits of the syllabus are commonly used and which aren’t
• practice with the specific types of caclulation, mechanism and see what the mark points are from their schemes etc
• get use their own mark schemes to improve and develop wording of answers (no more “I lnow what I want to say but I don’t know how to say it!”)
• desensitise you to tfeel of the paper to reduce anxiety of the day
• develop your ability to concentrate for the duration of the exam period
• help you develop strategies eg section B first … a certain type of question because you like it … a certain type of question because you hate it … etc
• prevent you from learning a pile of stuff you don’t need on the day

Simply put, by looking at papers early in your preparation will guide you to focussing your efforts where they are needed … and that means a higher return in the exam.

If you find yourself asking “what if they ask something else?” then ask yourself this … if you knew a certain driving examiner nearly always did a reverse aroundthe corner at  the jinction of High Street and Low Street, where would you practice your reverse around the corner? Everywhere other than that?

The first rule of combat is KNOW THE ENEMY, know their strengths and weaknesses and in the case of exam boards, these are all out on display in the past papers.

It’s never too early to start so prepare wisely and good luck!

Biodegradability

Methods of disposal or re-use

Cramming is a throw back to GCSE days. GCSE exams are designed to be learn … regurgitate … forget. A-level on the other hand is understand … apply … develop. This is why many of you have stuggled in A-level, it’s not because the material is so much harder, it’s that your mindset is wrong.

If you’re in AS you may find that this approach gets you through the first January exam, but it becomes less useful in June or in A2 as the material changes to more application. Also, the A2 material starts to contain synoptic elements (pieces of information that are key to understanding how the subject connects at all levels). If you manage to forget everything as soon as your exam is over, these elements are never given a chance to enter your long-term understanding of the subject which cause problems in June of A2.

If your in A2 you may now be finding that that a higher proportion of material is demandng a knowledge of previous studies. I’ve had many students that are annoyed at this as they feel it’s unfair. But is it unfair to expect an A-level student to have a grasp of the fundamentals of the subject he or she chose to study? I don’t think it is. I mean, if you studied languages for example, you’re ability to carry out a conversation in French will improve over the years as your comprehension of the grammar and lirary of vocabulary increases. It’s the same for sciences. 

One of the reasons the A-levels were changed was to get students to a more unified level in readiness for university studies. There wa also a drive to make a more thorough understanding of the subject endemic. I see the bottom line as this – if you try to cram and regurgitate in the June exams, you will crash and burn. Start NOW by refamiliarising yourselves with the key areas that I identify in other posts, on a regular basis. Try to link together the concepts to put your study in perspective. Think about the fundamentals, why does a reaction bother to take place, how does it do it, what actually happens and in what sequence, why does one thing do but another not? Answering these questions will help you to build a better grasp of the synoptic nature of your chosen subject.

Oxidation reactions are hugely important in chemistry, in that they relesae large amounts of energy in the form of heat. In biology too, we’ve learned that oxidation reactions form the basis of provide energy for growth (respiration and the production of ATP). But the concepts can be confusin and its easy to get muddled up. This post will hopefully sort it out a little and help you get a clearer picture of how to tackle oxidation states and half-equations.

Many of my students get confused at some point by oxidation, reduction and half equations. The topic gets recapped in CH5 so it is important to get the key points right:

• Remember, oxidation is a process of electron loss and reduction is a process of electron gain, OILRIG (Oxidation Is Loss, Reduction Is Gain).
• The Reducing agent is the electron donor and gets oxidised. the Oxidising agent is the electron acceptor and gets reduced.
• The number of electrons lost (donated) by the reducing agent equals the number of electrons gained (accepted) by the oxidising agent.
• Oxidation number is essentially the amount of charge an element brings to a compound (eg Na brings +1 and Cl brings -1 to NaCl)
• Oxidation numbers of all elements is zero, of all ions alone (Na+, Cl-, Ca2+, O2-) is its charge (+1, -1, 2+, -2 respectively), H is usually +1 and O is ussually -2
• Oxidation numers of elements in ions can be calculated by cancellation of charge, eg Mn in MnO4- must be giving +7 as 7 pluses cancel out seven of the 8 minuses from O (4x-2) leaving – left over.

Then how do we construct half equations?

1. Write the formula of the two species changing from the question across the arrow and identify their oxidation umbers.
2. Check the number of atomic species is the same (both 1 or 2 etc)
3. Balance electrons on the HIGH oxidation number side only, by subtractng High oxidation number – low oxidation number.
4. Balance any oxygen on the opposite side with H2O.
5. Balance any hydrogen on th eopposite side with H+
6. Check the net charge on both sides is equal and feel smug!

Worked example:

Construct a half equation for the oxidation of Cl- to ClO3-

1. Cl- -> ClO3- [Cl in Cl- is -1 and Cl in ClO3- is +5]
2. one chlorine species on either side
3. [High - Low = 5 - -1 = 6 electrons on high side, so Cl- -> ClO3- + 6e-
4. 3H2O + Cl- -> ClO3- + 6e-
5. 3H2O + Cl- -> ClO3- + 6e- + 6H+
6. -1 net charge on left and -1 net charge on right!!

Construct a half equation for the oxidation of Cl- to Cl2

1. Cl- -> Cl2
2. 2Cl- -> Cl2
3. 2Cl- -> Cl2 +2e- [high - low = 0--1 = 1 but it happens twice as there are two chlorine species]
4. END as there are no oxgens or hydrogens

This is a simple and robust method which gets the job done, practice it and it’ll soon become second nature.

Try some common ones like:

1. Cr2O7 2- to Cr3+
2. MnO4- to Mn2+
3. Cl2 to ClO-

• Primary alcohols are oxidised gently (using warming and distillation at temperature of less than 50C) to aldehydes and more vigorously (boil under reflux) to carboxylic acids.
• Secondary alcohols are oxidised vigorously (by boiling under reflux) to ketones
• Tertiary alcohols are resitant to oxidation (don’t say cannot be oxidised- they can, to CO2 and H2O on combustion!)

Note that for oxidation to occur, the acohol must lose its H attached to the -OH group and also a H on the carbon atom on which the -OH sits. Since tertiary alcohols do not have this H-atom, they are resistant to oxidation.

When attempting equations using structural formula, a [O] is acceptable so. Notes notice that the two H atoms that are removed join with the [O] to form water.:

Primary to an aldehyde

CH3-CH2-CH2-OH + [O] –> CH2-CH2-CHO + H2O

Primary to a carboxylic acid

CH3-CH2 -CH2-OH + 2[O] –> CH2-CH2-COOH + H2O

Secondary to a ketone

CH3-CH(OH)-CH3 + [O} –> CH3-CO-CH3

• Mass spectroscopy is a process which is used to identify the masses and abundances of isotopes of an element in a sample.
• It can also be used to find the relative molecular mass of an organic compound.
•  There are four stages: ionisation à acceleration à deflection à detection
• Ionisation: Vaporised sample is placed in an ionisation chamber and bombarded with high energy electrons fired from an electron gun. These knock off an outer shell electron forming a M⁺ ion.
•  Acceleration: The ions are attracted towards a negative charged plate which accelerates them through the vacuum.
• Deflection: The beam of ions is deflected by a magnetic field which deflects ions with a high mass/charge ratio less than those with a low mass/charge ratio.
• Detection: The strength of the magnetic field is steadily increased until the beam is focussed on the detector. As the ions strike the charged plate they set up a current whose magnitude is proportional to the number of ions that strike.
• In the mass spectrum of an element below: The number of peaks = the number of isotopes in the element, the relative abundance (sometimes expressed as % abundance) is the relative amount of each isotope in the sample and the m/z ratio is the mass to charge ratio of each isotope. As the charge is usually +1 in a mass spectrometer, it is the same as the relative isotopic mass.
• Ions with a charge of +2 would give a peak with low abundance at a value of ½ the m/z ratio of the parent ion. E.g. ⁶³Cu⁺ has an m/z of 63, but ⁶³Cu²⁺ has an m/z of 31.5. The abundance is low because the amount of energy needed to form 2+ ions is normally too large for the mass spectrometer.

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Surviving Maths in AS Biology

AQA A2 Biology: Writing the Synoptic Essay

Go on! You know you want to!