what's the website address for the revision guides ... thought i had it but it turned out to be something else !

Welcome to all Year 12 chemists!

Welcome to our A-level Chemistry blog, now for all students studying AS and A2 Chemistry at Cedars!

You can find links to useful websites and resources to download on the right-hand side of the blog page. Also, you can all post comments and respond to other comments on the blog, so please make use of this to get help with things you don't understand, to check when homework is due in if you've forgotten(!) or simply to share your deep and meaningful ruminating about chemistry with the rest of us as and when it happens!

If you fancy checking your knowledge and understanding of chemistry from GCSE, have a go at the Royal Society of Chemistry online 'check-up' which you can find at: http://www.rsc.org/education/teachers/learnnet/olympiad_l6checkup.htm. Please use my school email address (L.Sudbery@cedarsupper.beds.sch.uk) when asked for a teacher's or school email address, then I get to see how you've got on! The deadline to complete this by is Sunday, 18th October so you've got plenty of time but it would be good if as many of you as possible had a go!)

Year 13.... Don't forget the RSC olympiad! (Link to website is on the previous post from 16th July.)

RSC Olympiad for Year 13 chemists

Here is the link to the Royal Society of Chemistry's olympiad website, as promised!

www.rsc.org/education/teachers/learnnet/olympiad_L6.htm

You will first need to register as a student. Please use my email address when prompted for your teacher's details. You can then download practice olympiad papers to give you an idea of what to expect in the real thing!

Have a really great summer..... and don't forget that piece of homework that's due in on the first day back in September!

Mass Spectrometry and Infrared Spectroscopy

Do we need to know *how* they work. I looked on syllabus and as far as I could tell we do not need to know how we get the graphs, just how to use them, but wanted to make sure . .

What is evaporation?

If volatily is the ease with which a liquid turns into a gas, then does that mean all liquids evaporate? Is evaporation the state change from liquid to gas? Does this happen at below the boiling point (eg leaving some water at room temperature) because of the Boltzmann distribution, i.e. even at a temperature of 20 degrees celsius, some molecules have enough energy to turn into a gas, and eventually all molecules will leave the container in which the liquid is held?
Is it only liquids that do this (can sublimation be spontaneous, without a heat source?), and is there any point below which liquids do not evaporate, except melting point? Or does it just become so slow near the melting point that it can not be observed?
Also, when you're boiling a kettle, some of the water is turning into steam (a gas) which you can't see, immediately next to the spout, but what you can see is water vapour, a little further away from the spout, where the steam has hit the (relatively) cold air and condensed into water droplets, liquid suspended in air. Is this correct?

. . . . .

When drawing mechanisms for reactions is it necessary to show lone pairs of electron like in the book on page 158.
Also, do we need to know how to use bond enthalpies for substances that are not gases yet?

Carbon double bond

In a carbon double bond is there two pi bonds, one above the sigma bond and one below.
Or
Is that one pi bond.

Halides dissolving in polar solvents

A simple molecules structure e.g. Cl2, does not dissolve into polar solvents but does dissolve into non-polar solvents, but when it becomes an ion, e.g. Cl-, can it then dissolve into polar solvents but not non-polar solvents?

Non polar molecules with polar bonds

CCl4 is a non polar molecule with polar bonds because the dipoles acting in different directions cancell each other out . . .
I was wondering whether if one of the Cl atoms was replaced with another atom, say fluorine, would it still be a non polar molecules because the resulting dipole means fluorine is still slightly negative, or will it now be polar.
Im thinking it is polar because they the charges a no longer equal, so it is no longer electronically symmetrical, but I wanted to check.

Lone pairs

Just read that the reason they have more repulsive force is because whilst the bonded pairs are attracted to two nucleus's, they are attracted only to the central atom. This means it repel bonded pairs more as it is more concentrated around the central atom.
This right?

SO2

Several times when doing a dot and cross diagram I have done it differently to the actual answer, ever though I swear mine makes more sense.
Using the octet rule, even though I know there is the expansion of the octet, it wants to have a full shell (even though I realise 8 electrons is not a full shel for the sulfur) and be stable etc.
So I put in a dative covalent bond between on oxygen atom; and then a double covalent bond between the other oxygen atom and the sulfur atom.
This means all the atoms have stable shells, so why does it instead form two double covalent bonds? Is 8 not a very stable number when it is to do with the third shell? Does the oxygen form a double covalent bond because less energy is required to do that than the sulfur to form a dative covalent bond?

Also, the final shape of this is non-linear and 120 degrees. In this case then does the lone pair have the same repulsion power of the two covalent bonds?
All this came up for me in the question 3 c) on page 59.

Titration question

100cm3 of IO3- (aq) has a concentration of 0.023 mol/dm3
Calculate the maximumn mass of iodine which could be obtained from this solution.
Assume all IO3- can be converted to I2.

When it says, assume all can be converted to I2, does it mean I can assume the number of moles of IO3- is the number of moles of I2 I can get?

Is oxidation endothermic or exothermic?

I was reading the tick list for Enthalpy Changes, and the third bullet point is
Define the importance of oxidation as an endothermic process in:
-the combustion of fuels
-the oxidation of carbohydrates (such as glucose) in respiration
However, both of these processes are exothermic. Is that just a mistake, then?
Also, have we done anything on this?

Equilibrium and acids

As part of the reading on equilibrium, I came across a description of the dynamic equilibrium that occurs when ethanoic acid is dissolved in water, so I now understand more about the difference between strong and weak acids, that I didn't understand before.
Here are just a few questions:
Given that pH is the negative logarithm of the number of H+ ions in solution, is that related to the strength, rather than the concentration of the acid? Also, how does this change with a diprotic acid with two H+ ions (eg for each mole of H2SO4, there will be two moles of dissociate H+ ions and one mole of SO4 2- ions). When H2SO4 reacts halfway to form HSO4, will that have a different pH?
Also, when we've learnt about a metal reacting with an acid, it always says with an excess of acid. What happens if the metal is in excess?

Esterification

Earlier something randomly came into my head involving esterification. I know we've only lightly touched on it but something got me thinking;

What determines the final smell? does the smell depend on the alcohol or carboxylic acid used? so can you have a different smell for every combination of acid and alcohol or does each particular acid/alcohol always give the same smell no matter what is added to it?

Steve

Conclusions from rates data

For experiment 6 (powdered chips) I was surprised to see that the rate of reaction was lower than experiment 1 (in a certain amount of time, less mass had been lost), but also less mass was being lost generally compared to experiment 1, which is strange, because the same mass of chips and volume of acid were being used.
This is contrary to what I would have expected, because a greater surface area would mean more collisions => more successful collisions.
Can anyone explain this result?
Thanks.

could not spot the data needed for the graphs if it is on there?

Salt and Ice

I know it doesn't really have anything to do with anything we're doing now but something, a question in fact, came into my head due to recent weather activity.

Is the process that makes salt dissolve in water the same as the process that makes salt melt ice? or is it something completely different?

I know it's not particularly constructive in terms of organic chemistry or enthalpy but it's been playing on my mind.

Steve

Energy in bonds

This is how I understand the link between bonds and enthalpy.
Bonds contain energy. So to break bonds requires an energy input, and to make bonds, releases energy. ATP is energy currency and when it breaks down to form ADP, bonds are broken but why is energy released rather than needing to be inputted?
However, in a thermal decomposition reaction, a substance, such as calcium carbonate requires energy to break down, it decomposes only when heated, because energy is needed to break the bonds. This is why things don't spontaneously decompose to their simplest parts.
From another point of view, it's because atoms 'want' to have a full outer shell. Surely that would mean compounds with bonds contain less energy, because nature favours things with lower energy, but then bonds contain energy, so this doesn't quite fit. What have I missed here? I know we're going to do this in more detail in Year 13 when we do the Kreb's cycle, but is it the making or breaking of ATP that releases energy?
Is the enthalpy of something with bonds greater or smaller than the enthalpy of something without bonds?

New Resources

I am currently in the process of uploading new resources and aim to complete this by the end of this week. Please click on the links below to find resources for the new module that we have started this term, for both Mr Beaumont's/Mrs Braithwaite's and Mrs Sudbery's/Dr Everett's work.

http://cedarschemistry.edublogs.org/mrs-sudbery-and-dr-everetts-work/
http://cedarschemistry.edublogs.org/wp-admin/page.php?action=edit&post=93

Please let Mrs Sudbery know if there are any areas of AS Chemistry in particular that you would like some revision or practice resources for.