ENTHALPY CHANGE

Da, da, DAAAAA!!!! 20th post. Anyway this is one of my favourite stuff on F322, so hope you'll learn quick (or I'll find you and eat you - joking I've got better things to eat).

First things, first. You need to understand that for a reaction to happen the particles need to have enough energy so that when they collide their bonds will break. However when their bonds reform energy is given out.

• So now it is obvious to note that if the energy REQUIRED to break the bonds is greater than the energy given out when bonds are formed then the temperature of the surroundings will drop. This is an ENDOTHERMIC reaction. ENDO meaning energy taken in.
• When the energy RELEASED when bonds form is greater than energy taken in to break the bonds then the temperature of the surroundings increases. This is an EXOTHERMIC reaction. EXO meaning given out.

So that sums up the two enthalpy changes, remember the conservation of energy. YOU CANNOT DESTROY OR CREATE ENERGY. So when doing your calculations make sure the energy given out by a compound is the same that the surroundings receive.

Now that that is clear you might begin to wonder about combustion and respiration. They both release energy, do they not? They are indeed exothermic reactions, where a compound is oxidised (reacted with oxygen).

As an exmple of endothermic reaction, we have thermal decomposition of carbonates or even melting (you require heat to melt ice - it is a reaction).

ENTHALPY PROFILE DIAGRAMS: 

When one of these crosses your path do not turn the other way and run - bad idea, never turn your back on your enemy. These are free marks in the exam. Think about what happens in EXO and ENDOthermic reactions.

We know that in EXOthermic reaction energy is give out, which means that the products have less energy than reactants. Simple.

In ENDOthermic reactions energy is taken in so the products will have more energy than the reactants. 

What's the little hills, you ask?? Remember when I had mentioned that bonds need to be broken first? This is the MINIMUM energy required to do this.

ACTIVATION ENERGY: Minimum energy required to break bonds to start a reaction.

STANDARD CONDITION: A pressure of 100kPa (1 atmosphere) and a stated temperature (25 degrees Celsius).

ENTHALPY CHANGE OF REACTION: Enthalpy change that accompanies a reaction in the molar quantities expressed by chemical equation under standard conditions at standard state.

ENTHALPY CHANGE OF FORMATION: The enthalpy change that takes place when one mole of a substance is formed from it's constituent elements, under standard conditions at standard states.

ENTHALPY CHANGE OF COMBUSTION: The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, at standard states.

Sorry, you just need to memorise them. Try saying them in reverse (not the letters in a word but the whole sentence) and in different order, it helps.

When you are given experimental values you have to use this equation to calculate the enthalpy change.

Pretty self explanatory.

BOND ENTHALPY: The enthalpy change that takes place when one mole of bonds is broken by homolytic fission in gaseous state.

This will always be positive as you are breaking bonds so energy is taken in. When working these out the total bond enthalpy of the reactants equals the bond enthalpy of the products. So when working out the enthalpy change you should do the following:

Enthalpy change = Total bond enthalpy   -   Total bond enthalpy
                   of braking bonds           of making bonds

In short?? Broken - Made

Hess's law - that old guy - If a reaction can take place by more than one route and the initial and final conditions are the same then the enthalpy change for each route is the same.

Here is how it all works. So try to remember the rules and understand the process. First identify if you're given formation or combustion values. Then you'll know how to work the numbers using the rules.

That's it folks.

MODERN ANALYTICAL METHODS

Hello everyone! Nearing the end slowly, and with Breaking Benjamin a comforting sound in my ears, it will fly by.

All molecules absorb infrared radiation. This energy that they absorb causes the bond to oscillate in a certain way. It could either be stretching back and forth or a bending of the bonds of to the side.

This absorption shows up on infrared spectrum, and enables us to identify the presence of functional groups. Cool, eh??

You will be given a table in the exam which gives you absorption data and identifies the responsible functional group. Handy. Learn to understand it and identify what's where.

Learn it, learn it and once again learn it! Not of by heart of course but make sure you know what you see when you do see it.

Infrared spectroscopy is very useful and is now used in breathalysers to measure how much alcohol is in your breath.

Next we have mass spectrometry.

It is dead easy. An electron gun fires at a molecules and breaks it up. Just like glass shattering different fragments are formed, big and small. These show up as peaks on the graph. However, unlike glass, the fragments are cations (positive ions), which get acted on by a huge magnet. The further they get pushed away the smaller their mass was (if a person gets hit by a truck, they go flying but if the truck hits a tree - it'll move but slightly). How far they get pushed enables scientists to determine their mass. 

Now the peak furthest to the right (biggest m/z value) is the molecular ion. This bad boy is pretty lucky as the gun managed to hit one of the inner electrons and hence the molecule didn't break up at all. It is the molecule you were testing for but charged. Simple??

Once you have the mass you play around with numbers and figure out what it is. 

Hey! No sad faces... Not on my watch.

Any questions? Ask, I don't bite... Much.

HALOGENOALKANES

Oh my sweet oranges! I am starting to go crazy with all this revision. You too?? Too bad.

We have already covered many things about halogenoalkanes so here is a short summary.

• Electrophilic addition of hydrogen halides.
• Addition of halogens.

Now you will learn how to name them too. Isn't that epic??

Halogenoalkanes (geez, what a mouthful) contain only single bonds. We name them by first identifying where the halogen is attached. Count on which carbon it is on (smallest number counts). Write the number of the carbon then a relevant prefix. Finish off with the name of the alkane.

Halogen:           Prefix:

F                  flouro-

Cl                 chloro-

Br                 bromo- 

I                  iodo-

NUCLEOPHILE: An electron pair donor. 

NUCLEOPHILIC SUBSTITUTION/HYDROLYSIS of HALOGENOALKANES:

Because it's a substitution, you know that on top of the halogenoalkane you are going to have something else left over. That which has been replaced.

Think about it this way. In electrophilic addition we added the halides. Here we will remove them. See? Opposite.

You might come across questions where it will ask which hydrolysis reaction will take place faster. This rate is affected by two factors:

• Polarity - C-F bond is the most polar (greatest difference in electronegativity) so the C delta+ atom attracts the nucleophile more readily - So faster reaction.
• Bond enthalpy - The C-I bond is the weakest (I don't think you need to know why but I'll include it later in this post - probably at the end somewhere) so it is broken more easily. This means a faster reaction.

Faster as you go up. Then faster as you go down. One point contradicting the other? It's not a mistake, I assure you. The think is that in hydrolysis bond enthalpy is more effective than bond polarity. So you should keep the polarity in mind but focus on the bond enthalpy.

• Industries had a craze when it come to compounds containing chlorine. Such as, chloroethene and tetraflouroethene (these coming from radical substitution), especially CFC (ChloroFlouroCarbons).
• These were used for aerosols, air conditioners. This is because they were relatively unreactive, non-toxic and volatile. But every silver lining has a cloud to it (Sorry, I just had to). CFC's had a huge impact on the ozone layer (about that later - I promised).

There are now biodegradable alternatives to the killer CFC's, such as HCFC's and even CO2 as a blowing agent for expanding polymers.  

Now back to bond enthalpy. The C-I bond is the weakest because the outer electrons of Iodine are very far away, this means nuclear attraction is very weak and the electrons can be easily stolen away. You knew this was coming, didn't you?

ALCOHOLS

Don't get any ideas, this isn't going to be a page filled with different types of alcohols. Well, maybe a little. But not in a way that you think.

First up: PROPERTIES OF ALCOHOLS.

It is, oh, so clear that alcohols can form hydrogen bonds. You know, the electrostatic attraction between the electron deficient Hydrogen and lone pair on the oxygen atom. Course you do. 

Hydrogen bonds are the strongest intermolecular (between molecules) forces, this results in relatively high melting/boiling point.

This also means that alcohols have a relatively low viscosity (so difficult to form a gas).

The hydrogen bonds give alcohols another property. Alcohols are soluble in polar solvents. This is because a hydrogen bond can form between the lone pair on oxygen and the electron deficient hydrogen on a water molecule.

As the chain length increases the solubility decreases. This is because a larger proportion of the molecule is not polar.

Next up: PRODUCTION OF ALCOHOLS.

Alcohol can be produced in two main ways, that you need to know.

Hydration of ethene. Sounds scary? It's not. It is a simple reaction between ethene and steam (recognise it?). A H3PO4 catalyst, 300 degrees and 60 atm are required. This is a reversible reaction (more about this later).

Fermentation. We have a carbohydrate that is acted upon by yeast to form the beloved alcohol - not personally - (wine for example) and carbon dioxide.

C6H12O6 -----> 2C2H5OH + 2CO2

CLASSIFYING ALCOHOLS: This is my method of remembering this.

Primary alcohols: When the OH group is on the end carbon in a chain, and no groups are attached.

Secondary alcohols: OH group is in the middle somewhere but no group is attached to that carbon.

Tertiary alcohol: OH group in the middle with an alkyl group attached to the same carbon.

The textbook phrases it differently and actually talks about alkyl groups attached but this works for me and actually gets me the marks.

Here is what the textbook says, if you think about it hard it does make sense. My way seems simpler.

PRIMARY: OH attached to a carbon with no alkyl groups or bonded to one alkyl group.

They take the hydrocarbon chains as alkyl groups.

SECONDARY: OH group attached to a carbon bonded to two alkyl groups.

TERTIARY: OH group attached to carbon with three alkyl groups.

SO here there is no hydrogen next to the carbon.

COMBUSTION OF ALCOHOLS:

Alcohol + O2 ------> Carbon dioxide + Water

OXIDATION OF ALCOHOLS:

This is a summary of all the oxidations. The R here means the rest of the chain.

When alcohols are oxidised during distillation only the first product is formed. During reflux (allowing the reaction to continue so all reactants can react).

The alcohols are oxidised using Metal dichromate XCr2O7 under the presence of sulphuric acid catalyst. 

We can get rid of the OH group to form alkenes, in a reverse reaction by adding an acid catalyst and heat. This is called dehydration reaction - so many names.

Finally, you need to understand how esters are formed. This is dead easy - if you know the rules. 

Alcohol + Carboxylic acid -----> Ester

During esterification (who comes up with these original names? Hah! The blog doesn't recognise half of the chemistry words) the Hydrogen on the alcohol and the OH group on the carboxylic acid break off and form water. This leaves the COO group for esters.

Esters have a fruity smell. - just a little side comment.

ALKENES

Now on to Alkanes brother ALKENE! No excitement? How about this:

Anybody hugging the screen yet? Geez, you people are strange.

As I'm sure you're all aware, because you're smart you all, is that alkenes contain a double bond. This means that they are unsaturated. A double bond means there are TWO pairs of electrons. 

First pair forms a sigma bond (that is an overlap of s orbitals - remember those bad boys?). each carbon donates one electron to the SIGMA bond.

Second pair of electrons forms a PI bond (this is sideways overlap of p orbitals on each carbon).

The PI bond fixes the carbon atoms in position, at either end of the double bond. This prevents any rotation of the bond.

Now when we think about the number of bonded regions, we have two on each carbon that is the bond C-H and One C-C bond (even though there's two bonds, it's one region of bonding). This means that the angle is 120 degrees, forming a trigonal planar shape.

Oh, and another thing. The PI bond is a very small region of space, this means there will be a high electron density (almost like pressure). This means that PI bond is weaker as electrophiles will attack that region first.

Next up we have reactions of alkenes. There's many so be prepared to take notes.

Addition of HYDROGEN / hydrogenation: 150 degrees Celsius

Addition of HALOGENS / halogenation: 

Addition of Hydrogen halides:

Addition of STEAM: H2PO4 catalyst, high temperature and pressure

Make sure that you know when what fission takes place. Keep in mind that when both elements have the same electronegativity, it will be HOMOLYTIC fission. Different elements always have a difference of electronegativity so it will be a HETEROLYTIC fission.

Electrophile: Electron pair acceptor (positive ion).

There are other things for which alkenes are used for. That is addition polymerisation. It is very simple: monomers of an alkene are added together. During polymerisation the double bond is broken to form a long chain made up of the subunits that were used in the reaction. Ensure you remember that the double bond breaks and draw brackets around that subunit to show it is only a section of the polymer.

Continuing on the topic of polymers is a section of waste. Remember that recycling is important to conserve raw resources, as well as reduce CO2 emission. This needs to be done carefully as some plastics contain Chlorine, which has an effect on ozone layer (about that later). Furthermore, chlorine is toxic and can form HCl, which we know is very corrosive. 

To reduce waste biodegradable and decomposable plastics are developed.

ALKANES

I know the exams are close and you're all sick of it all but we need to finish it. Here's something to cheer you up: 

Alkanes are hydrocarbons, which means that they are a compound made of hydrogen and carbon only. Common sense, eh??

You know from GCSE that crude oil is used as a source of hydrocarbons. During fractional distillation you are able to separate the fuels because each chain has a different boiling point.

Moving on to more difficult part is the effect of branching and chain length on boiling points.

CHAIN LENGTH: As the length increases, there are more electrons and hence more points of contact. This means more Van der Waals's forces form and more energy is required to break them. So as chain length increases the boiling point increases.

BRANCHING: Branched molecules have fewer points of contact (same number of electrons but different shape - isomerism, remember?). they are also further apart as molecules cannot get close together. This means less Van der Waal's forces and less energy required to break them. So as a molecule becomes more branched it's boiling point decreases.

When producing alkanes, you should keep in mind that they are fuels. Therefore they will be combusted. 

In real life complete combustion is not possible, especially since most cars have limited supply of oxygen. This results in incomplete combustion, which produces dangerous gases.

C4H10 (l) + 9/2 O2 (g) ----> 4 CO + 5 H2O

Carbon monoxide is poisonous, it prevents haemoglobin from binding with oxygen in red blood cells, so body tissues become deprived of oxygen. This can lead to death. Simply put.

It is much easier to combust branched alkanes than straight chains of alkanes. This is because oxygen only attacks the outside of the chain, so when the alkane is branched it is easier to attack the molecule and more of the molecule will be combusted.

Because of this companies crack the long chains of alkanes into smaller ones, produce branched alkanes or cyclic hydrocarbons, which combust more efficiently.

Time to step up the game now. It's radical substitution time! Yeah! Am I the only one happy? Nevermind. 

• We use ultraviolet light in a process called INITIATION to break Cl2, Br2, etc. into two radicals by homolytic fission (rings a bell?).

Important to note that radicals are very aggressive (reactive in chemistrian speak), so they will attack anything to form a bond.

Usually in a reaction bonds are first broken, using energy and then reformed releasing energy. In this case the radicals do not require any energy as their bonds are already broken.

• During PROPAGATION the radical attacks an alkane, steals a hydrogen and donates it's extra electron to the left over molecule, to form a HCl (or whatever halogen is there) and that radical alkane, minus one hydrogen.
• That radical alkane now wishes to attack everything, when it chances upon a Cl2 molecule it steals one Cl atoms and donates it's extra electron to the other Cl atom forming a Cl radical. We have formed the radical we started off with a ChloroAlkane (or whatever halogen used).
• This will go on to form many compounds, until no reactants are left. The problem is compounds may form that you do not wish to form.
• Final step is TERMINATION. During this process two radicals react together and share their extra electrons to form a covalent bond (shared pair of electrons, NO??) 

Cl. + Cl. ----> Cl2

CH3. +CH3. ----> C2H6

CH3. + Cl. ----> CH3Cl

Here are termination products when dealing with methane.

Ensure you understand that radical substitution forms a mixture of products.

BASIC CONCEPTS

I know, I know. The title is very vague. But to be honest this section is very vague, it has all the gibberish required for F322 but, seriously it's, well, basic. To start of with: 

Because why not??

First of all we have some definitions, which you may not necessarily need to remember but you do need to understand them.

EMPIRICAL FORMULA: The simplest whole number ratio of atoms of each element present in each element.

As it says in the name, remember it's a ratio. So you need to find the moles of each stuff and divide by the smallest to get the simplest WHOLE number ratio. Simple!

MOLECULAR FORMULA: Actual number of atoms of each element in a molecule.

GENERAL FORMULA: Simplest algebraic formula of a member of a homologous series.

That sounds depressing, doesn't it?? It only means a general formula for any compound of a specific type of molecule. E.g. An alkane, alkene, alcohol. Each of these is a homologous series, meaning each member of that group has similar properties (alkenes have double bonds, alcohols have OH group).

STRUCTURAL FORMULA: The arrangement of atoms in a molecule.

Write it out as it looks. E.g. Propane: CH3CH2CH3

DISPLAYED FORMULA: Relative positioning of atoms and bonds between them.

This one is how it actually looks. So its a drawing - you display drawings don't you?? Structural you write what it looks like, displayed you draw what it looks like.

SKELETAL FORMULA: Simplified organic formula, without showing hydrogen. Leaving just carbon skeleton and associated functional groups.

HOMOLOGOUS SERIES: A series of organic compounds having the same functional group, but with each successive member differing by CH2.

Alright, all this means is that they have the same general formula, same functional group but as you go along the group of that series each member (of that group) has an extra CH2.

FUNCTIONAL GROUP: A group of atoms responsible for the characteristic reactions of a compound.

Easy enough??

Here are the first ten member of the alkanes homologous series:

1. Methane
2. Ethane
3. Propane
4. Butane
5. Pentane
6. Hexane
7. Heptane
8. Octane
9. Nonane
10. Decane

Notice all end with the suffix "ane".
Akenes end with the suffix "ene".

The next part is a pain. A pain in the backside, at least for me it is. You need to be able to identify, draw and name different types of isomerisms.

Okay. So there are two main types of isomerisms:

• Structural isomerism: Same molecular formula but different structural formulae.
• Stereoisomerism: Same structural formulae but with different arrangement in space.

Yeah! Well done me, I freaked you out.

Structural isomerism (as in the name) has different structural formula. Remember structural formula is the one where you write what you see. So this means that the number of C, O, H, etc. is the same but the OH group (for example) could be attached to a different carbon. This is the easy one, just think about where all the functional groups are and voila!

Stereoisomerism is the tricky one. Here we have the structural formula the same. So none of the groups are moved about. This means that there MUST be a double bond present, because this prevents the molecule from twisting in the air (and being a nuisance) and therefore a functional group could be on the same carbon (structure the same) but on the top OR bottom of the double bond! Got it??

Then we have an example of stereoisomerism, which is E/Z isomerism (that bad guy - yes).

As I said a double bond has restricted rotation. Now for E/Z isomerism two different groups have to be attached to each carbon atom on the double bond.

• E isomerism is when the two (same) groups are on opposite sides.
• Z isomerism is when the two (same) groups are on the same side.

To make matters more complicated we have a special case for E/Z isomerism, which is cis-trans isomerism. In cis/trans isomerism there must be two non-hydrogen groups and two hydrogen around the double bond. So kind of like E/Z but this time the two different groups could be a Cl and H on one carbon and Cl and H on another.

• Cis isomerism is when the two other (non hydrogen) groups are on the same side of the double bond.
• Trans isomerism is when the two other (non hydrogen) groups are on the different sides of the double bond. Think of trans as being changing gender... so different.

Next up we have different types of covalent bond fission ( breaking of covalent bonds in different way).

During HOMOLYTIC fission the two products have the same charge, they are two radicals as the shared pair (the covalent bond) is split equally so each atom gets one electron each.

During HETEROLYTIC fission the two products have different charges. A cation (positive ion) and anion (negative ion) are formed. This is because one of the two atoms is greedy (more electronegative) and steals both electrons from the covalent bond. This type of fission happens during electrophilic addition (about that later).

We can use CURLY arrows to show where the pair of electrons (from covalent bond) have moved from and to.

When drawing these diagrams, think....

1. Which is more electronegative?
2. Then you'll know if it's hetero/homolytic fission.
3. Next you know that cations are electrophiles (attracted to electrons/ or electron acceptors) so they will attack any double bonds of a compound and steal on electron to become neutral and another one to form a covalent bond.
4. This leaves that carbon a cation (its extra electron was stolen - the one from double bond).
5. This means the left over anion will then attack the cation carbon and donate it's electrons, one to make carbon neutral so the anion becomes neutral too, and one to form a covalent bond.

Ta dah!

Worst for last. You really should keep in mind percentage yield and atom economy, and for crying out loud! Do understand them too, memorising isn't everything.

In real life situations, it is not always possible to get the calculated amount of product in a reaction:

• Reversible reactions may not go to completion
• Some product may be lost when it is removed from the reaction mixture
• Some of the reactants may react in an unexpected way

The yield of a reaction is the actual mass of product obtained. The percentage yield can be calculated:

Percentage Yield =    Actual mass of product obtained
                   -------------------------------------   * 100
                    Maximum theoretical mass of product

So in short percentage yield shows you how efficient that reaction is, how close you got to what you wanted to get.

Atom economy is very similar, and that what gives me a headache. It looks at how much of all the products is what you really want. Ugh! I'll explain in a bit.

Atom economy = Molecular mass of the desired product
               -------------------------------------  *100
                  Total molecular mass of products

Now... You could have a high percentage yield (as you get nearly the same mass of - I don't know! butanol, for example - as you expected to get) but the atom economy is low as there are many waste products produced along with butanol (e.g. carbon dioxide and water).

To clear up. % yield looks at the what you wanted to get and actually do. And atom economy creates a ratio of what you actually got over all products - so how efficient the reaction is. You do not want a low atom economy as it wastes raw resources.

Phew! That's it. Hope you got that, it took me a LOOOOONG white to get my head round it.

F322

YEAH! I have managed to start with the right thing this time!. Anyway, Tuesday is the exam and I got a bit carried away by my Very important things that I have to do. So lets get cracking with F322. Here is the list of topics I will cover:

• Basic concepts
• Alkanes
• Alkenes
• Alcohols
• Halogenoalkanes
• Modern analytical techniques
• Enthalpy changes
• Rates and equilibrium
• Chemistry of the air
• Green chemistry

Don't get put off by the titles of these sections, they just make my life much easier, it enables me to put all the work into chunks, not tiny chapters. Well, what are we waiting for?? 

Lets GO!!

F321

I think it is a bit too late for that but I thought I'd mention the different topics that need to be known for the F321 module. (I know, sorry, should have done it ages ago but I sort of forgot). As a little remedy here is the list of topics for F321 and a very snazzy picture.

• Atoms
• Moles and equations
• Acids
• Redox
• Ionisation energy
• Bonding and structure
• Periodicity
• Group 2
• Group 7

Summary

Hello everyone.

I hope you have all had a wonderful exam, though the questions were sooo dull. Now I wanted just to say that since the F321 exam is over and done with I will now comcentrate on F322.