Wednesday, 28 February 2018

GCSE OCR Gateway Organic Chemistry C6.2p Electrochemical Cells

C6.2p To be able to recall that a chemical cell produces a potential difference until the reactants are used up
Electrochemical Cells

Some metals are more reactive than others for example zinc is more reactive than copper.

You will have heard of the reactivity series and you can find out more about it here.

Reactivity is about the gain or loss of electrons from the metal.

Very reactive metals lose electrons easily,  less reactive metal tend to hold on to their outer shell electrons.

Zinc atoms tend to lose their two outer shell electrons

Zn        Zn2+     +   2e

But copper atoms tend to lose electrons less easily than zinc.

Cu        Cu2+     +   2e

So if we place a zinc rod in an aqueous solution of copper sulphate containing blue Cu2+ ions, an exchange of electrons takes place with zinc giving its outer shell electrons to the copper ions.

This is called a displacement reaction and I have discussed them here

Here is the equation for that reaction:

Zn(s)       +         Cu2+ (aq)             Zn2+ (aq)    +      Cu (s)

And you would observe the blue colour fades after a time (Cu2+ (aq) is gaining electrons) just as the zinc rod becomes thinner (Zn(s) dissolves as Zn2+ (aq), each ion losing 2 electrons in the process).   As well, a brown solid (copper) forms on the zinc since each copper ion has gained 2 electrons from the zinc atom and become a copper atom.

Now if instead you put a zinc strip and a copper strip (both highly polished) in a solution of dilute sulphuric acid nothing happens until you connect them together with a wire, then changes appear at the metal surface: bubbles especially. 

This is called a Voltaic Cell after Alessandro Volta who first built a chemical cell (you can see it in the photo below to the right) and from whom we have the unit of potential difference: the Volt.  

Now connect both metal strips to a voltmeter and it reads a voltage; a potential difference must have been established between the two metals.

(We'll get to the porous vase in a second!!)

This is the basis of all chemical cells, two metals of different receptivity connected electrically produce a potential difference.  

The more reactive metal is releasing electrons and so it is the negative terminal of the cell and the metal receiving the electrons in this case copper is the positive terminal.

Chemical cells produce a potential difference until the reactants (the metals) are used up.  In the copper—zinc cell the potential difference falls as the metal zinc dissolves.

The chemical cell involving copper and zinc was one of the first to be built and is called the Daniell Cell. Here is a picture of typical set up shown in cut away:

And here is the schematic:

If the solution of sulphuric acid is 1M then the cell’s potential difference is 1.1volts.

These were the first electrical cells where chemical energy is converted into electrical energy.  Here is an old picture of one of the original Daniell Cells with some labelling in French, I believe.

The next picture shows how the solutions are kept apart yet electrically connected using a porous pot container.  Ions can pass between the solutions via the holes in the porous pot.

And then here is the set up on the lab bench showing a voltage of 1.09v—must have been a little resistance in the set up to bring the voltage down by 0.01v. 

Other cells are possible as the technology has moved on considerably from the Daniell Cell.  The lithium ion cell in your smart phone is one of the latest incarnations of a chemical cell.

Here is a typical dry cell (dry because not aqueous solutions used in its construction.)

Friday, 23 February 2018

GCSE OCR Gateway Organic Chemistry C6.2o Cracking

C6.2o To be able to describe the production of materials that are more useful by cracking and to give the conditions and reasons for cracking and some of the useful materials produced
Crude oil is an essential resource at the moment for the production of many fuels, lubricating oils and bitumen for road surfaces.
What cracking does is make some of the larger molecule fractions more useful.
Cracking does that in two ways
Cracking produces more reactive hydrocarbons and cracking produces smaller molecule hydrocarbons.

For example cracking duodecane C12H26 could produce octane C8H18 and and butene C4H8
Both these molecules are more useful superficially than duodecane since octane could be a constituent of higher grade fuel and butene is a reactive intermediate.

C12H26      C8H18  +  C4H8

From this simple example we can see what cracking essentially does: it breaks up larger hydrocarbon molecules in the crude oil fractions into smaller molecules.

Note that the sum of carbon and hydrogen atoms on the right hand side of the equation equals the number of carbon and hydrogen atoms on the left hand side.

No hydrogen or carbon atoms are theoretically “lost” in the process.

Industrially, catalytic cracking is achieved using a zeolite type catalyst and a high temperature (450oC) and pressure.

Thermal Cracking by the name uses heat only processing the hydrocarbon fraction at between 700 and 1000K and a high pressure.

In cases of cracking the products are separated using further fractional distillation.

You can run a mock up experiment in the laboratory that mimics the industrial catalytic cracking process.

Here is a diagram of the apparatus you might have used:

Liquid paraffin is usually used as the long chain hydrocarbon that is going to be cracked.  The paraffin is usually absorbed on mineral wool to stop it slopping about in the test-tube.

Porcelain chips, crushed brick or aluminium oxide beads are used for the catalyst. 

The catalyst is held in place using mineral wool.

Heating the catalyst rather than the paraffin generates sufficient heat to warm and boil the paraffin and this expands over the catalyst and cracks up.

A gas issues from the delivery tube.  It is insoluble in water and can be collected over water.

This gas is very flammable unlike paraffin burning with a yellow flame and it also decolorises bromine water unlike paraffin.

The catalyst turns black on heating.

So how do we explain what is going on?

Essentially, alkane hydrocarbons crack into an alkane and an alkene.

It is the alkene that is the more reactive product of cracking because it contains a double carbon-carbon bond.

You can find out more about alkenes here

The catalyst turns black because the cracking process does produce atomistic carbon. 

The bromine water test indicates the presence of a double carbon–carbon bond in a molecule.

These alkenes can be polymerised and plastics of many different varieties result from addition polymerisation.


Wednesday, 21 February 2018

GCSE OCR Gateway Organic Chemistry C6.2l and n Crude Oil: a finite resource

C6.2n To be able to explain how modern life is crucially dependent upon hydrocarbons and recognize that crude oil is a finite resource
C6.2l To describe the fractions as largely a mixture of compounds of formula CnH2n+2 which are members of the alkane homologous series
Crude oil has come to be one of the most essential resources that support life in the 21st Century.  Crude products are essential to every nation on earth. 

Think of it like this: remove crude oil and its products from life as we know it and what is lost: heating, fuel for all forms of transport vehicles and freight, aircraft and shipping, all forms of lubrication of engine moving parts, petrochemicals for the production of plastics and other material, gases for lighting and heating.

The next picture shows this variety of products derived from crude oil and emphasises just how dependent we still are on oil. 

Hydrocarbons are crucial to life on planet earth.

What are these hydrocarbons?

These hydrocarbons are usually saturated, that is the molecules only contain single covalent bonds.  No molecule contains any double or triple carbon—carbon covalent bonds.

The formulae of these hydrocarbon molecules follow a simple pattern CnH2n+2
The number of hydrogen atoms in each hydrocarbon is twice that of the carbon atoms plus two. This pattern is known as the general formula of the hydrocarbon.
All hydrocarbon molecules whose formula fits this CnH2n+2 pattern are part of a family of molecules called Alkanes.

These alkane molecules are the feedstock of today’s petrochemical industry.

There are still vast resources and reserves of fossil fuels on earth but they are decreasing in size dramatically.  here is a chart showing the proven oil reserves by region:

But and it is a big but it is likely that we have passed the point of peak oil: that is the moment when the maximum resources of crude oil existed on earth. 

At present (2018) crude oil, despite new discoveries, is a reducing resource, its use is outstripping new discoveries.  In other words, there are only a finite number of barrels of oil that can be extracted from the earth’s crust. 

Or to put it more dramatically the time will come one day in the future when the last drop of oil will dribble out of a hole in the ground and that will be it, forever!!  It will all be gone for good.

Who knows when that day will be?  The above charts suggests a date around the mid 21st century!

We might be able to predict it given the rate at which we sue crude oil resources today but really we should be arguing for the conservation of this precious resource for example the development of alternative fuels renewable fuels so that crude oil products are not burnt but reserved to make useful products.

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