Monday, 21 May 2012

Contact Process-Manufacture of sulfuric acid

d) The industrial manufacture of chemicals
This part is for SINGLE SCIENCE.

5.25 recall the raw materials used in the manufacture of sulfuric acid
The raw materials are:

  • sulphur
  • air (oxygen)
  • water

5.26 describe the manufacture of sulfuric acid by the contact process, including the
essential conditions:
i a temperature of about 450 °C
ii a pressure of about 2 atmospheres
iii a vanadium(V) oxide catalyst

Stage 1: making sulfur dioxide

You can either burn sulfur in air:
S(s) + O2(g) à SO2(g)

or heat sulfide ores strongly in air:
4FeS2(s) + 11O2(g) à 2Fe2O3(s) + 8SO2(g)
(FeSis pyrite or iron pyrite)
Iron pyrite crystals

Stage 2: Making sulfur trioxide

Now the sulfur dioxide is converted into sulfur trioxide using an excess of air from the previous processes. 
2SO2(g) + O2(g) 2SO3(g)     H= -196 kJ/mol 
An excess of oxygen is used in this reaction, because it is important that as much sulfur dioxide as possible is converted into sulfur trioxide. Having sulfur dioxide left over at the end of the reaction is wasteful, and could cause possibly dangerous pollution. (Remember sulfur dioxide can dissolve in water and form acid rain, this can kill plants and animals-by altering pH of water fish live in for example. It will corrode limestone which is basically calcium carbonate. It can also leach nutrients from the soil.)

As the forwards reaction is exothermic, there would be a higher percentage conversion of sulfur dioxide into sulfur trioxide at a low temperature. (Remember your equilibrium stuff, go to my equilibrium post if you want to revise that first.)

However, at a low temperature, the rate of reaction would be very slow. 450°C is a compromise. Even so, there is already about a 99.5% conversion. 

There are 3 gas molecules on the left-hand side of the equation, but only 2 on the right. Reactions in which number of gas molecules decrease are favoured by high pressures. (Remember Le Chatelier's principle where you're trying to remove the change, if you increase pressure, moving the equilibrium to the side with less gas molecules would decrease pressure.) In this case though, the conversion is so good at low pressures already that it isn't economically worthwhile to use higher ones. So a pressure of 2 atmospheres is sufficient.

The catalyst, vanadium (V) oxide, has no effect on the percentage conversion, but helps to speed up the reaction. Without the catalyst, the reaction would be extremely slow. 

Remember, catalysts remain chemically unchanged at the end of the reaction. They help to speed up the rate of reaction, by providing an alternative pathway with a lower activation energy. Activation energy is the minimum amount of energy needed for a reaction to take place. So if the activation energy is lowered, more particles will have the required activation energy so a greater number of the collisions will be effective. Effective collisions are ones where reactions actually take place. Sometimes particles collide without reacting because they don't have the minimum activation energy required.

Sulfur dioxide is converted into sulfur trioxide

Stage 3: Making the sulfuric acid

In principle, you can react sulfur trioxide with water to make sulfuric acid. 
SO3(g) + H2O(l) à H2SO4(aq)

In practice, this produces an uncontrollable fog of concentrated sulfuric acid. Instead, the sulfur trioxide is absorbed in concentrated sulfuric acid to give fuming sulfuric acid (also called oleum). 
H2SO4(l) +SO3(g) à H2S2O7 (l)

This is converted into twice as much concentrated sulfuric acid by careful addition of water. 
H2S2O7(l) + H2O(l) à 2H2SO4(l)

I'm not sure which equation you guys have learnt, so I've included both the principle and the real life one. :)

5.27 recall the use of sulfuric acid in the manufacture of detergents, fertilisers and paints

Sulfuric acid has a wide range of uses throughout the chemical industry. The highest single use is in making fertilisers (including ammonium sulfate and 'superphosphate'-essentially a mixture of calcium phosphate and calcium sulfate).

It is also used in the manufacture of detergents and paints. If you look at the list of ingredients on any industrial or domestic detergents (including shampoos and liquid 'hand-soaps) and find the words 'sulfate' or 'sulfonate', then sulfuric acid was used in the manufacturing process. Even those simply labelled as containing 'anionic surfactants' almost certainly contain these sorts of ingredients, even if they don't name them.

In paint manufacture, sulfuric acid is used in extracting the white pigment titanium oxide, TiOfrom titanium ores.

Sunday, 20 May 2012

Group 1 elements-lithium, sodium and potassium

Update: I've included the trends, physical and chemical properties of Group 1 elements as requested by someone. Some information taken from Edexcel Chem textbooks. :) 

Section 2: Chemistry of the elements
part b) Group 1 elements-lithium, sodium and potassium

2.6 describe the reactions of these elements with water and understand that the reactions provide a basis for their recognition as a family of elements
2.7 recall the relative reactivities of the elements in Group 1

Alkali metal
Hydroxide solution produced
Gas produced
Rate of gas produced
Lithium hydroxide
Fairly vigorous
Sodium hydroxide
Potassium hydroxide
Very vigorous
Rubidium hydroxide
Caesium hydroxide
Extremely explosive

As you can see, the reactions get more violent as you go down the group, telling you that the metals are more reactive down the group. 

Describe what happens when sodium is added to water:
When sodium is added to water, it reacts very quickly and vigorously. The reaction is exothermic and the heat produced melts the sodium. The molten sodium darts around the water surface and a yellow flame is seen. You may see a bit of fizzing/bubbling (effervescence) as hydrogen is evolved. 
Remember MM-FF. Melts, moves, floats, fizzes 
Li, Na and K are all less dense than water, hence they float on water. 

Write word equations to show the reactions of lithium, sodium and potassium with water:

Lithium + water à lithium hydroxide + hydrogen
Sodium + water à sodium hydroxide + hydrogen
Potassium + water à potassium hydroxide + hydrogen

So you see they all form hydroxides, and since all group 1 metal compounds are soluble it dissolves to form an alkali. (All alkalis are just soluble bases, but not all bases are alkalis, for example metal oxides are bases but not all of them are soluble to form alkalis.. e.g. Copper (II) oxide.)

If they ask you about what colour the solution will turn if universal indicator is added, it will turn blue or purple. This is because the solution is alkaline, and if they ask you to state what ion causes this, it’s the OH- ion (hydroxide ion). They are called alkali metals for a reason!

2.8 explain the relative reactivities of the elements in Group 1 in terms of distance between the outer electrons and the nucleus. (single science)

As you go down the group the metals become more and more reactive. This is because their atoms get bigger, so the outer shell electrons are further away from the nucleus. So the electrostatic force between the nucleus and the outer shell electron (OSE) is weaker, hence it is easier to lose the OSE (alkali metals have only 1 OSE). The atoms want to lose the OSE to form full outer shells so they are more stable and unreactive after that. 

Also, you can think of how as the atoms get bigger, there are more electron shells in between the OSE and the nucleus. You can think of them as 'shields', so the force of the nucleus on the OSE is weaker the bigger the atom. 

The following is extra information someone has requested. 

Physical Properties

Melting Point (°C)
Boiling Point (°C)
Density (g/cm3)
Lithium - Li 
Sodium - Na
Potassium - K
Rubidium - Rb
Caesium - Cs

  • You will notice that the melting and boiling points of the elements are very low for metals, and decrease as you go down the group.
  • Their densities tend to increase, but potassium has a lower density than sodium, so the densities don't increase that tidily. Lithium, sodium and potassium are all less dense than water, hence will float on it. 
  • The metals are very soft and can be easily cut with a knife. They get softer as you go down the Group. They are shiny and silver when freshly cut, but tarnish within seconds on exposure to air. 
Storage and handling
All these metals are extremely reactive, and get more reactive as you go down the Group. They all react quickly with air to form oxides, and react between rapidly and violently with water to form strongly alkaline solutions of the metal hydroxides.

To prevent them from reacting with oxygen/water vapour in the air, lithium, sodium and potassium are stored under oil. Rubidium and caesium are so reactive that they have to be stored in sealed glass tubes to stop any possibility of oxygen getting to them. 

Great care must be taken not to touch any of these metals with bare fingers. There could be enough sweat on your skin to give a reaction producing lots of heat and a very corrosive metal hydroxide. 

Compounds of the alkali metals
All Group 1 metal ions are colourless. That means that their compounds will be colourless or white unless they are combined with a coloured negative ion. For instance,  Potassium dichromate (VI) is orange, because the dichromate (VI) ion is orange. Group 1 compounds are typical ionic solids and are mostly soluble in water.

Summary of the main features of the Group 1 elements:
Group 1 elements:
  • are metals
  • are soft with melting points and densities very low for metals
  • have to be stored out of contact with air or water
  • react rapidly with air to form coatings of the metal oxide
  • react with water to produce an alkaline solution of the metal hydroxide and hydrogen gas
  • increase in reactivity as you go down the Group
  • form compounds in which the metal has a 1+ ion
  • have mainly white compounds which dissolve to produce colourless solutions

Saturday, 19 May 2012

Haber Process

Section 5: part d) The industrial manufacture of chemicals
-for double award you only need to know the Haber Process, Single Award people, you also need to know the Contact Process for the manufacture of Sulphuric Acid!

5.21 recall that nitrogen from air, and hydrogen from natural gas or the cracking of hydrocarbons, are used in the manufacture of ammonia
So if they ask what are the raw materials used in the Haber process, you know it. It's nitrogen from the air and hydrogen either from natural gas, which is methane (CH4); or from the cracking of hydrocarbons. (Cracking is in the Crude Oil post.) 

5.22 describe the manufacture of ammonia by the Haber process, including the essential conditions:
i. a temperature of about 450°C
ii. a pressure of about 200 atmospheres
iii. an iron catalyst

Remember these conditions!! And remember that the reaction is reversible. Also, the forwards reactions is exothermic
N2 + 3H2  2NH3

So decreasing the temperature would actually increase the yield, however, it is still done at a fairly high temperature to speed up the reaction. It makes the rate of reaction faster so the manufacturers get their ammonia quicker, as they say, time is money. The reaction would be too slow otherwise at low temperatures. It would be useless to have a low temperature and achieve a high yield of ammonia if it's going to take ages. You need the gases to reach equilibrium within the very short time that they will be in contact with the catalyst in the reactor.So 450°C is a compromise, and still produces a reasonably high proportion of ammonia. 

The catalyst does NOT affect the amount of products made. The yield of ammonia stays the same, you just get it faster because it speeds up the reaction by lowering the activation energy needed for the reaction. :) 

Increasing the pressure would favour the forwards reaction which is what is wanted, to get more ammonia. This is because if you look at the balanced equation, there are 4 moles of gas on the reactants side (left) but 2 moles of gas (ammonia) on the right hand side (products). So according to Le Chatelier's principle where you try to remove the change, if you increase pressure, the equlibrium would move to the right hand side to decrease pressure. And the products have less pressure because there are only 2 moles there.

Also, just as an extra, just thought that this will be useful to know and is very logical: :)

Credit for the information goes to
And no plagiarism was intended! 

Rate considerations
Increasing the pressure brings the molecules closer together. In this particular instance, it will increase their chances of hitting and sticking to the surface of the catalyst where they can react. The higher the pressure the better in terms of the rate of a gas reaction.

Economic considerations
Very high pressures are very expensive to produce on two counts.
You have to build extremely strong pipes and containment vessels to withstand the very high pressure. That increases your capital costs when the plant is built.
High pressures cost a lot to produce and maintain. That means that the running costs of your plant are very high.

The compromise
200 atmospheres is a compromise pressure chosen on economic grounds. If the pressure used is too high, the cost of generating it exceeds the price you can get for the extra ammonia produced.

5.23 understand how the cooling of the reaction mixture liquefies the ammonia produced and allows the unused hydrogen and nitrogen to be recirculated

Separating the ammonia
When the gases leave the reactor they are hot and at a very high pressure. Ammonia is easily liquefied under pressure as long as it isn't too hot, and so the temperature of the mixture is lowered enough for the ammonia to turn to a liquid. The nitrogen and hydrogen remain as gases even under these high pressures, and can be recycled.

At each pass of the gases through the reactor, only about 15% of the nitrogen and hydrogen converts to ammonia. (This figure also varies from plant to plant.) By continual recycling of the unreacted nitrogen and hydrogen, the overall conversion is about 98%.

5.24 recall the use of ammonia in the manufacture of nitric acid and fertilisers

So ammonia is used to make nitric acid and fertilisers, as you know from bio, plants need nitrates to grow.

Just in case you want to know, here are some properties of ammonia:
  • alkaline gas (turns damp red litmus paper blue, which is the test for ammonia!)
  • extremely soluble in water--it forms a weak alkali-->ammonia solution
  • less dense than air
  • colourless gas with pungent odour

Metallic Crystals

1.45 describe a metal as a giant structure of positive ions surrounded by a sea of delocalised electrons

That's what a metal is, a lattice of positive ions in a sea of delocalised electrons. Basically delocalised electrons is as the name suggests, it's not attached to any particular atom and is able to move freely. 

1.46 explain the malleability and electrical conductivity of a metal in terms of its structure and bonding

Metals can conduct electricity because the delocalised electrons are free to move and carry the charge. The key words here are that the electrons are free to move/mobile and can move through the structure. 

Metals are malleable and ductile because they are a lattice structure and arranged in layers, which are able to slide off each other easily when a force is applied. [The layers are the sheets of positive ions.] Thus making it easy to bend and shape-->malleable. Easily pulled out into wires-->ductile.  

Metals can be made harder by alloying them with other metals. An alloy is a mixture of metals-for example, brass is a mixture of copper and zinc. In an alloy, the different metals have slightly different sized atoms. This breaks up the regular arrangement and makes it more difficult for the layers to slide. 

a pure metal, with neat layers that would easily slide over each other

alloy: the atoms have different sizes, so it disrupts the regular packing and makes it much more difficult for particles to slide over each other when a force is applied. this tends to make alloys harder than the individual metals that make them up. 

A common example of an alloy is stainless steel, which is iron mixed with chromium and nickel. The chromium and nickel form strong oxide layers which protect the iron. This is why stainless steel is so resistant to corrosion. Obvious uses include kitchen sinks, saucepans, knives and forks and gardening tools. But there are also major uses for it in the brewing, dairy and chemical industries where corrosion-resistant vessels are essential.

I'm sure you all know the difference between elements, compounds and mixture by now. So I'd just like to point out that alloys are considered mixtures rather than a compounds because of the totally variable proportions of the metals. Whereas in water (a compound), every single water molecule has two hydrogen atoms combined with one oxygen atom. In the alloys, the metals can be mixed in any proportions...