Cracking!03 September 2015 Learning objective: To know what cracking is and why it’s so useful! Starter Name fractions from the fractional distillation column which are MOST useful? Explain Fractions

Cracking All (Grade C)Most (Grade B)Some ( Grade A/A*) State the reasons for cracking and how it is carried out Describe the conditions for cracking and how to test for alkenes Analyse word equations to write a balanced symbol equation for cracking Draw the molecular structure and write the chemical formula for:- A ) Decane. Now, split it up into Octane and ethene. What is this process called? KEYWORD: Cracking

Learning objective: To know what cracking is and why it’s so useful!

Cracking Hydrocarbons Large hydrocarbons are broken into smaller molecules using heat and a catalyst. This process is known as catalytic cracking. The small molecules produced are then separated by distillation. Catalytic cracker Heat to vaporise Distillation tower pressure Big Molecules Smaller molecules Molecules break up

Catalytic Cracking In the catalytic cracker long chain molecules are split apart or ‘cracked’. An example of such a reaction is: C 8 H 18  C 6 H 14 + C 2 H 4 + ethene Octane hexane Ethene is used to make plastics Heat pressure catalyst Used as a fuel

 Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient. They do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand - this is where cracking comes in. Cracking is a useful process that converts useless heavy fractions into smaller useful small ones

Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are vaporised and passed over a hot catalyst. This breaks chemical bonds in the molecules, and forms smaller hydrocarbon molecules. Cracking is an example of a thermal decomposition reaction. Some of the smaller molecules formed by cracking are used as fuels, and some of them are used to make polymers in plastics manufacture. Conditions for cracking: 1.High temperature (300 o C) 2.Use a catalyst (aluminum oxide)

Demo of Cracking Watch the demo that your teacher will carry out. While the practical is being carried out, make careful observations to be able to answer these questions. Label the apparatus 1 ) What are the conditions needed for cracking to take place? 2) What type of reaction is taking place? 3) Describe the test for alkenes. State the chemical used and the result you should expect.

Learning objective: To know what cracking is and why it’s so useful!

Catalysts are special substances that make reactions go faster But they are still present at the end of the reaction ! Catalyst allow reactions to happen at lower temperatures and pressures that otherwise would be needed Processes are therefore cheaper to carry out Some catalysts are expensive metals like Platinum It doesn’t matter though, as catalysts are never used up! Catalysts in the body and in biological situations are called enzymes

Alkanes : Supply and Demand Methane gas Octane – found in the petrol fraction Long chains – found in the residue fraction What is the relationship between the amounts of the alkanes in crude oil and their demand? Can you think of a way of sorting out this problem? Q

Cracking...what’s it all about? Using the molymods, try to model the process of cracking Try to answer the questions below: Q1 – What conditions are needed for this process Q2 – Why are the shorter chains more useful? Q3 – What can be done with “leftovers”? EXT: What process is cracking the opposite of?

Polymers Alkenes can be used to make polymers. Polymers are very large molecules made when many smaller molecules join together, end-to-end. The smaller molecules are called monomers. Lots of monomer molecules → a polymer molecule. polymeruse polyetheneplastic bags and bottles polypropenecrates and ropes polychloroethene water pipes and insulation on electricity cables Examples of polymers and their uses

chemical structureball-and-stick model Structural formula ethene C 2 H 4 Propene C 3 H 6 What is the formula for Alkenes?

Learning objective: To know what cracking is and why it’s so useful! What products could be made from cracking C 8 H 18 ?

Learning objective: To know what cracking is and why it’s so useful! long-chain alkaneshort-chain alkane + alkene 300°C aluminium oxide

Ethene C H H C H H Ethene is the first member of the alkene series Ethene contains a carbon to carbon double bond Ethene contains four carbon to hydrogen single bonds C H H C H H or

Addition reaction – ethene and bromine C H H C H H Bromine molecules can react with unsaturated hydrocarbons with carbon to carbon double bonds as follows: Br The bromine atoms from the bromine molecule have added on to the carbon atoms by breaking the carbon to carbon double bond Bond breaking Bond forming Bond breaking H H H H Two new carbon to bromine single bonds formed Orange-brown coloured bromine solution changes to a colourless solution in this reaction 1,2-dibromoethane is the new product

A catalyst is a substance that is ____________in a reaction, but makes the reaction go __________. Catalysts are often expensive metals, e.g. gold or platinum. Why do you think the long-term costs are low? Catalysts

1. What is cracked? 2. What are the products formed during cracking? 3. Name the 2 conditions needed for cracking. 4. Name the catalyst involved in cracking 5. What is a catalyst? 6. Give an example of a catalyst. 7. Draw the structural formula when propane is cracked in methane and …………

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