6.1 Cracking Learning outcome Explain what thermal and catalytic cracking are Task: answer recap questions

6.1 Cracking Good if you can: Identify the two types of cracking Great if you can: Define the conditions needed for thermal and catalytic cracking Awesome if you can: Explain the economic reasons for cracking

6.1 Cracking Crude oil has different quantities of each fraction. Which fractions do you think are needed the most? Rank them from most needed to least.

6.1 Cracking The smallest fractions are in most demand because they are used to make petrol. Crude oil contains more of the larger fraction, which people don’t want. So we have to “crack” the long chains into smaller ones.

6.1 Cracking Think pair share What is cracking?

What is cracking? 6.1 Cracking Long chain hydrocarbons are broken down into shorter chain hydrocarbons

Cracking in the classroom Students need to condition necessary for cracking The hydrocarbons are heated to vapourise them Thermal decomposition then occurs

a Place about a 2 cm3 depth of mineral wool in the bottom of the boiling tube and gently press it in place with a glass rod. Drop about 2 cm3 of liquid paraffin on to the wool, using a dropping pipette, Use enough paraffin to completely soak the mineral wool, but not so much that the paraffin runs along the side of the tube when it is placed horizontally. b Clamp the boiling tube near the mouth so that it is tilted slightly upwards, as shown in the diagram below. Place a heap of catalyst (pumice stone or porous pot fragments) in centre of the tube and fit the delivery tube. c Fill the trough about two-thirds full with water and position the apparatus so that the end of the delivery tube is well immersed in the water. d Fill four test-tubes with water and stand them inverted in the trough. Also place the the test-tube bungs, upside down, in the water, . e Strongly heat the catalyst in the middle of the tube for a few minutes, until the glass is up to a dull red heat. Avoid heating the tube too close to the rubber bung. f While keeping the catalyst hot, flick the flame from time to time to the end of the tube for a few seconds to vaporise some of the liquid paraffin. Try to produce a steady stream of bubbles from the delivery tube. Be careful not to heat the liquid paraffin too strongly or let the catalyst cool down. To avoid suck-back do not remove the flame from heating the tube while gas is being collected. If suck-back looks as if it is about to occur, lift the whole apparatus by lifting the clamp stand. g When a steady stream of gas bubbles is established, collect four tubes full of gas by holding them over the Bunsen valve. Take care not to lift the water-filled tubes out of the water when moving them, to avoid letting air into them. Seal the full tubes by pressing them down on the bungs, then place them in a rack.

Products of cracking When a long chain alkane is cracked the products are an alkane and alkene. The alkene is often ethene.

Thermal cracking Thermal cracking involves heating alkanes to a high temperature 700-1200K using high pressure (up to 7000kPa) Carbon – carbon bonds break and one electron from each pair goes to each carbon atom. So two shorter chains are made each ending in a carbon atom with an unpaired electron. These are called free radicals and are very reactive. There is not enough H atoms to produce to alkanes so one of the new chains forms a carbon-carbon double bond and is an alkene The chain does not necessarily break in the middle and this type of cracking produces a high proportion of alkenes. The alkanes are kept in these conditions for a very short time to prevent too much thermal decomposition

Catalytic cracking Catalytic cracking takes place at lower temperatures (approx 720K) and lower pressure A zeolite catalyst is used. This is made of silicon and aluminium oxide. It has a honeycomb structure, this has a very large surface area so increases the rate of reaction. It is also acidic We use catalytic cracking mainly for motor fuels. The products are mainly branched alkanes and cycloalkanes

Plenary Why is cracking necessary? What conditions are required for cracking? Write word and formulae equations for the cracking of Octane Dodecane (C12) 3. Write the equations for the cracking reaction if the products were heptane and ethene what