Cracking

Approx. no. C atoms per mol. % from Distillation % Demand Main Usage Name if Fraction Approx. no. C atoms per mol. % from Distillation % Demand Main Usage Refinery Gas 1-4 2 4 Refinery fuel Gasoline 5-8 12 22 Petrol Naphtha 8-10 12 5 Cracking Kerosene 10-14 12 8 Jet fuel Gas Oil 14-19 19 23 Diesel Fuel Oil 19-35 43 38 Power station fuel supply exceeds demand  cracked to satisfy shortfall

Cracking involves the breaking of C-C bonds in alkanes Longer chain fractions are not useful and therefore a lower value economically. Longer chain fractions are broken into shorter lengths (cracked). Shorter chains are produced (e.g. petrol) Some products are alkenes which are more reactive than alkanes Cracking involves the breaking of C-C bonds in alkanes

Alkenes and Cracking Used as chemical feedstock Converted into a huge range of other compounds including polymers and product like drugs and paints Ethene is the starting material for polyethene and a wide range of other everyday materials.

Ways to Crack Alkanes Are alkanes reactive or unreactive? Why? They are very UNreactive so harsh conditions are required to break them down. Several different ways – you need to know 2. You must LEARN the key details. 1. Thermal Cracking 2. Catalytic Cracking

1. Thermal Cracking – Conditions (need to know) Heating alkanes at a high temperature (700-1200K) High pressure (up to 7000kPa) Produces a high number of alkenes Can also produce hydrogen Chain can break at any point Mechanism is not needed at AS. The red highlighted points are the conditions on the spec.

2. Catalytic Cracking – Conditions (need to know) Takes place at a high temperature (720K) – note that this is slightly lower than thermal cracking, but STILL classed as high temperature. Slight pressure (more than atmospheric) Uses zeolite catalyst (made of silicon dioxide and aluminium oxide) Zeolite has a honeycomb structure, so has a large surface area Zeolite is an acidic catalyst This form of cracking produces motor fuels Products include branched alkanes, cycloalkanes and aromatic compounds. Products of cracking are then separated by fractional distillation The products are mostly gases (less than C5) The red highlighted points are the conditions on the spec.

Summary Questions Complete the word equation for the following possibilities: Decane  octane + ____________________ Decane  propane + __________________ Nonane  ethane + ____________________ Nonane  butane + _____________________

Summary Questions Cont. 2. In the laboratory cracking of alkanes, how can you tell that the products have shorter chains than the starting materials? 3. Why would we not crack octane industrially? 4. How can the temperature required for cracking be reduced? 5. Give two economic reasons for cracking long chain alkanes

Reforming

What is reforming? Straight chains do not burn as efficiently as branched chains and ones with rings. Reforming converts straight-chain alkanes into branched-chain alkanes or cyclic hydrocarbons. Heated up using a catalyst e.g. platinum Helps them too burn more easily in the engine. Pentane  Cyclopentane and hydrogen Heptane  methylbenzene and hydrogen