Cracking - DF

Question 1

Why do we do cracking

Answer 1

from crude oil we get more of the heavier (long chain) fractions than we can use, and less of the lighter (short chain) fractions than we need - cracking solves this as the long chain alkanes are split giving shorter chain alkanes and shorter chain alkenes

Question 2

Catalyst definition

Answer 2

catalysts speeds up rate of reaction without getting used up by providing an alternative route with a lower activation energy. They emerge chemically unchanged but they may change physically. They do not alter the how much product is given they only alter the rate its produced

Question 3

Catalysis definition

Answer 3

process of speeding up a reaction using a catalyst

Question 4

2 types of catalysts

Answer 4

heterogeneous and homogeneous

Question 5

Heterogeneous catalyst definition

Answer 5

the catalyst is in a different state to the reactants

Question 6

How does a heterogeneous catalyst work

Answer 6

the reactions happens on the catalyst surface:
1. the reactant molecules are adsorbed onto the catalyst surface - make bonds with it which weakens the bonds within the reactant molecules.
2. bonds within the reactant molecules break which takes less energy as the bonds are weakened hence the activation energy has been lowered.
3. new bonds form making the product molecules
4. the product molecules leaves the catalyst surface (desorbs)

Question 7

Why is it important for the catalyst to have a large surface area in heterogeneous catalysts

Answer 7

as the reaction happens on the catalyst surface, if it has a large surface area more molecules can be adsorbed and collisions will be more frequent. Heterogeneous catalysts are often a wire mesh or a fine powder on a solid support which stops the catalyst crumbling away

Question 8

Cracking long chain alkanes experiment method

Answer 8

1. Set up apparatus, tube A is for collecting liquid products and tube B is for gaseous products which are collected over water (cracking will give a mixture of varying chain lengths hence some will be liquid and some gas)
2. The catalyst is heated strongly to ensure that when the alkane gasses pass over, the temperature is high enough for cracking reactions to take place. The long chai alkanes (liquid paraffin) are heat gently - just enough to vaporise them
   3.Have serval tubes of water ready in water trough B for collecting the gases. Discard the first tube of gas collected as that will just be displaced air.
3. To avoid suck-back continue to heat while changing the collection tubes and et the end remove the delivery tube from the water trough before stopping heating.
4. The liquid in tube A and gas in tube B can be tested for the presence of alkenes using bromine water
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Question 9

Homogenous catalysts definition and examples

Answer 9

catalyst and the reactants are in the same state.
acid or base catalysis, enzyme catalysed reactions

Question 10

Catalyst poisoning meaning

Answer 10

some substances may ‘poison’ a catalyst stopping it from functioning. In heterogeneous catalysis poison molecules are ones which adsorb more strongly to the catalyst surface than the reactant molecules and once adsorbed they don’t react. So they bond to the catalyst surface blocking it and preventing the reactant molecule from adsorbing and reacting