Catalysts

Question 1

Why do we crack long chain alkanes?

Answer 1

Shorter chain alkanes & alkenes are more useful than long chain alkanes

Question 2

What are the products of cracking?

Answer 2

Shorter chain alkanes & shorter chain alkenes

Question 3

What are the conditions for cracking?

Answer 3

The longer chain alkanes are heated (500 degrees celcius) with a zeolite catalyst (Al2O3 is used in the lab)

Question 4

Draw the apparatus for cracking long chain alkanes in the lab

Question 5

What is the method for cracking long chain alkanes in the lab

Answer 5

1. set up apparatus
2. Heat catalyst strongly & the long chain alkanes gently
3. Have several tubes ready to collect the gases (discard the first tube as it will be displaced air)
4. To avoid ‘suck back’ continue to heat while changing the collection tubes & at the end remove the delivery tube from the water trough before stopping heating
5. The liquid & gas products can be tested for the presence of alkenes using bromine water

Question 6

What is the definition of a catalyst?

Answer 6

A catalyst increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is chemically unchanged at the end of the reaction

Question 7

What’s the definition of catalysis?

Answer 7

Catalysis means speeding up a chemical reaction by using a catalyst

Question 8

Are the products of cracking the same every time?

Answer 8

No, the products of cracking are random & the same molecule can give different cracking products

Question 9

What is the definition of a heterogeneous catalyst?

Answer 9

A catalyst that’s in a different state to the reactants. Usually a solid catalyst for a gaseous or aqueous reaction

Question 10

How does a heterogeneous catalyst work?

Answer 10

1. The reactant molecules are adsorbed onto the catalyst surface- they make bonds with it, this weakens bonds within the reactant molecules
2. bonds within the reactant molecules break(this tales less energy because the bonds were weakened - hence activation energy is lower than route w/out a catalyst)
3. New bonds form, making the product molecules
4. The product molecules leave the catalyst surface (desorption)

Question 11

Why are zeolite catalysts so widely used?

Answer 11

Their crystalline structure with its many channels gives them a huge surface area

Question 12

What is the definition of a homogeneous catalyst?

Answer 12

A catalyst that is in the same state as the reactants- usually both the reactants & catalyst are in solution or all are gases

Question 13

Why do heterogeneous catalysts often get poisoned?

Answer 13

The poison clings to the catalysts surface more strongly than the reactant does. So the catalyst is prevented from getting involved in the reaction its meant to be speeding up