Alkanes

Question 1

Characteristics of alkanes

Answer 1

• General formula: C_nH_2n
• Saturated (with single C-C bonds)
• every C atom has 4 single bonds with other atoms
• cycloalkanes have 2 fewer hydrogens than other alkanes. Their general formula is CnH2n.

Question 2

What shape makes up alkane molecules?

Answer 2

Question 3

Describe the boiling points of alkanes.

Answer 3

• smaller alkanes, e.g. methane are gases at r.t.p - they’ve got very low boiling points
• larger alkanes are liquids - they have higher boiling points

Question 4

Explain the boiling points of alkanes.

Answer 4

The boiling point of an alkane depends on ts size and shape.

Question 5

.Describe the combustion of alkanes.

Answer 5

If you burn (oxidise) alkanes with oxygen, the products are carbon dioxide and water only.

Question 6

When can combustion reactions take place?

Answer 6

Combustion reactions happen between gases, so liquid alkanes have to be vaporised first.

Question 7

Write the equation for the complete combustion of propane.

Answer 7

C₃H₈ + 5O₂ ⇒ 3CO₂ + 4H₂O

Question 8

How does the size of an alkane affect its combustion?

Answer 8

Smaller alkanes turn into gases more easily (they’re more volatile), so they burn more easily too.

Larger alkanes release more energy per mole as they have more bonds to react - this makes them good fuels.

Question 9

How can you use volumes of gases reacting together to work out what hydrocarbon is combusting?

Answer 9

All gases at the same temperature and pressure have the same molar volume. This means you can use the ratio of their volumes to calculate molar ratios, and then work out what hydrocarbon is combusting.

Question 10

What is the method for using the ratio of volumes between two gases to work out what hydrocarbon is combusting?

Answer 10

1. using the volumes provided , write the reaction equ. Multiply each term by its volume. One of the product volumes is unknown.
2. Simplify the reaction equation by ÷ing by the volume of the hydrocarbon
3. Look at how many moles of each product (CO₂ and H₂O) the hydrocarbon combusts to form (from the equ. you’ve just written). You need to find the no. of moles of the product with the unknown vol.
4. Any oxygen atoms that don’t end up in CO₂ must be in H₂O (or vice versa), so the unknown coefficient/no. of moles = the total no. of moles of the reactant you know, oxygen, (coefficient multiplied by subscript) - the total no. of moles of the known product.
5. Now you have the moles of each of the R (O₂) and P (CO₂ and H₂O), exc. the hydrocarbon, you can use this to work out the no. of moles of the hydrocarbon.
6. All the C atoms end up in CO₂ and all the H atoms into H₂O - so the no. of moles of each to give you the molecular formula of the hydrocarbon.

Question 11

30cm³ of hydrocarbon x combusts completely with 240cm³ of oxygen. 150cm³ of carbon dioxide is produced. What is the molecular formula of hydrocarbon X?

Answer 11

Question 12

What is the product of the incomplete combustion of alkanes?

Answer 12

carbon monoxide

e.g.

2CH_4(g) + 3O₂ ⇒ 2CO_(g) + 4H₂O_(g)

Question 13

Why is carbon monoxide dangerous?

Answer 13