Alkanes

Question 1

What is the enthalpy change of complete combustion of an alkane?

Answer 1

The enthalpy change for the complete combustion of 1 mol of reactants under standard conditions. Typically, excess oxygen is available to ensure complete combustion into H2O and CO2.

Question 2

Describe free radical substitution of Methane to Dichloromethane

Answer 2

1. Initiation
   - A Cl2 molecule will break down by homolytic fission to form two free chlorine radicals under UV light.
2. Chain propagation
   - The Chlorine free radical will collide with an alkane, causing the alkane to lose a Hydrogen and thus become a free radical. The Cl radical –> HCl
   - The alkane radical will then react with a Cl2 molecule –> regenerating a Cl free radical and CH3Cl (Chloromethane)
   - Chloromethane reacts with Cl radical to form chloromethane radical and HCl.
   - Chloromethane radical and Cl2 react to form dichloromethane and regenerate the chlorine radical.
3. Termination
   It can occur between any two radicals colliding

Question 3

How does acid rain form?

Answer 3

When fossil fuels are burned in engines, Nitrogen monoxide is formed due to the high heat and pressure.
Also, sulphur impurities react to form SO2 and further oxidise to SO3 in the upper atmosphere.

Both Nitrogen and Sulphur oxides dissolve in rainwater to form acid rain.

CO2 reacts with water to form HCO3- ions, which dissociate into H+ and CO3^2-. H+ causes a more acidic pH and is thus believed to have detrimental impacts on surface waters of the ocean.

Greenhouse gases but do not cause ozone depletion

Question 4

What are the effects of acid rain?

Answer 4

1. Damage to the vegetations’ waxy cuticle layer that protects the leaf and prevents the plant from taking in CO2 to photosynthesise. Acid rain removes most essential nutrients and activates heavy metals in the soil, like aluminium.
2. Aquatic life may die when water is too acidic. If bacterial decomposers die below pH 5.5, undecomposed food and waste will remain at the bottom of the ocean.
3. Erodes limestone and copper, causing damage to historical sites as well as statues to the point where they need repairs; otherwise, they pose a danger to human life.
4. Human lung problems have been linked to acid rain pollution. People with asthma are at higher risk. Acid rain creates tiny particles of pollution and can cause severe health problems when inhaled.

Question 5

How do we reduce acid rain?

Answer 5

1. Find renewable alternative energy sources which reduce Fossil fuel dependency.
2. Oil can be desulphurised before burning, and sulphur can be sold to offset the costs.
3. Neutralise acidic gases before releasing them into the atmosphere by introducing them to a chamber filled with alkaline material.

Question 6

How does Carbon monoxide form and what are its associated issues?

Answer 6

Fossil fuels burned in a limited oxygen supply = incomplete combustion, forming carbon monoxide.

• Highly toxic and odourless
• Combines with haemoglobin, reducing the blood oxygen-carrying ability.
• Many short-term health complications may result in death from extreme inhalation.
  -Forms photochemical smog and releases soot, which clog stomata and cause asthma in animals.