Hydrocarbon Reactions (HSC SSCE Chemistry): Revision Notes

Implications of Obtaining and Using Hydrocarbons

Introduction

Hydrocarbons play a central role in modern life. We encounter them daily in many forms, from the plastics in our electronic devices to the fibres in our clothing. Hydrocarbon-based fuels power our vehicles, heat our homes, and cook our food. Many everyday products including paints, adhesives, solvents, medicines, and cosmetics originate from simple alkanes, alkenes, and alkynes. However, obtaining and using these valuable compounds comes with significant environmental and social implications.

Sources of hydrocarbons

Crude oil

The primary source of hydrocarbons is crude oil, a fossil fuel formed by the decomposition of prehistoric living organisms. This process takes millions of years, making crude oil a non-renewable resource. Crude oil is a complex mixture of many different hydrocarbons, ranging from small molecules with just a few carbon atoms to large molecules containing dozens of carbon atoms.

Fractional distillation

Because crude oil is a mixture, it must be separated into useful components through a process called fractional distillation. This technique separates substances based on their different boiling points.

The fractional distillation process works as follows:

• Crude oil is heated to approximately $400^{\circ}\text{C}$ at the bottom of a tall distillation column
• The hot mixture vaporises and rises up the fractionating tower
• The tower has a temperature gradient, being hottest at the bottom and coolest at the top
• As the vapours rise and cool, different fractions condense at different heights
• Each fraction condenses when it reaches the part of the tower matching its boiling point
• The condensed liquids collect in trays at various levels and are drawn off

Catalytic cracking

Many heavier fractions from crude oil can be further processed to increase the yield of lighter, more valuable fractions that are in highest demand. In Australia, heavier fractions with carbon chains of 15-25 carbon atoms undergo catalytic cracking.

Catalytic cracking is a decomposition reaction where larger hydrocarbon molecules break down into smaller, more useful molecules. This process uses:

• A catalyst (typically zeolites, which are aluminosilicate compounds containing aluminium, silicon, and oxygen)
• A temperature of approximately $500^{\circ}\text{C}$
• Moderately low pressure

Thermal cracking

Thermal cracking is an alternative method that also breaks down large hydrocarbons into more useful products. This process uses:

• High temperatures ranging from $450^{\circ}\text{C}$ to $750^{\circ}\text{C}$
• High pressure (approximately 70 atmospheres)
• No catalyst

The reactions in thermal cracking and catalytic cracking often produce the same products, but the mechanism by which the reactions occur differs.

Mining and transporting crude oil and natural gas

Extraction locations

Mining for crude oil and natural gas frequently occurs in ocean environments, where millions of years of sedimentation have created ideal conditions for fossil fuel formation. Natural gas is a mixture of simple alkanes, predominantly methane ($\text{CH}_4$), with smaller amounts of ethane, propane, butane, and pentane.

Crude oil extraction also takes place on land, sometimes in remote areas of significant environmental importance. Long-standing controversy surrounds mining operations in Alaska, the Arctic, and Antarctic regions due to the potential environmental impacts of extraction and transport activities.

Oil spills and their impacts

One of the most serious environmental risks associated with crude oil is the possibility of spills during transport. Several major incidents demonstrate the devastating consequences:

These examples demonstrate that oil spills have severe consequences across multiple dimensions:

• Environmental damage: Destruction of ecosystems and wildlife
• Economic losses: Job losses and business failures
• Social impacts: Community stress and displacement
• Long-term effects: Recovery can take many years or even decades

The greenhouse effect

Carbon dioxide as a greenhouse gas

One of the primary uses of organic compounds is as fuel. Common hydrocarbon fuels include:

• Octane and ethanol for automobiles
• Methane and ethane for domestic heating and cooking
• Butane for portable BBQ gas bottles

When these fuels undergo combustion, they produce carbon dioxide ($\text{CO}_2$), a greenhouse gas that absorbs infrared radiation in the atmosphere. This trapped heat contributes to the enhanced greenhouse effect, where human activities increase the natural warming of Earth's atmosphere beyond normal levels.

Evidence of increasing carbon dioxide

The increase in atmospheric carbon dioxide over the past 50 years is undeniable. The US National Oceanic and Atmospheric Administration (NOAA) has maintained measurements at the Mauna Loa Observatory in Hawaii, showing a steady and significant increase in $\text{CO}_2$ levels.

International response

Recognition of the climate change threat has led to international cooperation:

Intergovernmental Panel on Climate Change (IPCC)

The IPCC was established in 1988 as an international body to provide clear, scientific information on climate change. It is open to all member countries of the United Nations and the World Meteorological Organisation. The IPCC's knowledge is highly respected due to the range of contributing scientists and rigorous review processes.

United Nations Framework Convention on Climate Change (1992)

In 1992, Australia and 150 other countries signed this agreement at the United Nations Conference on Environment and Development in Rio de Janeiro, Brazil. Countries recognised the importance of working together to reduce greenhouse gas emissions and combat climate change.

Kyoto Protocol (1997)

This international agreement set emission reduction targets:

• First commitment period (2008-2012): 37 industrialised countries committed to reducing greenhouse gas emissions to an average of 5% below 1990 levels
• Second commitment period (2013-2020): Target increased to 18% below 1990 levels

Consequences of the enhanced greenhouse effect

The enhanced greenhouse effect produces numerous observable and measurable environmental changes:

Physical changes to ice and water

• Glacier shrinkage: Glaciers in the Arctic and other regions have experienced significant reduction in size
• Changed ice formation patterns: In polar regions, ice now forms later in winter and breaks up earlier in spring, affecting animal feeding behaviour and plant flowering and breeding cycles
• Declining Arctic sea ice: NASA satellite data shows the area of Arctic sea ice has fallen from approximately 7 million square kilometres in 1980 to just over 4 million square kilometres recently
• Rising sea levels: NASA satellite readings show an average rise of 85 mm since 1993, while ground-based measurements indicate a rise of nearly 200 mm since 1870. In the long term, this will result in land loss and flooding of coastal cities

Weather pattern changes

While harder to link definitively to increased carbon dioxide and global temperatures, several trends are occurring:

• More frequent and prolonged heat waves and droughts in summer
• More severe snowstorms in winter
• Increased frequency and intensity of hurricanes, severe storms, and cyclones

Ocean acidification

Measurements show a decrease in pH in the world's oceans since the industrial revolution. This phenomenon can be attributed to increased atmospheric carbon dioxide through a series of chemical equilibria.

Temperature effects on carbon dioxide solubility

The solubility of carbon dioxide in water depends on both concentration and temperature. As temperature increases, carbon dioxide molecules gain enough energy to overcome attractive forces with water molecules and escape as gas.

Impact on coral reefs

One of the most visible impacts of decreasing pH and increased water temperatures is damage to coral reefs. The Great Barrier Reef in Australia has been the subject of significant study, showing increasing acidity and obvious coral damage over the past 10-15 years.

The comparison shows healthy, vibrant coral (left) versus damaged, bleached coral (right). Coral bleaching and death result from the combined stresses of warmer temperatures and more acidic conditions, threatening these vital marine ecosystems.

Dealing with the problem

Post-combustion carbon capture

There are several methods for reducing carbon dioxide emissions from combustion. Coal-fired power stations currently produce 40% of Australia's greenhouse gas emissions. One promising approach is post-combustion capture, already widely used in the oil, gas, and chemical industries.

How post-combustion capture works

Normally in a coal-fired power station, exhaust gases containing 10-15% carbon dioxide are released directly to the atmosphere. Post-combustion capture can reduce up to 85% of this $\text{CO}_2$ before release.

The process works as follows:

1. Exhaust gases pass through an absorber unit containing a liquid (typically an amine solution)
2. The amine chemically binds with $\text{CO}_2$, removing it from the gas stream
3. The $\text{CO}_2$-rich amine solution moves to a stripper unit
4. Heat separates the $\text{CO}_2$ from the amine
5. The cleaned amine solution returns to the absorber for reuse
6. The captured $\text{CO}_2$ is cooled and compressed into liquid form
7. Waste gases with greatly reduced $\text{CO}_2$ are released to the atmosphere

Carbon sequestration

The captured liquid carbon dioxide must be stored to prevent its release into the atmosphere. This storage process is called carbon sequestration. The liquid $\text{CO}_2$ is injected into suitable geological formations deep underground, where it becomes trapped in porous rock layers.

Polymer pollution

Plastic production and consumption

Many organic compounds, including alkenes, alcohols, carboxylic acids, and amines, are used to create polymers - large molecules formed by linking many small molecules (monomers) in repeating units. Common polymers include plastics, resins, coatings like teflon, and fibres like nylon and rayon.

The scale of plastic production has grown dramatically:

• By the 21st century, global plastic production exceeded total metal production by volume
• Worldwide plastic production now exceeds 300 million tonnes per year
• The amount of plastic manufactured in the first 10 years of this century equalled the total produced during the entire previous century
• Only about 10% of all plastic is recycled
• Australia ranks in the top five waste-producing nations per capita

Recycling challenges

While plastic packaging provides excellent protection and is lightweight compared to metal or cardboard alternatives, it creates major environmental problems. In the past, more than one-third of plastic consumed in Australia went to landfill. Today, many local councils offer recycling facilities for plastic materials.

Plastic identification codes

Since most plastics are incompatible when mixed, different types must be separated before recycling. Manufacturers use a plastic identification code - a triangle with a number inside - to help identify plastic types. This differs from the recycling symbol, which is a triangle made of three arrows with no number inside.

Benefits of recycling

Recycling benefits the environment in several important ways:

• Reduces landfill: Less plastic waste accumulates in dumps
• Energy efficiency: Recycling plastic uses only 30% of the energy needed to make the original product
• Energy savings: Recycling 1 tonne of plastic saves enough energy to run a refrigerator for a month
• Resource conservation: Reduces raw materials needed for plastic production
• Product creation: It takes 125 recycled plastic milk bottles to manufacture one 140-litre wheelie bin

Current recycling rates and environmental impacts

Currently, Australians recycle approximately 23% of plastic packaging products. The majority goes to landfill, creating a major waste management problem. Plastic debris has accumulated in natural habitats from the poles to the equator, with substantial quantities in marine environments.