chemistry - topic 6: global challenges

What are the first five prefixes?

Meth-

Eth-

Prop-

But-

Pent-

Define an alkane.

• A saturated hydrocarbon

• General formula C_nH_2n+2

• Colourless compound

• Less reactive

Define an alkene.

• Un-saturated hydrocarbon

• Functional group is C=C (Carbon double bond)

• General formula C_nH_2n

• More reactive because it can make more bonds by breaking up its double bond

What is an addition reaction and what undergoes it?

An alkene opens up it’s double bond, causing other atoms to bind to it.

What is the bromine water test?

A test to distinguish alkenes and alkanes:

• Bromine water would change to colourless if there are double bonds

What bonds hold together monomers?

Covalent bonds

What conditions are required for polymerisation?

High pressure and a catalyst.

What are some examples of polymers?

Resin, plastic, polystyrene.

How do alkenes become polymers?

• The double bond breaks and forms a bond with the adjacent monomer

• Brackets are added

• A subscript ‘n’ is added to represent the repeat unit

• Poly(alkene)

What is the functional group of alcohols?

OH

What is the general formula of alcohols?

C_nH_2n+1OH

What are the uses of some alcholols?

Ethanol - Beer and wine

Methanol/Ethanol - Solvents (dissolves solute) in perfume

How can we make alcohol (renewable materials)? [4]

• Fermentation (glucose → carbon dioxide + ethanol)

• Relies on yeast cells that catalyse the reaction

• 35C and normal atmospheric pressure in a lab

• Limewater during fermentation turns cloudy and forms white precipitate (calcium carbonate)

• Low percentage yield

How can we make alcohol (non-renewable materials)? [4]

• Hydration (ethene + steam ⇌ ethanol)

• Maximum yield: low temperature and high pressure

• However this isn’t efficient, so it is done at 300C and 60 atmospheres to maximise rate (with a phosphoric catalyst)

• High percentage yield

What is the functional group of carboxylic acids?

COOH

What is the general formula of carboxylic acids?

C_nH_2n+1 COOH (n is one less than it should be)

Why are carboxylic acids considered acids?

Because their atoms disassociate when dissolved in water to form H+ ions.

What is crude oil?

A mixture of hydrocarbons.

How is crude oil formed?

• Organisms die and fall onto the seabed (high pressure anaerobic env)

• Formation of sediment on top

• Millions of years later - crude oil forms

• Crude oil is finite

Describe the method that separates hydrocarbons in crude oil.

Fractional Distillation:

• Crude oil is vaporised through a blast furnace and sent it through a fractionating column

• Gases with higher BPS escape at the bottom of the column because that is where it is hot

• Gases with lower BPS escape at the top of the column because that is where it is cold - these gases rise, cool, and condense.

• Each section of the column is a fraction

• As the size of the hydrocarbon increases, the boiling point increases because the intermolecular forces get stronger and require more energy to break

What are the characteristics of short and long chains of hydrocarbons?

Short - Low BPS, more likely to evaporate (volatile), easier to ignite, less viscous.

Long - Higher BPS, less volatile, harder to ignite, more viscous.

What is cracking and why is it done?

The breakdown of long hydrocarbons into shorter hydrocarbons through thermal decomposition.

This is done because short hydrocarbons have high demand and low supply, while long hydrocarbons have low demand and high supply.

How do we carry out cracking?

• Heat between 600-700C

• Catalysts (silica or alumina)

• Two products: a shorter alkane and an alkene (for polymers)

How do we make a chemical cell and what are its uses?

• Zinc sulfate in a beaker with a zinc strip

• Copper sulfate solution in another beaker with a copper strip

• Connect the solutions with filter paper soaked with potassium nitrate solution

• Connect a voltmeter to measure p.d.

• Use: Batteries in phones/torches

How do we make a hydrogen-oxygen cell and what are its uses?

• Hydrogen is put inside the cell and gives up electrons

• Electrons are donated, which move up and around the circuit

• Hydrogen ions diffuse across the electrolyte

• Oxygen enters the other side which reacts with the H⁺ ions and electrons

• Overall equation: 2H₂ + O₂ → 2H₂O

• Use: Turns hydrogen and oxygen into electricity and water.

Pros and cons hydrogen-oxygen fuel cells?

Pros: No pollution, more energy, no power lost in transmission

Cons: Expensive materials

How was the Earth’s atmosphere formed?

Volcanic activity

What did the early atmosphere consist of and how does it compare to the modern atmosphere?

Early atmosphere:

• Little oxygen (ancient rocks form with iron compounds)

• Carbon dioxide, water vapour (released by the volcano)

• Ammonia

Now: Less water vapour, less co2, more oxygen (78% nitrogen).

• Water vapour condensed due to Earth cooling

• CO2 dissolved as oceans formed and due to photosynthesis

• Oxygen is a by-product of photosynthesis

What are the three main g’house gases?

Carbon dioxide, water vapour, methane

Describe the greenhouse effect process.

• Short wavelength radiation (UV, VL) heats up the Earth’s surface

• The Earth emits a longer wavelength radiation (IR)

• Radiation is absorbed by greenhouse gases

• Radiation is re-radiated in all directions incl. back to Earth

What are the consequences of climate change?

• Ocean warming/marine heatwaves

• Glacier melting - declines in sea ice

• Extreme weather

Why is the science for climate change not completely certain?

Some scientists disagree

What are the differences of high density and low density polyethene?

HDPe - High pressure, moderate temps, side chains, low MPS

LDPe - Low pressure and temp, catalyst, no side chains, stronger attractions, high MPs

What are the differences between thermosetting and thermosoftening?

Softening: No crosslinks, low MP, melt when heated

Setting: Crosslinks, high MP, does not melt when heated

What are alloys and name 2 examples.

A mixture of 1+ metals (and other elements).

Examples: Brass (copper and zinc), bronze (copper and tin), steel (iron and carbon).

Define corrosion.

The gradual destroying of a metal by reacting it with substances in the environment.

Rusting can only occur on // and // .

Iron and steel.

What are the products of complete combustion?

Carbon dioxide and water

What are the products of incomplete combustion?

Carbon monoxide, water, and carbon.

What is the differene between potable water and pure water?

Potable - Water that is safe to drink, containing minerals and micro-organisms

Pure - Chemically only H2O

How do we make water potable from freshwater sources?

• Water collected in a reservoir

• Water is screened

• Sedimentation and coagulation to form sludge - uses aluminium sulfate and lime

• Fine filtration

• Sterilised with chlorine or UV light

• Storage

How do we make salt water potable?

1. Simple Distillation

• Water heated and evaporated

• Condenses and collected in a beaker

2. Reverse osmosis

• Low WP to high WP passing through a semi-permeable membrane

• Pressure is applied which stops salt passing through

How do we make waste water potable?

• Water is screened

• Sedimentation and coagulation

• Effluent is aerobically digested

• Sludge is anaerobically digested and used as fertiliser

How do we extract copper?

Reduction with carbon

1. Copper sulfide + oxygen → copper oxide + sulfur dioxide

2. Copper oxide + carbon → carbon dioxide + copper

How do we extract iron?

Blast Furnace

1. Iron ore, coke, and limestone is sent through a blast furnace.

2. Coke reduces carbon dioxide to make carbon monoxide

3. Carbon monoxide reduces iron ore to make iron

4. Limestone removes impurities to form slag

How do extract aluminium?

Electrolysis

1. Graphite cathode and anode

2. Molten aluminium is mixed with cryolite to lower MP

3. Aluminium ions are reduced to form aluminium at the cathode, and oxygen is oxidised to form oxygen gas at the anode.

4. The anode reacts with the oxygen, so it needs to be replaced frequently.