SC20 - Fuels ✓ Flashcards by Apurva Bhattacharyya

SC20a - What is a hydrocarbon?

A compound containing only hydrogen and carbon atoms

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SC20a - What is crude oil?

A mixture of hydrocarbons that can be separated into useful fractions

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SC20a - How are crude oil and natural gas formed?

They are formed from the ancient remains of microscopic animals and plants that once lived in the sea
These remains became covered by layers of sediment, and over millions of years the remains turned into natural gas and crude oil.

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SC20a - Why is crude oil considered a mixture?

It contains different types of hydrocarbons (Different lengths and different amount of carbon atoms) and these different hydrocarbons aren’t chemically bonded

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SC20a - What are petrochemicals?

Substances formed from crude oil

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SC20a - What is the main gas in natural gas?

Methane (CH₄)

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SC20b - How is crude oil separated?

Using fractional distillation

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SC20b - What occurs in fractional distillation (of crude oil)?

Crude oil is heated strongly till it evapourates and the hot vapours are pumped into a fractionating column
The fractionating column is hottest at the bottom and coldest at the top
As vapours rise, they will cool down and will condense when they reach the fraction at a temperature lower than their boiling point
Liquid from each fraction falls into a tray and is pumped away
The vapours with the lowest boiling points don’t condense and leave as a mixture of gases
Bitumen has the highest boiling point and leaves the bottom as a hot liquid

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SC20b - What are the products of each fraction (in a crude oil fractionating column), and their uses?

Top to bottom:

Gases: domestic heating/cooking
Petrol: fuel for cars
Kerosene: fuel for aircrafts
Diesel oil: fuel for cars/trains
Fuel oil: fuel for ships
Bitumen: surfacing roads and roofs

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SC20b - How does the number of carbon atoms change from top to bottom of the crude oil fractions?

It increases from top to bottom (Gases is smallest, bitumen is longest)

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SC20b - How does boiling point change from top to bottom of fractions (Gases to bitumen)

It increases from top to bottom (Gases is lowest, bitumen is highest)

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SC20b - How does ease of ignition change from top to bottom of fractions (Gases to bitumen)

It decreases from top to bottom (Gases is easiest, bitumen is hardest)

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SC20b - How does visocsity change from top to bottom of fractions (Gases to bitumen)

It increases from top to bottom (Gases is lowest, bitumen is highest)

Viscosity is ‘how thick’ the fluid is. high viscosity (bitumen) flows with more difficulty than low viscosity (gases)

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SC20c - What are the first four alkanes?

Methane (CH₄)
Ethane (C₂H₆)
Propane (C₃H₈)
Butane (C₄H₁₀)

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SC20c - What are alkanes?

Hydrocarbons with only single covalent bonds between carbon atoms

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SC20c - What are the compounds of crude oil, mostly?

Alkanes

SC20c - What does the family of alkanes form and what are the features of this?

The alkane homologous series has the features:

Molecular formula differs by CH₂
Formula of C_nH_2n+2
Gradual variation in physical properties (e.g boiling points)
Similar chemical properties

SC20c - What does the structural formula of alkanes look like?

*Just a pic*

SC20c - How does the boiling point of alkanes change a they get longer?

The boiling points increase. The longer they get, the less the difference in boiling point

SC20c - How do alkanes react with oxygen?

Combustion: Alkane + Oxygen → Carbon dioxide + Water (With excess oxygen)

SC20d - What is combustion?

When a hydrocarbon burns and oxidises with the oxygen in the air

SC20d - What is complete combustion?

When a hydrocarbon burns in excess oxygen to produce only carbon dioxide and water (as well as energy).
This can be considered oxidation of the hydrocarbon

SC20d - What is incomplete combustion?

When a hydrocarbon burns in a limited supply of oxygen producing carbon and/or carbon monoxide as well as carbon dioxide and water. energy is also produced (although this is less than complete combustion)

SC20d - What is the test for carbon dioxide?

Lime water will turn milky in the presence of carbon dioxide

SC20d - What are the problems with complete combustion?

* Carbon dioxide can dissolve in atmospheric water to produce dilute carbonic acid (H₂CO₃) which can cause weak acid rain * Rising levels of carbon dioxide in the atmosphere leads to global warming

SC20d - What are the problems with incomplete combustion?

* Carbon is black soot or smoke which can block pipes carrying away waste products and can blacked buildings and cause breathing problems * Carbon monoxide is an odourless, colourless, toxic gas that binds to the haemoglobin of red blood cells reducing the amount of oxygen that can bind causing sleepiness and unconsciousness and sometimes even death

SC20e - Describe how combustion of hydrocarbons can lead to acid rain

* Hydrocarbons contain impurities of sulfur that will oxidise with oxygen creating sulfur dioxide * Sulfur dioxide dissolves in water to form sulfurous acid: * SO₂ + H₂O → H₂SO₃ * Sulfurous acid can be oxidised forming sulfuric acid: * 2H₂SO₃ + O₂ → 2H₂SO₄ * Sulfurous acid and sulfuric acid can fall as acid rain

SC20e - How can 'internal combustion' lead to acid rain?

* Internal combustion engines in cars reach temperatures high enough that air inside the engine can cause the nitrogen to be oxide * This forms NO_x (x number of oxygen) which is an atmospheric pollutant * Nitrogen oxides dissolve with atmospheric water and form nitric acid: * NO_x + H₂O → HNO₃ (unbalanced) * Nitric acid can fall as acid rain * Catalytic converters tend to reduce the NO_x produced

SC20e - What pH is acid rain?

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SC20e - What are the effects of acid rain?

* Crops don't grow well in low pHs and so will die * Excess acidity in rivers and lakes can kill fish/insects and prevent eggs from hatching * Increased rate of metal's corrosion: * Metal + sulfuric acid → metal sulfate + hydrogen * Weathering of limestone (calcium carbonate) buildings due to neutralisation: * Limestone + Sulfuric acid → Calcium sulfate + water + carbon dioxide

SC20f - What is the process of breaking down a hydrocarbon and how does it occur?

* Cracking is the process of breaking down a large hydrocarbon (often an alkane) into smaller more useful hydrocarbons (often alkane and alkene) * Crude oils are heated to evaporate and the vapours are passed over a catalyst and heated to 〰️650 C

SC20f - What are alkenes?

* Alkenes are a type of hydrocarbon * Their general formula is C_nH_2n

SC20f - Why are alkanes/kenes saturated/unsaturated?

* Alkanes are saturated as they ONLY contain single (C-C) bonds between carbon atoms * Alkenes are unsaturated as they contain ONE double (C=C) bond between their carbon atoms

SC20f - How does supply and demand vary between fractions of crude oil?

* While fuel oil and bitumen are in much higher supply than demand, the other are in much higher demand than supply * As fuel and bitumen are the two largest hydrocarbons in the fraction, this is the reason for cracking to occur

SC20f - What are alkenes from crude oil used for?

Creating polymers

SC20f - What are the pros and cons of using hydrogen rather than hydrocarbons as fuel for cars?

**Pros**: * Naturally is a by product of cracking and can be easily obtained by reacting methane with steam * Produces water but no carbon dioxide (no greenhouse gas emissions) **Cons**: * Hydrogen is a gas at room temperature making it hard to store/transport unless kept at high pressure/liquified

How do we test for water during a combustion investigation?

* White anhydrous copper sulfate in the U-shaped tube turns into blue hydrated copper sulfate

How do we test whether a gas is an alkene or alkane?

- Alkenes turn bromine water colourless