Reactivity 1.3 - Energy from fuels

What is combustion and what does it require

an exothermic process of burning something. It requires a fuel, oxygen and a source of ignition.

All metals can oxidise but not all metals can (...) why?

combust due to reactivity

Metal + Oxygen -> ....

metal oxide

1. Equation of combustion of Magnesium and Oxygen

2. What is observed during this reaction

3. What is oxidized

4. What is reduced

5. What do we use to work out what has been oxidized

1. 2Mg(s) + O2(g) -> 2MgO(s)

2. Bright white flame

3. Magnesium

4. Oxygen

5. Using Oxidation States

1. Equation of combustion of Sulphur and Oxygen

2. What is observed during this reaction

3. What is oxidized

4. What is reduced

5. What do we use to work out what has been oxidized

1. S(s) + O2(g) -> SO2(g)

2. Blue flame

3. Sulphur

4. Oxygen

5. Using Oxidation States (Sulphur will have a positive oxidation state)

For ionic and covalent compounds, oxygen is the (....) and the metal/non-metal is the (....)

Oxygen is the OXIDIZING AGENT that GAINS electrons (reduction) and the metal/non-metal is the REDUCING AGENT that LOSES electrons (oxidation)

Why are organic compounds used as fuels

- Release large amounts of energy

- don't spontaneously combust due to a high activation energy and therefore is easy and safe to transport and store.

What are the typical organic compounds used in fuels and combustion

- Hydrocarbons (alkanes)

- Alcohols

What is complete combustion?

- When a fuel burns COMPLETELY, due to enough oxygen being present

- carbon dioxide and water are the products due ALL carbon and hydrogen being OXIDISED

What always occurs in complete combustion

- It breaks the carbon chain

- results in the formation of CO2 and H2O

Equation for combustion of Methane

methane + oxygen → carbon dioxide + water

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)

Equation for combustion of Ethanol

ethanol + oxygen → carbon dioxide + water

C2H5OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (l)

What is incomplete combustion

If there's not enough oxygen, and produces particulate carbon soot or carbon monoxide gas and always produces water.

Why is carbon monoxide dangerous?

It binds with haemoglobin, reducing the blood's oxygen carry-capacity

It is toxic

It contributes to smog

What is the link between the carbon content/percentage and combustion?

The higher the carbon content of the fuel, the more incomplete the combustion is e,g more carbon = more soot produced and less energy released

If more soot is produced what is the appearance of the flame

Dirtier/Smokier

What math formula can be used work out what fuel will produce the dirtiest flame

Percentage by mass. Using this we can work out the percentage of carbon in a compound as the higher the percentage of carbon, the smokier it is.

Name three fossil fuels

coal, oil, natural gas

What is coal

A solid fossil fuel formed from plant remains. It has 80% -90% of carbon

What are three pros and three cons of coal

Pros

- Abundant

- Long lifespan

- Products from the combustion of coal have other uses, e.g. ash can be used to make roads

Cons

- Produces lots of pollution (CO2, SO2, soot)

- Mining can destroy habitats

- Difficult to transport

What is crude oil

a non-renewable mixture of compounds made from dead sea creatures

What are three pros and three cons of crude oil

Pros

- Easy to store and transport in pipelines and tankers

- Impurities can be easily removed

- High energy density (Releases lots of energy per kg)

Cons

- Oil spills destroy ecosystems/habitats

- Produces lots of pollution (CO2, SO2, soot)

- Uneven world distribution

What is natural gas

a mixture of methane and other gases

What are three pros and three cons of natural gas

Pros

- Cheapest of the fossil fuels

- Easy to store and transport in pipelines and pressurized containers

- Large amounts of energy per unit mass (Known as high specific energy)

Cons

- Produces lots of pollution (CO2)

- Expensive and time-intensive to produce

- Expensive to store

Fuels undergo combustion to release what? What is this what?

Specific energy. Specific energy is measure of the energy stored in a substance.

How do you calculate specific energy and energy density

Specific energy(kJ/g) = kJ/mol (bond enthaply) / molar mass

Energy density(kJ/cm^3) = kJ/g (specific energy) x density

Link between carbon content, hydrocarbon chain and specific energy

The longer the hydrocarbon chain, the higher the carbon content and therefore the more carbon dioxide produced. This means a lower specific energy (less energy released.)

Link between state of matter and specific energy

If the fuel/hydrocarbon is a liquid at room temperature, it has a higher energy density due to having a higher boiling point.

Higher the carbon content, the (...) and the (....)

Higher the carbon content, the lower the specific energy and the bigger the tendency to not complete combustion

Higher the oxygen content, the (...)

lower the specific energy

Calculate the mass of carbon dioxide produced when 1.00 g of butane undergoes complete combustion.

1. Start with the balanced chemical equation:

C4H10(g) + 6½O2 (g) → 4CO2 (g) + 5H2O (l)

2. Calculate the moles of butane, using moles = mass/Mr

n (C4H10 (g)) = 0.0172 moles

3. Use the balanced chemical equation to deduce the moles of carbon dioxide produced:

1:4 - carbon dioxide = 4 x 0.0172 = 0.0688 moles

4. Calculate the mass of carbon dioxide produced, using mass = moles x Mr

Mass of CO2 = 0.0688 x (12.01 + (2 x 16.00) = 3.03g

What is a carbon footprint

The carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event.

What are the greenhouse gases and some of their sources?

- Carbon dioxide - produced when living organisms respire and when fuels are BURNED

- Methane - produced by LIVESTOCK and landfill sites as well as being released during mining

- Nitrous oxides - commonly released during the COMBUSTION of impurities in fossil fuels

- Water vapour - produced during combustion as well as by EVAPORATION of the oceans and lakes as part of the water cycle

What is the greenhouse effect

- shortwave radiation from the sun strikes the Earth's surface and is ABSORBED AND RE-EMITTED from the surface of the Earth as INFRARED radiation

- The infrared radiation passes through the atmosphere where SOME thermal energy passes straight through and is

emitted into space

- But some infrared radiation is ABSORBED by greenhouse gases and RE-EMITTED in all directions

- This reduces the thermal energy LOST into space and TRAPS it within the Earth's atmosphere, keeping the Earth warm

What are some impacts of increasing greenhouse gases on the climate

As the concentration of greenhouse gases in the atmosphere increases due to human activity, more thermal energy is trapped within the Earth's atmosphere and emitted, causing the Earth's average temperature to rise

What are renewable sources and an example

Resources that won't run out in the future. e.g Biofuels

What are non-renewable sources and an example

Resources will run out in the future. e.g Coal

Why are biofuels being developed

- depletion of fossil fuels

- reduce pollution and global warming

- uneven distribution of energy

What is a bio fuel?

What are the benefits?

What are the limitations?

A fuel that has been formed through photosynthesis.

- Renewable and sustainable as more of the fuel can be made in a short space of time

- Photosynthesis is not an efficient as a very small % of the solar energy is captured as plants don't cover the earth. Therefore not as much energy is released by biofuels as fossil fuels.

What are the three biofuels you need to know

Wood

Ethanol

Methane

Wood is mainly coal that has been under pressure for millions of years

How does the % of oxygen compare between wood and coal and therefore how does their specific energy compare and why?

The % of oxygen in coal is much lower than in wood, this means the specific energy of coal is much higher than that of wood (3x).

What happens when wood/fuels are exposed to high pressures

Oxygen and water vapour are lost (photosynthesis)

PROS of Wood

CONS of Wood

Pros

- Less polluting

- Renewable/Carbon neutral

Cons

- Produces soot, carbon dioxide when burnt

How is Ethanol made after photosynthesis and what conditions are required

Made through fermentation using the glucose and it requires a temp of 37 degrees and an anaerobic area.

Compared to a bigger organic compound like octane how does its specific energy compare.

Ethanol has a higher % of oxygen than octane and there fore a lower specific energy

PROS of ethanol

CONS of ethanol

Pros

- Since it is partially oxidised, it has a higher tendency to complete combustion so produces less CO and nitrogen oxides

- Reduces a country's dependence on oil

Cons

- can corrode engines/cars

- Lower specific energy than fossil fuels

What 3 ways can methane be produced

- Bacterial breakdown of plant matter

- Breakdown of carbohydrates into a 50% methane mixture

- Breakdown of fats into a 72% methane mixture

What must you remember about fuels cells and electrolysis

In fuel cells, the charges on the electrodes are switched, the anode becomes negative and the cathode becomes positive

Anode always = ...

Cathode always = ...

Anode = Oxidation

Cathode = Reduction

What is a fuel cell and how does it work?

A fuel cell is an electrochemical cell with an alkaline electrolyte in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode

What happens at the anode

Hydrogen/ the fuel is oxidised and its oxidation number increases. It combines with the OH- from the alkaline electrolyte to form:

2H2 (g) + 4OH- (aq) → 4H2O (l) + 4e-

What happens at the cathode?

Oxygen is reduced and gains electrons reacting with water to form OH-:

O2 (g) + 2H2O + 4e- → 4OH- (aq)

All together the overall equation for the reaction at the fuel cell is...

2H2 (g) + 4OH- (aq) + O2 (g) + 2H2O + 4e- → 4H2O (l) + 4e- + 4OH- (aq)

Cancel out like terms

2H2 (g) + O2 (g) → 2H2O (l)

Pros of fuel cells

- Water is the only reaction product, so fuel cells present environmental advantages over other types of cells

- The product can be used as drinking water

Cons of Fuel cells

- Hydrogen is a highly flammable gas

- Very thick walled cylinders and pipes are needed to store hydrogen