C6 - chemistry out there Flashcards
Why are fuel cells used in spacecraft?
The water produced is not wasted -the astronauts drink it.
They are lightweight; normal batteries are heavier, so the spacecraft can use the saving to carry a bigger payload.
They are compact.
There are no moving parts.
Why does an electrolyte have to be a liquid?
Electrolytes are ionic. The charge moves through the molten liquid by the movement of ions.
If the electrolyte solidifies then the ions are in fixed positions are cannot move and the current cannot flow.
Electrolysis is the flow of charge by loving ions. These moving ions discharge at electrodes.
Why is the car industry developing fuel cells?
- there are no carbon dioxide emissions when fuel cells are used in cars.
- fossil-fuels are non-renewable, so at manufacturers are researching alternatives that are renewable and cause less pollution.
- the main product of a hydrogen-powered fuel cell is water, which is not a pollutant.
- there is a large source of hydrogen available by decomposing water.
What are the advantages of hydrogen-oxygen fuel cells?
- there is direct energy transfer. Energy is converted directly from chemical energy in the fuel into electrical energy. The energy does not have to be converted into heat first.
- there are fewer production stages. Every new stage in any process increases the energy losses. In the fuel cells themselves the energy conversion is all done in a single stage.
- they all more efficient as there are fewer stages and almost all the energy is converted.
- they are less polluting. A hydrogen-oxygen fuel cell produces water and no nitrogen oxides.
- fuel cells last longer than conventional rechargeable batteries before they need to be replaced.
- fuel cells weigh less than the current style of batteries in electric vehicles.
What are the problems with hydrogen-oxygen fuel cells?
- poisonous catalysts are used that have to be disposed of at the end of the life-time of the cell.
- fossil fuels are burnt to produce the hydrogen and oxygen needed.
How have scientists attitudes to CFCs changed since when they were first discovered?
When CFCs were first discovered they were thought to be totally safe.
- scientists now know the CFC molecules slowly move upwards into the stratosphere when they attack the ozone layer
- society agreed with scientists’ views that it is CFCs that deplete the ozone layer. This is why the use of CFCs has been banned in the UK but it is a global problem.
What can CFCs be replaced with?
CFCs can be replaced with alkanes or hydrofluorocarbons (HFCs) which will not damage the ozone layer.
How does the ozone limit the amount of UV radiation we receive?
- the UV part of the EM spectrum has exactly the right frequency to make ozone molecules vibrate
- the energy of the UV radiation is converted into movement energy inside each molecule
- the thicker the ozone layer, the more UV radiation is absorbed
How do CFCs deplete the ozone layer?
In the stratosphere, the UV radiation from the sun breaks down the CFC molecules to give highly reactive Cl atoms. A single chlorine atom is called a chlorine radical.
1) these chlorine atoms react with ozone molecules, turning the ozone back into oxygen gas and depleting the ozone layer
2) chlorine atoms are regenerated so they can react with more ozone molecules
3) UV light breaks down CFCs very slowly, so they last a long time
What is the equation for the depletion of the ozone layer?
CF2Cl2 –> (UV above the arrow) CF2Cl + Cl•
What does redox mean?
•redox is a word made up from two processes that work together.
-the two processes are reduction and oxidation.
Give an example of a redox reaction
Rusting is a redox reaction:
iron + water + oxygen –> hydrated iron(III) oxide
•galvanising protects iron from rusting by covering it with a layer of zinc.
- the layer of zinc stops water and oxygen from reaching the surface of the iron.
- zinc also acts as a sacrificial metal, as it is more reactive than iron.
