C6 - Chemistry Out There Flashcards
Write the half equations of the electrolysis of aluminium oxide and the electrode it takes place at.
Anode:
2O2- - 4e- –> O2
Cathode:
Al2+ + 2e- –> Al
What is electrolysis?
The decomposition of a liquid by passing an electric current through it
What are the products formed during the electrolysis of aqueous sodium hydroxide (NaOH) and dilute sulfuric acid (H2SO4)?
Cathode: hydrogen
Anode: hydroxide (water and oxygen bubbles form)
Write the half equations for the electrolysis of dilute sulfuric acid, containing H+, SO42- and OH- ions
2H+ + 2e- –> H2
4OH- - 2e –> OH-
What affects the rate of electrolysis?
The current and the time taken - each one is directly proportional to the amount of product made during electrolysis
What is the cation and the anion in the electrolysis of lead bromide? What’s formed at each electrode? What are the half equations?
Cation: Pb2+ Anion: Br-
Lead formed at cathode, bromine formed at anode
Pb2+ + 2e- –> Pb
2Br- - 2e- –> Br2
In an electrolysis experiment using a current of 1.0A, 1g of copper was deposited in 60 minutes. 1) How long will it take to deposit 3g if the current is 2A? 2) What current needs to be used to deposit 10g of copper in 4 hours?
1) 1.5 hours
2) 2.5A
Describe the energy level diagram for an exothermic reaction (used in a hydrogen fuel cell).
The reactants have greater energy than the products - energy is given out to the environment during the reaction
Describe the energy level diagram for an endothermic reaction.
The reactants have less energy than the products - energy is taken in from the environment during the reaction
Describe how a hydrogen fuel cell works.
The fuel used is hydrogen and as it enters the cell at the anode. The hydrogen is forced through the catalyst and splits into two H+ ions and two electrons. The protons diffuse through the proton exchange membrane. The unused hydrogen leaves the cell before going through. The electrons can’t diffuse through so go through the electric circuit and produce the current to power the cell. The hydrogen ions that diffuse through react with the oxygen that enters at the cathode to produce water, which is the waste product. A hot phosphoric acid catalyst is needed in this reaction.
What’s the overall reaction in a hydrogen fuel cell?
2H2+O2 –> 2H2O
What are the half equations that take place in hydrogen fuel cell?
Anode: H2 - 2e- –> H+ 2H2O
What are the advantages of fuel cells in space?
- They’re lightweight
- The water (waste product) can be drunk by the men on a manned spacecraft
- They’re compact
- They have no moving parts
Compare the use of fuel cells to hydrocarbon fuels (diesel, petrol).
- Fuel cells don’t release harmful waste products (such as CO2), so don’t contribute to global warming and the greenhouse gases
- There’s a large potential source of hydrogen - the water can be decomposed by electrolysis to produce even more hydrogen
- The stages in producing a fuel cell are more efficient than conventional fuels - electrical energy transferred directly when required
- The catalyst in hydrogen fuel cells are often poisonous
- Most of the world’s hydrogen production is done using fossil fuels (reacting steam with coal or natural gas) - this produces CO2 emissions
Explain a thermite reaction.
A vigorous exothermic reaction between aluminium and iron (III) oxide that produces aluminium oxide and iron.
The iron (III) oxide loses oxygen, so is reduced to iron - this melts. The aluminium gains oxygen, so is oxidised to aluminium oxide.
2Al2 + Fe3O3 –> Al2O3 + 3Fe
What is a reducing and oxidising agent?
Reducing Agent - substance that causes the other to be reduced (gain electrons/lose oxygen)
Oxidising Agent - substance that causes the other to be oxidised (lose electrons/gain oxygen)
What is the order of reactivity (most to least reactive) of the 5 common metals?
- Magnesium
- Zinc
- Iron
- Tin
- Copper
A reaction takes place between Iron(III) Nitrate and Magnesium. What is the overall reaction and the half equations? What are the reducing and oxidising agents?
Fe(NO3)3 + Mg --> Fe + Mg(NO3)2 Fe3+ + 3e- --> Fe - reduced Mg - 2e- --> Mg2+ - oxidised Specator ions = NO3- The reducing agent is Magnesium and the oxidising agent is Iron.
What two things does rusting require?
Oxygen and water
What is the equation for rusting? What happens to the iron and the oxygen?
iron + oxygen + water –> hydrated iron (III) oxide
The iron is oxidised to form Fe3+ ions, and the oxygen is reduced to form oxide (O2-) ions
What are the methods of preventing rust?
- Coat with a layer of paint
- Coat with a layer of oil or grease
- Galvanising - add a layer of zinc
- Using alloys of the iron - adding carbon or chromium
What is sacrificial protection?
A layer of either zinc or tin is added to the iron. This ensures that when the material is exposed to air and water, the air and water reacts with the layer instead of the iron. Therefore, the zinc or tin is rusted instead of the iron - thus sacrificing itself. However, if the tin layer is scratched and the iron exposed, the iron will rust faster as iron is more reactive than tin. If zinc is used then the iron won’t be reacted with regardless, as zinc is more reactive than iron and is reacted with in preference.
What are some uses of ethanol?
- Common drinking alcohol
- Fuel
- Solvent in perfumes and aftershaves
What does it mean if a substance is organic?
It contains carbon, excluding CO2 and carbonate substances
Explain the fermentation reaction and what’s needed.
Yeast are needed as they contain enzymes to convert glucose to CO2 and ethanol - oxygen can’t be present because yeast anaerobically respire.
C6H12O6 –> CO2 + C2H5OH
A warmer place is optimal as the enzymes have more energy to work. Limewater is used in experiment at school to prove CO2 is made and act as an airlock so oxygen can’t get back in once it’s out.
The ethanol eventually kills the yeast so it sinks to the bottom, and the ethanol is purified using fractional distillation.
Explain the purification of ethanol.
The ethanol is extracted from the yeast to increase the purity of ethanol. Heat is applied to the ethanol-water mixture. A fractionating column is on top of the ethanol-water mixture, which helps the ethanol vapour separate out when heated. The vapour is condensed in a condenser to become purified ethanol liquid
What’s the equation that take places in a hydration reaction?
ethene + steam –> ethanol
C2H4 + H20 –> C2H5OH
What’s needed in a hydration reaction?
Temperature of 300C
A hot phosphoric acid catalyst
Compare the fermentation and hydration reaction.
- Fermentation is more sustainable because it uses a natural renewable resource (yeast), unlike Hydration that uses a non-renewable crude oil (ethene)
- Hydration is more expensive than Fermentation because of the conditions needed - high temp, catalyst
- The atom economy and percentage yield of Hydration is 100%, Fermentation is 51% atom economy and 15% percentage yield
- Fermentation produces a low-purity product, Hydration produces a high-purity product - no waste`
What do all alcohols contain?
A hydroxyl group - OH
Their general formula is CnH2n+1OH
What are the three steps in CFC’s reaction with the ozone layer?
- Initiation
- Propagation
- Termination
Explain UV light’s involvement with the ozone layer.
The ozone layer absorbs UV light to produce ozone molecules O3 from O2.
The reaction is: 3O2 –> 2O3
How is the ozone layer affected by CFCs?
- INITIATION STEP:
There is a carbon-chlorine covalent bond within a CFC molecule. UV light breaks this covalent bond and a chlorine radical is formed. They’re highly reactive because they’re unpaired. - PROPAGATION STEP:
The chlorine radical reacts with ozone molecules to form another chlorine radical and oxygen:
:Cl + O3 –> :ClO + O2
:ClO + O3 –> :Cl + 2O2
It damages the ozone and sets up a chain reaction - one chlorine molecule can damage 100,000 ozone molecules - 2O3 –> 3O2 - TERMINATION STEP:
The chlorine radical is reacted with another chlorine radical to form a chlorine molecule:
:Cl + :Cl –> Cl2
Where did CFCs used to be found?
Aerosols
Refrigerants
Air conditioners
Why did scientists change their minds about CFCs?
At the 1987 Montreal Protocol they limited production and use of CFCs. They are soluble in water, have a low boiling point and are unreactive. These were seen as positives, but when they realised the effect on the ozone layer they became negatives. It being unreactive means it can’t be removed from the stratosphere easily.
What’s an alternative to CFCs?
HFCs - hydrofluorocarbons
Doesn’t have the carbon-chlorine bond, so no harm to the ozone
What is cataracts?
It can be caused by excess UV light exposure. The proteins in the eyes are damaged, so the lens becomes cloudy
What is soft water?
Water containing a low concentration of calcium and magnesium ions. It can form a lather with soap easily.
What is hard water?
Water containing a high concentration of calcium and magnesium ions. It can’t form a lather with soap easily. ‘Scum’ is also formed
Why do some areas have soft water?
It depends on the rock in the areas. Most are igneous rocks (like granite) and aren’t susceptible to dissolving and absorbing calcium ions
Why do some areas have hard water?
Carbon dioxide in the air dissolves into rain to produce a weak acid. Chalk and limestone rock contain calcium carbonate, which reacts with the rainwater and CO2 to produce calcium hydrogencarbonate (temporary hard water). If the rock contains dissolved calcium sulfate then this makes permanent hard water
What are the equations for temporary and permanent hardness?
Temporary:
2CaCO3 + H2O + CO2 –> Ca(HCO3)2
Permanent:
CaSO4 + BaCl2 –> BaSO4 (white ‘scum’ solid) + CaCl2
How does boiling affect permanent hard water?
It doesn’t
How does boiling affect temporary hard water?
The calcium hydrogencarbonate breaks down when heated to form calcium carbonate, water and carbon dioxide. Calcium hydrogencarbonate is soluble in water but calcium carbonate is insoluble, so it forms limescale and removes the Ca2+ ions from the water, and thus the hardness.
Ca(HCO3)2 –> CaCO3 + H2O + CO2
How can limescale be removed?
Using a weak acid, such as ethanoic acid (vinegar) or citric acid. They don’t react with the unit but rather the limescale to form a calcium salt, water and carbon dioxide: calcium carbonate + citric acid –> calcium citrate + water + carbon dioxide
How does limescale affect a unit?
The greater the thickness of limescale, the more energy required by the unit
Describe the water in a steam iron.
The water is boiled before being put in the iron so that limescale won’t ruin the unit if it were boiled in the unit
How can you remove water hardness?
- ion-exchange resin
- washing soda (Na2CO3)
- Boil temporary hard water (but limescale is produced)
How does washing soda react with hard water?
Washing soda is sodium carbonate, which is soluble in water. The calcium and magnesium ions are substituted by the sodium ions in the water, so the hard water is softened.
Ca(HCO3)2 + Na2CO3 –> CaCO3 (solid precipitate) + 2NaHCO3
How does an ion exchange column work?
The hard water passes through an ion-exchange column, which can be plumbed into the water supply in a house. The columns contain ion-exchange resins (tiny peads packed into a metal/plastic tube).
The calcium and magnesium ions in the water are swapped with the sodium ions on the resin. The sodium ions go into the water and the calcium and magnesium ions attach to the resin
How can you regenerate an ion exchange resin?
Adding salt (sodium chloride, NaCl) to remove the Ca2+ and Mg+ ions from the resin and swap back in the Na+ ions
Why is the water in dishwashers and washing machines softened?
The water is softened using an ion-exchange column to stop the build up of limescale on the plates and clothes
What is the difference between fats and oils?
At room temp, fats are solid and oils are liquid
What can vegetable oils be used for?
- Biodiesel - renewable resource, unlike diesel
- Soap
- Part of a healthy diet
What are esters?
Compounds that form when a carboxylic acid (organic acid - COOH) reacts with an alcohol
What is the structure of fats and oils?
They consist of fatty acids that are chemically joined to glycerol (has three hydroxyl (-OH) groups rather than one)
They have long chains of carbon atoms
Describe an unsaturated fat/oil.
At least one double bond within the carbon-carbon chain.
Turns colourless in its reaction with bromine water
Describe a saturated fat/oil.
No double bond within the carbon-carbon chain.
Remains orange in its reaction with bromine water
How does bromine water react with an unsaturated fat/oil?
It reacts with the double bonds within the carbon-carbon chain in an addition reaction to produce a colourless dibromo compound. The double bond breaks into a single bond and another single bond formed with bromine per carbon atom
How many fatty acids react with glycerol?
For every molecule of glycerol, three fatty acids react with it
What are immiscible liquids?
Two or more liquids that don’t dissolve into one another, such as oil and water
What is an emulsion?
Tiny droplets of one of the liquids dispersed throughout the other liquid
What is an oil-in-water emulsion? Give an example.
Tiny droplets of oil dispersed throughout water.
Example: Milk - watery liquid with butter fat droplets dispersed throughout
What is a water-in-oil emulsion? Give an example.
Tiny droplets of water dispersed throughout oil.
Example: Butter - butterfat with watery liquid dispersed throughout
What is margarine?
A water-in-oil emulsion that consists of a blend of vegetable oils mixed with water
What is a spreadable solid?
Unsaturated vegetable oils have relatively low melting points, so must be blended with saturated vegetable oils to reach the desired consistency. They’re turned into saturated vegetable oils in their reaction with hydrogen (hydrogenation reaction)
What is a hydrogenation reaction?
Can also be referred to as an addition reaction. It’s the reaction between a vegetable oil and hydrogen to form a spreadable solid. The hydrogen breaks down the double bonds into single bonds, making it a saturated fat/oil
How is soap made?
Oils (often olive, coconut or palm) or fats react with sodium hydroxide solution
The ester bond is broken and split into glycerol, and sodium salts of the fatty acids which are the soap. This is SAPONIFICATION
What is hydrolysis?
When a substance is broken down through its reaction with water
What happens during saponification?
This is an example of a hydrolysis reaction. The OH- ions from the NaOH solution break down the oil or fat molecule in a similar way to a water molecule. The ester bond between the fatty acids and the glycerol break: fat + sodium hydroxide --> soap + glycerol The ester (carboxylic) reacts with the Na+ ions in NaOH instead of the H+ ions, which are removed because NaOH neutralises them as it's an alkali
What do each of the washing powder ingredients do?
Active detergent - does the cleaning
Optic Brightener - gives a ‘whiter than white’ look
Bleach - removes coloured stains
Enzymes - remove stains in cool washes
Water softener - prevents ‘scum’ building on the clothes
What do each of the washing liquid ingredients do?
Active detergent - does the cleaning
Water - thins the liquid so it pours easily
Rinse agent - helps the water drain off
Colouring agent and fragrance - makes the product attractive to use
Describe the structure of a detergent molecule
Hydrophilic head and hydrophobic tail
How does a detergent molecule work to remove stains?
The hydrophobic tail forms strong intermolecular bonds with the fat in the grease stain, surrounds it and pulls it out from the clothes into the water to be washed out. The hydrophilic head forms strong intermolecular bonds with the water rather than the grease.
Why is rinsing essential with biological washing powders?
To remove the enzymes from the clothes so they don’t irritate your skin
Why is cleaning clothes at low temperatures an advantage?
- enzymes can be used - wider variety of stains can be removed
- less energy required, so cost is lower
- wider variety of fabrics can be washed as they won’t be damaged by low temp (e.g shrunk, ripped)
What is dry cleaning? How does it work?
It’s a way of cleaning clothes without using water. A solvent such as tetrachloroethene is used instead of water to dissolve the greasy stains that don’t dissolve in water.
The clothes are cleaned in a machine that’s fitted to stop the solvent fumes escaping. They’re washed with solvent, rinsed with fresh solvent (so there’s no grease in the rinse) and then dried. The used solvent is distilled to remove the stains and then reused.
Describe the forces in dry cleaning.
The intermolecular forces between the grease molecules and the solvent molecules independently are weaker than between the grease and solvent molecules. The solvent molecules surround the grease molecules and pull it off the fabric and into the solvent. The solvent is then dirty so is renewed for the rinse.
