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Flashcards in Industrial Chemistry Deck (37):

what is electrolysis?

the process by which ionic substances are broken down into simpler substances using electricity. During electrolysis, metals and gases may form at the electrodes.


what happens during electrolysis?

- positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced
- negatively charged ions move to the positively charged electrode during electrolysis. They lose electrons and are oxidised


what is an ionic substance?

an ionic substance forms when a metal reacts with a non metal. They contain charged particles called ions. For example, sodium chloride forms when sodium reacts with chlorine. It contains positively charged sodium ions and negatively charged chloride ions. Ionic substances can be broken down by electricity


what has to happen for electrolysis to work?

for electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or molten (melted).


what is an experiment to investigate electrolysis?
- set up apparatus like so
- the lead (metal) will be attracted to the negative cathode where it will gain electrons
- the bromine (non metal) will be attracted to the positive anode where it will lose electrons
- lead and bromine formed


how do you write ionic half-equations representing the reactions at the electrodes during electrolysis?

At the positive electrode, electrons will be lost: to show this we write the lost electrons as products:
2Br- = Br2+2e-
make sure the charges are equal on both sides: 1->1-
At the negative electrode, electrons will be gained so we write them as reactants:
2H+ + 2e- = H2


what is the order of the reactivity series?

- potassium
- sodium
- calcium
- magnesium
- aluminium
- zinc
- iron
- tin
- lead
- copper
- silver
- gold
- platinum


what do the more reactive metals do?

give up their electrons more easily so make compounds more readily


what is oxidation?

loss of electrons


what is reduction?

gain of electrons


what is a dynamic equilibrium?

a chemical equilibrium between a forward reaction and the reverse reaction where the rate of reactions are equal


what is ammonia?

ammonia (NH3) is a compound of nitrogen and hydrogen. It is a colourless gas with a choking smell, and a weak alkali which is very soluble in water.


what is ammonia used for?

ammonia is used to make fertilisers, explosives, dyes, household cleaners and nylon


how is ammonia manufactured?

ammonia is manufactured by combining nitrogen and hydrogen in an important industrial process called the haber process


how is ammonia used to manufacture nitric acid?

- ammonia is oxidised with oxygen from air using a hot platinum catalyst to form nitrogen monoxide and water
- the gas is cooled and reacted with more oxygen to form nitrogen oxide
- this is reacted with more oxygen and water to form nitric acid


what is nitric acid used for?

- used to make nitro-aromatic compounds from which dyes are made
- it is also used in the manufacture of artificial nitrogenous fertilisers (like ammonium nitrate)
- the fertiliser salts are made by neutralising ammonia solution with the appropriate acid
- the resulting solution is heated, evaporating the water to crystallise the salt


what is the test for ammonia?

- ammonia has a characteristic sharp, choking smell
- It makes damp red litmus paper turn blue
- ammonia forms a white smoke of ammonium chloride when hydrogen chloride gas, from concentrated hydrochloric acid, is held near it
- it will put out a lit splint


why are very reactive metals harder to extract?

the oxides of very reactive metals, such as aluminium, form stable oxides and other compounds. A lot of energy is needed to reduce them to extract the metal


why are less reactive metals easier to extract?

the oxides of lesser reactive metals, such as iron, form less stable oxides and other compounds. Relatively little energy is needed to reduce them to extract the metal


how are very reactive metals (above carbon) extracted?

by electrolysis


how are less reactive metals (below carbon) extracted?

displaced from its ore by reaction with carbon or carbon monoxide


why are the least reactive metals not extracted?

e.g gold, because it is so unreactive, it is found as the native metal and not as a compound, so it does not need to be chemically separated. However, chemical reactions may be needed to remove other elements that might contaminate the metal


how is aluminium extracted from purified aluminium oxide by electrolysis?

- aluminium ore is called bauxite. The bauxite is purified to yield a white powder, aluminium oxide, from which aluminium can be extracted
- the extraction is done by electrolysis
- but first, the aluminium oxide must be made molten so that electricity can pass through it
- aluminium oxide has a very high melting point (over 2000°), so it would be expensive to melt it
- Instead, it is dissolved in molten cryolite, an aluminium compound with a lower melting point than aluminium oxide
- The use of cryolite reduces some of the energy costs involved in extracting aluminium


what happens in an aluminium oxide electrolysis tank?
- aluminium metal forms at the negative electrode and sinks to the bottom of the tank, where it is tapped off
- oxygen forms at the positive electrodes. This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide, and they gradually burn away
- consequently, the positive electrodes have to be replaced frequently, which adds to the cost of the process


why is the extraction of aluminium so expensive?

because of the amount of electricity used up in the reaction process


what are the ionic half-equations for the reactions at the electrodes in aluminium extraction?

- during electrolysis, metal ions, which are positive, gain electrons from the negative electrode (cathode) to form neutral metal atoms: Al3+ + 3e− → Al
- at the positive electrode (anode), non-metal ions, which are negative, lose electrons to from neutral atoms. These atoms join to make molecules of the non-metal element, such as a molecule of oxygen gas: 2O2− → O2 + 4e−
- the removal of electrons from the cathode and addition of electrons to the anode means that an electrical current is passing through the electrolyte


how is iron extracted from its ore?

iron is extracted from its ore in a huge container called a blast furnace. Iron ores such as hematite contain iron oxide. The oxygen must be removed from the iron oxide to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions.
Carbon is more reactive than iron, so it can push out or displace the iron from iron oxide. Equations: iron oxide + carbon → iron + carbon dioxide/ 2Fe2O3 + 3C → 4Fe + 3CO2
In the blast furnace, it is so hot that carbon monoxide can be used to reduce the iron oxide in place of Fe2O3 + 3CO → 2Fe + 3CO2 oxide + carbon monoxide → iron + carbon dioxide/


what is the function or iron ore in the extraction of iron?

it is a compound that contains iron


what is the function of coke in the extraction of iron?

it contains carbon and burns in the air to produce heat, and reacts to from carbon monoxide (needed to reduce the iron oxide)


what is the function of limestone in the extraction of iron?

it contains calcium carbonate, which helps to remove acidic impurities from the iron by reacting with them to form molten slag


what is the function of air in the extraction of iron?

it contains oxygen which allows the coke to burn, and so produces heat and carbon monoxide


what are the uses of aluminium?

- it has a low density, which makes it light for its size, and it's strong. This makes it good for the bodies of planes, light vehicles and ladders
- it has a very thin layer of oxides in its surface, preventing water and oxygen from getting to it, making it a metal which resists oxidation
- it is malleable and its protective oxide layer prevent it from reacting with chemicals in food (tin foil)
- it conducts heat and electricity well, so it is used in power cables and cooking materials


what are the uses of iron?

- it is one of the three magnetic metals, so it is usually used in electromagnets (e.g scrapyards to pick up cars)
- it is malleable
- used for frames of large buildings, bridges because it is very strong
- used in cables for cranes because it is very strong under tension (when stretched)
- used to reinforce concrete as it expands and contracts at the same rate when heated and cooled, and it adds strength and flexibility to the concrete


what does the manufacture of ammonia use?

- nitrogen from the air
- hydrogen either from natural gas or cracking of hydrocarbons


what is the haber process?

the raw materials for this process are nitrogen and hydrogen. Hydrogen is obtained by reacting natural gas-methane-with steam, or through the cracking of oil. Nitrogen is obtained by burning hydrogen in air. Air is 80% nitrogen, all the rest is oxygen. When hydrogen is burned in air, the oxygen combines with the hydrogen, leaving nitrogen behind


what are the conditions required during the haber process?

- a high temperature (about 450°)
- a high pressure (about 200 atmospheres)
- an iron catalyst


how does the cooling of the reaction mixture after the haber process allow unused hydrogen and nirtogen to be recirculated?

when the gases leave the reactor they are hot and at a very high pressure. Ammonia is easily liquefied under pressure as long as it isn't too hot, and so the temperature of the mixture is lowered enough for the ammonia to turn to a liquid. The nitrogen and hydrogen remain as gases even under these high pressures, and can be recycled