Salts and Electrolysis Flashcards
What is an acid?
Substances with a pH less than 7
They are proton donors, so produce H+ ions when added to water
They are the chemical opposite of an alkali
What is a base?
Proton acceptors with a pH value above 7
They will react with acids to neutralise them and produce salt as one of the products
What is an alkali?
Bases that dissolve in water
They have a pH value greater than 7
They produce OH- ions when in solution
Which metals in the reactivity series will acids react with?
Those above hydrogen
Why wouldn’t you react sodium or potassium with an acid in a lab?
Because the reaction is too violent to be done safely
What is the reaction that occurs when metals react with acids?
Metal + Acid → Salt + Hydrogen
MASH
What is produced in a neutralisation reaction?
Acid + Alkali/Base → Salt + Water
How can salt be made from a metal or base that is insoluble in water?
- It is added a little bit at a time to the acid until the acid has reacted
- The mixture is then filtered to remove the excess solid reaction
- This leaves a solution of the salt
- The solid salt is made when water is evaporated from the solution so that is crystallises
How can salts be crystallised from solutions?
By evaporating off water
How can we make soltuble salt?
By reacting an acid with an alkali
What compound is produced in every neutralisation reaction?
Water
Why do we need to use an indicator to show what neutralisation is complete?
Because there is no visible change
What is ammonia solution?
An alkali that does not contain a metal
What is the chemical formula for ammonium nitrate?
NH4NO3
What are ammonium salts used for?
As fertilisers
How can insoluble salts be made?
- By mixing solutions of soluble salts that contain the ions needed
- This forms a precipitate
- The precipitate can then be filtered from the solution, washed with distilled water, and dried
Give a useful application of precipitation
It can be used to remove some pollutants, such as metal ions, from water.
The water is treated by adding substances that react with the pollutant metal ions dissolved in the warer to form insoluble salts
What is electrolysis?
A process that uses electricity to break down ionic compounds into elements
What must happen to ionic compounds before they can be electrolysed and why?
They must be molten or in a solution so that their ions are free to move to the electrodes
What is an electrolyte?
A substance containing free-moving ions that is broken down by electrolysis
Describe what happens during electrolysis/how it works
- The electrical circuit has two electrodes that make contact with the electrolyte.
- The ions in the electrolyte move to the electrodes where they are discharged to produce elements
- Positively charged ions are attracted to the negative electrode, where they are reduced to form either metals or hydrogen
- Negatively charged ions are attracted to the positive electrode where they are oxidised to form non-metallic elements
What is reduction and oxidation?
Reduction is losing electrons (positive ions)
Oxidation is gaining electrons (negative ions)
Oxidation Is Loss
Reduction Is Gain
(OIL RIG)
Write out the haf equation for the oxiation of lead at the negative electrode
Pb2+ + 2e- → Pb
Complete the half equation for the reduction of bromine at the positive electrode
2Br → Br2 + 2e-
When aqueous solutions are electrolysed, what is produced at the positive electrode?
Oxygen gas is always produced unless the solution contains halide ions, in which case the halogen will be produced
When aqueous solutions are electrolysed, what is produced at the negative electrode?
Hydrogen gas is always produced unless the solution contains ions of a metal that is less reactive than hydrogen
What is aluminium oxide mixed with before it is electrolysed and why?
Cryolite
This lowers its melting point from 2000°C to 850°C
This means less energy is used
What is produced by the electrolysis of aluminium?
Aluminium and oxygen
What are the positive electrodes used in the electrolysis of aluminium made of and why does this cause problems?
Carbon
At the high temperatures at which the reaction takes place, the oxygen reacts with the carbon to produce CO2
This means that the electrodes gradually burn away and have to be replaced
What is brine?
A solultion of sodium chloride (salt) in water
What is produced during the electrolysis of brine?
Hydrogen at the negaive electrode
Chlorine at the positive electrode
Sodium hydroxide solution is left behind
Why is hydrogen produced when sodium chloride is electrolysed?
Because the solution contains hydrogen ions which are discharged in preference to the sodium ions as sodium is more reactive than hydrogen
What is the sodium hydroxide produced from the electrolysis of brine used for?
- Making soap
- Making paper
- Making bleach
- Neutralising acid
- Controlling pH
What is the chlorine produced from the electrolysis of brine used for?
- To kill bacteria in drinking water and swimming pools
- To make bleach
- To make disinfectant
- To make plastics
What is the hydrogen produced from the electrolysis of brine used for?
- To make margerine
- To make hydrochloric acid
Why might electroplating be done?
- To make an object look more attractive
- To protect a metal object from corroding
- To increase the hardness of a surface
- To reduce costs by using a thin layer of metal instead of pure metal
What is electroplating?
Using electrolysis to put a thin coating of metal onto an object
How is electroplating done?
- The object to be plated is used as the negative electrode
- The positive electrode is made from the plating metal
- The electrolyte is a solution containing ions of the plating metal
- At the positive electrode, ions of the plating metal lose electrons to form metal ions which go into the solution
- At the negative electrode, metal ions from the solution gain electrons to form metal atoms which are depositied on the object to be plated
