Electrolysis Flashcards
Definition of an electrolyte
The substance broken down by electrolysis is called an electrolyte. Electrolytes are made up of ions
Why can’t solid ionic compounds conduct electricity?
The ions are fixed in a giant lattice so are unable to freely move to its oppositely charged electrode.
Why can ionic compounds that are molten or aqueous form able to conduct electricity?
The ions are free to move in the giant lattice to their oppositely charged electrode
Which compounds will undergo electrolysis?
Ionic compounds will undergo electrolysis because it is made of ions which can carry charge. Has to be in molten or aqueous form so the ions are free to move to their oppositely charged electrode
How is electrolysis set up
Two electrodes that are connected to a power supply are dipped into the electrolyte
What is an electrode
A material which carries an electrical current
Why are electrodes often made of graphite
2 reasons
- conducts electricity
- unreactive (doesn’t react with electrolyte)
Which ions are attracted to which electrode
Positive cations attract to the negative cathode
Negative anions attract to the positive anode
What mnemonic helps to remember the names of electrodes and what does it stand for?
PANIC
Positive Anode Negative is Cathode
What is an oxidation reaction? How do we remember this?
Loss of electrons
OIL - Oxidation Is Loss
What is a reduction reaction? How do we remember this?
Gain of electrons
RIG - Reduction Is Gain
What happens when ions reach their oppositely charged electrode?
The ions lose their charge (discharged) and become elements/ atoms.
Which electrode does oxidation occur and why?
Positive anode
Oxidation is the loss of electrons which means the ions were negative as they had more electrons. The negative anions attract to the positive anode.
Which electrode does reduction occur and why?
Negative cathode
Reduction is the gain of electrons which means the ions were positive as they had less electrons. The positive cations attract to the negative cathode
What happens to the electrons lost at the anode?
The electrons flow around the circuit to the cathode
Definition of electrolysis
A process that uses an electric current to split up a substance that contains ions
What happens at the anode?
The negative ions are attracted to the positive anode. Each ion has to lose (e.g. 1) electron to become neutral. This is an oxidation reaction. Forms the atom/ element.
What happens at the cathode?
The positive ions are attracted to the negative cathode. Each ion has to gain (e.g. 1) electron to become neutral. This is a reduction reaction. Forms the atom/ element
Example of the equation at the anode with sodium chloride. What type of reaction?
2Cl- —–> Cl2 + 2e-
Oxidation
Example of the equation at the cathode with sodium chloride. What type of reaction?
Na+ + e- ——> Na
Reduction
Why is it sometimes easier to electrolyse the aqueous solution?
Most ionic compounds have high melting points do it is difficult to melt them and make then molten. It is easier to dissolve them in water and electrolyse the aqueous solution
What ions will be present in a solution
H2O —-> H+ + OH-
What apparatus do we use for the electrolysis of water?
Hoffmann Voltameter
What happens at the cathode for the electrolysis of water?
Equation?
H+ ions are attracted
Gains 1 electron
Reduction reaction
2H+ + 2e- —–> H2
What happens at the anode for the electrolysis of water?
Equation?
OH- ions are attracted
Loses 1 electron
Oxidation
4OH- —–> O2 + 2H2O + 4e-
Predicting products at the cathode?
Least reactive positive ions are formed
Metals lower than hydrogen in the reactivity series will go to the cathode
Predicting products at the anode?
Negative ions are attracted
If a halide ion is present a halogen is formed
Otherwise, OH- ions go to the anode and form oxygen
What are the four ions present in brine solution?
Na+ Cl- H+ OH-
What colour is the universal indicator for brine and what does it mean?
Green, neutral
What ions will move to the cathode for brine and why?
H+ ions
Less reactive than Na+
What is the product of brine at the cathode? What is the equation?
How can we confirm the product?
H2
2H+(aq) + 2e- —–> H2(g)
Squeaky pop test - lighted splint into the gas, if it pops there is hydrogen present
What ions will move to the anode for brine and why?
Cl- ions
Halide ion present
What is the product of brine at the anode? What is the equation?
How can we confirm the product?
Cl2
2Cl(aq) —–> Cl2(g) + 2e-
Blue litmus paper becomes pink then bleaches
How does the colour of the universal indicator change at the anode for brine? What does it mean?
Starts green and changes to yellow (acidic). Then it turns colourless because chlorine is a bleach. Chlorine is produced at the anode
What colour is the universal indicator for the remaining solution of brine? What does it mean? What is the solution?
Purple —> alkaline
OH- ions are alkaline
Forms NaOH(aq)
Equation for electrolysis for Brine
2NaCl + 2H2O —–> H2 + Cl2 + 2NaOH
Uses of products of electrolysis of brine
Chlorine- kills bacteria in swimming pools, bleach
Hydrogen- fuel, used to make margarine
Sodium hydroxide- soap, paper
What is electroplating?
An electroplated object is one that is coated with a thin layer of metal by the process of electrolysis
Uses if electroplated objects
4 reasons
Protect metal underneath from corroding
Looks more attractive
Increase hardness or resistance to scratching
Saves money by using a thin layer of precious metal
What is the anode for electroplating and what happens what reaction?
The anode is always the plating material
Oxidation - loses electrons to become positively charged
What is the cathode for electroplating and what happens what reaction?
The cathode is always the object that will be plated
Reduction - gains electrons to become negatively charged
What safety precaution should be taken if something is toxic
Do the experiment in the fine cupboard
How could a sample of gas be collected at the positive electrode be collected?
Collect the gas using displacement of the electrolyte in a test tube
Why might hydrogen gas be produced at the cathode?
Hydrogen ions are positive
Opposite charges attract
Less reactive than the other metal
