Chp 5 - Electricity And Chemistry Flashcards
What is electrolysis
When an electric current is passed through a molten ionic compound the compound decomposes or breaks down into its non ion state.
The process also occurs for aqueous solutions of ionic compounds
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Why do covalent compounds not undergo electrolysis
It’s because covalent compounds cannot conduct electricity hence they do not undergo electrolysis
Can ionic compounds conduct electricity in a solid state
Ionic compounds in the solid state cannot conduct electricity either since they have no free ions that can move and carry the charge
What is an electrode
its a rod of metal or graphite through which an electric current flows into or out of an electrolyte
What’s an electrolyte
Its the ionic compound in molten or dissolved solution that conducts the electricity.
What’s the anode
It’s the positive electrode of an electrolysis cell
It’s where anions go to
What’s an anion
An anion is a negatively charged ion which is attracted to the anode
What’s a cathode
A cathode is the negative electrode of an electrolysis cell
It attracts cations
What is a cation
A cation is a positively charged ion which is attracted to the cathode
For example in the electrolysis of lead bromide what happens
First Add Lead (II) Bromide into a beaker and heat so it will turn molten, allowing ions to be free to move and conduct an electric charge .
Negative bromide ions move to the positive electrode (anode) and lose two electrons to form bromine molecules.
There is bubbling at the anode as brown bromine gas is given off
Positive lead ions move to the negative electrode (cathode) and gain electrons to form a grey lead metal which deposits on the surface of the electrode
What do electrolysis in aqueous solutions always have
Aqueous solutions will always have water (H2O)
H+ and OH– ions from the water are involved as well
For a reaction in aqueous solutions what’s most likely to happen
OH– ions and non-metal ions attracted to positive electrode
Either OH– or non-metal ions will lose electrons and oxygen gas or gas of non-metal in question is released E.g. Chlorine, Bromine, Nitrogen.
H+ and metal ions attracted to the negative electrode but only one will gain electrons
Either hydrogen or metal will be produced
If the metal is above hydrogen in reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode
In electrolysis who goes first to give or take electrons to return back to its original form
The less reactive one does
What is the reactivity series
Potassium, sodium, lithium, calcium, magnesium, aluminium, CARBON, zinc, iron, HYDROGEN, copper, silver, gold
Or group 1, group 2, group 3, CARBON, zinc, iron, HYDROGEN, copper, silver, gold
What’s the difference between concentrated solution and dilute situation
Concentrated and dilute solutions of the same compound give different products
For anions, the more concentrated ion will tend to get discharged over a more dilute ion.
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What does binary compound mean
A compound consisting of only 2 elements
When you have a binary compound that’s metal and non metal during electrolysis where do they go
For a binary molten compound of a metal and a nonmetal, the cathode product will always be the metal
The product formed at the anode will always be the non-metal which is anion
In electrolysis what happens with concentrated sodium chloride which is an aqueous solution
Ions from Nacl: Na+ Cl-
Ions from H2O: H+ OH-
Anode reaction: 2CL —> CL2 + 2e-
Chlorine gas is released as it’s lost enough electrons to become chlorine again it’s original form
Cathode reaction: (2H+) + (2e-)—> H2
Hydrogen gas is released as it gains enough electrons each back to become its normal form again.
Ask teacher why they turn into gases
When looking at a chemical reaction what way do you look first to see the reactant and product
You go from left to right
E.g 2CL —> CL2 + 2e
You go from left to right meaning that the end result is that chloride loses its 2 electrons to become a gas again
E.g 2H + 2e —> H2
You go from left to right meaning that the end result becomes a hydrogen gas but it gains electrons again to become one
What happens to the solution dilute sodium chloride in electrolysis
Ions from NaCl : Na + Cl-
Ions from H20 : (H+) + (OH-)
Anode reaction:
(4OH-) —> O2 + H2O + 4e-
Oxygen is produced
Cathode reaction:
2H + (2e-) —> H2
Hydrogen gas is produced
Ask teacher how this works
Describe a chemical test for water
Anhydrous copper sulphate
It’s originally white but when it reacts with water it turns blue
White —> blue
How do you determine what gas is produce at an electrode
If the gas produced at the cathode burns with a ‘pop’ when a sample is lit with a lighted splint then the gas is hydrogen.
If the gas produced at the anode relights a glowing splint dipped into a sample of the gas then the gas is oxygen.
The halogen gases all produce their own colours (bromine is red-brown, chlorine is yellow-green and fluorine is pale yellow)
What do you do to copper refine CuSO4
The electrolysis of CuSO4 using graphite rods produces oxygen and copper
By changing the electrodes from graphite to pure and impure copper, the products can be changed at each electrode.
Electrolysis can be used to purify metals by separating them from their impurities
In the set-up, the impure metal is always the anode, in this case the impure copper
The cathode is a thin sheet of pure copper
The electrolyte used is an aqueous solution of a soluble salt of the pure metal at the anode, e.g: CuSO4
Copper atoms at the anode lose electrons, go into solution as ions and are attracted to the cathode where they gain electrons and form now purified copper atoms
The anode thus becomes thinner due to loss of atoms and the impurities fall to the bottom of the cell as sludge
The cathode gradually becomes thicker
How can you purify metals in electrolysis
Electrolysis can be used to purify metals by separating them from their impurities.
How do anions work in terms of being first to give or take electrons
We have seen that cations lower down on the reactivity series tend to be discharged in preference to more reactive cations
The same occurs for anions which can be arranged in order of ease of discharge:
More reactive SO42- → NO3– → OH– → Cl– → Br– → I– Less reactive
However if the solution is dilute only the OH– ion is discharged and so oxygen would be formed
In the cathode what is normally produced
either a metal or hydrogen gas is produced
What’s the difference between an inert anode and reactive anode
If the anode is inert (such as graphite or platinum), the ions lose electrons to the anode and form a nonmetal or oxygen gas
If the anode is a reactive metal, then the metal atoms of the anode lose electrons and go into solution as ions, thinning the anode
In electrolysis state what this equation is:
Pb2 + 2e- —> Pb
Reduction occurs at the cathode as the positive ions gain electrons to become a normal element
What’s an electrochemical cell
An electrochemical cell is a source of electrical energy
Give an example of how electrochemical cells work
The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external circuit
A common example is zinc and copper
Zinc is the more reactive metal and forms ions more easily, releasing electrons as its atoms form ions
The electrons give the more reactive electrode a negative charge and they then flow around the circuit to the copper electrode
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The difference in the ability of the electrodes to release electrons causes a voltage to be produced
The greater the difference in the metals’ reactivity, the greater the voltage
How is voltage produced in electrolysis
It’s produced by the difference in reactivity of electrodes
When a metal conducts it is the electrons that are moving through the metal. What happens if a salt solution conducts in electrolysis
When a salt solution conducts, it is the ions in the solution that move towards the electrodes carrying the electrons.
What is electroplating
Electroplating is a process where the surface of one metal is coated with a layer of a different metal
The metal being used to coat is a less reactive metal than the one it is covering
The anode is made from the pure metal that was used to coat
The cathode is the object to be electroplated
The electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode
What are the uses of electroplating
Electroplating is done to make metals more resistant to corrosion or damage, e.g: chromium and nickel plating
It is also done to improve the appearance of metals, e.g: silver plating cutlery
What are conductors
Conductors of electricity allow electrical charge to pass through them easily
Conductors can be:
Solids such as metals or graphite
Liquids such as molten lead bromide or molten metals
Solutions such as sodium chloride solution
What are insulators
Insulators are the opposite of conductors as they resist the flow of electricity and do not conduct
Most insulators are solids of plastic, rubber or ceramic
Plastics are used as insulators and are placed around electrical wiring and for some tool and machine handles
Ceramics are used in very high voltage lines where contact between the power line and the metal of the pylon would be dangerous
In earths crust are metals and metal compounds such as gold, iron oxide, aluminium oxide.
But when found these substances are often mixed with it her substances.
How do people extract the metal from the other substance
metals have to be extracted from their ore through processes such as electrolysis, using a blast furnace or by reacting with more reactive material
Metals which lie above carbon have to be extracted by electrolysis as they are too reactive
When metals are too reactive what do you do
Metals which lie above carbon in the reactivity scale have to be extracted by electrolysis as they are too reactive
How are potassium, sodium, lithium, calcium, magnesium, aluminium, carbon extracted
metals are extracted by electrolysis of molten chloride or molten oxide
This is a very expensive process though
How are zinc, iron, hydrogen and copper extracted
They are extracted by heating with a reducing agent( gives electrons to another element and that element gets reduced while the reducing agent gets oxidised) such as carbon such as carbon or carbon monoxide in a blast furnace
This is a cheap process as carbon is cheap and can also be used as a heat source
Do silver and gold need to be extracted
No they don’t as they are found as pure elements
What is cryolite used for
It’s used for dissolving of substances in electrolysis.
It acts an electrolyte with an other substance that has dissolve in its molten state.
What is bauxite
Aluminium ore
What’s the difference between 2O and O2
2O is the ion state of oxygen whereas O2 is the free natural gas state of oxygen
Why can carbon dioxide be produced in electrolysis
It’s can happen as some of the Oxygen Produced at the positive electrode (anode) can react with a Graphite (Carbon) electrode to produce Carbon Dioxide Gas:
C (s) + O2 (g) → CO2 (g)
This is what causes the graphite anodes to burn away so they must be reduced regularly
What is brine and how is the manufacture of chlorine, hydrogen and sodium hydroxide done
Brine is a concentrated solution of aqueous sodium chloride
When electrolysed it produces chlorine, hydrogen and sodium hydroxide
The electrolyte is a concentrated sodium chloride which contains the following ions: H+, Cl– and OH–
The H+ ions are discharged at the cathode as hydrogen gas
The Cl– ions are discharged at the anode as chlorine gas
The Na+ and OH– ions remain behind and form the NaOH solution
In the end the electrolyte turns into a sodium hydroxide solution:
NaOH
