Redox equilibria Flashcards
Electrochemical cells info
Can be made from 2 different metals dipped in salt solutions of their own ions and connected by a wire( the external circuit). The solutions are connected using a salt bridge. There are always 2 reactions within the cell, one is an oxidation and the other reduction reaction = redox process. Electrons flow through the wire from the most reactive metal to the least. A voltmeter in the external circuit shows the voltage between the 2 half-cells, called cell potential or e.m.f
Electrochemical cells make…
Electricity
Explain what happens in the zinc/copper electrochemical cell
Zinc loses electrons more easily than copper, so zinc (from the zinc electrode) is oxidised to form Zn2+ ions, releasing electrons into the external circuit. In the other half-cell the same number of electrons are taken from the external circuit reducing the Cu2+ ions to copper atoms
What is a salt bridge used for, what is it made from and what does it do
It is used to connect 2 half-cell solutions, it is made from filter paper soaked in potassium nitrate (KNO3). It allows ions to flow through and balance out the charges
Another example of half-cells
Involve solutions of 2 aqueous ions of the same element such as Fe2+/Fe3+. The conversion happens on the surface of the electrode. The electrode is made of platinum as it is an inert metal
How easily a metal is oxidised is measured using…
Electrode potential. A metal that’s easily oxidised = very negative potential, one that’s harder to oxidise has less negative/ positive potential
Important conventions when drawing electrochemical cells include…
The half-cell with more negative potential goes on left
The oxidised forms go in the centre of cell diagram
Cell potential calculation
Electrode potential of right hand side - electrode potential of left hand side
Why will the cell potential always be positive
Because you are subtracting the more negative value from the more positive vale
Half-cell reactions are reversible. This means the equilibrium position is affected by changes in…
Temp
Pressure
Concentration
Changing the equilibrium position in half-cell reactions changes the… How do you get around this
Cell potential
Get around it by using standard conditions to measure the electrode potentials, using these conditions means you will always get the same value for the electrode potential and you can compare values for different cells
What are electrode potentials measured against
Standard hydrogen electrodes
What is the standard electrode potential of a half-cell
The voltage measured under standard conditions where the half-cell is connected to a standard hydrogen electrode
In half-cells, when using a standard hydrogen electrode, what is the electrode made of and why, where is the electrode found and what are the standard conditions
Made of platinum as you can’t have a gas electrode
electrode is always shown on the left
Standard conditions…
Any solution = concentration of 1.00moldm-3 eg HCl or HNO3
Temp must be 298k
Pressure must be 100kPa
The standard hydrogen electrode half-cell has an electrode potential of…
0 volts
What example provides evidence that electrons are transferred
The electrochemical cell, due to oxidation and reduction reactions and the flow of electrons through the external circuit
What is the anticlockwise rule used for
To predict whether a reaction will occur and to show which direction it will go in
Electrochemical cells are used as….
Batteries
What is a common type of non-rechargeable cell and where are they found
Dry cell alkaline battery, found in things like TV remotes, torches, smoke alarms- gadgets that don’t use a lot of power or are only used for short periods of time
Why is it not practical to reverse the half equations/ recharge non-rechargeable batteries
They can be made to run backwards under the right conditions, but trying to do this can make it leak or explode. This is because the zinc anode forms the casing of the battery, so becomes thinner and zinc is oxidised. Another reason is that the ammonium ions produce hydrogen gas which escapes from battery. Without hydrogen the ammonium ions cannot be reformed by reversing the reactions
Examples of where rechargeable batteries are found, 3 types and how to recharge them
Mobile phones, laptops and cars.
Lead-acid, NiCad (nickel-cadmium) and L ion (lithium ion)
To recharge, a current is supplied to force electrons to flow in the opposite direction around the circuit and reverse the reactions . This is possible as none of the substances escape or are used up.
Advantages of non-rechargeable batteries
Cheaper than rechargeable batteries
Will work for longer than a rechargeable battery
Can be recycled, metal in them removed and used again
Less likely to contain toxic metals such as lead and cadmium = less hazardous in landfill if contents leak out and pollute water sources
Disadvantages of non-rechargeable batteries
Have to be replaced every time they run out, so rechargeables cheaper in long run
Have to be disposed of once used as cannot be recharged
Can’t supply as much power, no use for devices that use a lot of power
More produced as they only last once so use more resources creating more waste, ends up in landfill as hardly any recycled
Fuel cells info
In most cells the chemicals that generate the electricity are contained in the electrodes and the electrolyte that form the cell. In fuel cell, chemicals stored separately outside the cell and fed in when electricity required. Example is the hydrogen-oxygen fuel cell, can be used to power electric vehicles.
How does the hydrogen-oxygen fuel cell work
Hydrogen and oxygen gases are fed into 2 separate platinum containing electrodes. Electrodes separated by an ion-exchange membrane that allows protons (H+ ions) to pass through it, but stops electrons going through it. Hydrogen is fed to the negative electrode. Electrons flow from negative electrode through external circuit to positive electrode. H+ ions pass through ion-exchange membrane towards positive electrode. Oxygen fed to positive electrode. Overall effect is that H2 and O2 react to make water
What is the major advantage with fuel cells
They don’t need electrical recharging. As long as hydrogen and oxygen is supplied, the cell will continue to produce electricity. Another benefit = only waste product is water, so no nasty toxic chemicals to dispose of and no CO2 emissions from cell itself.
Disadvantage of fuel cells
You need energy to produce the supply of hydrogen and oxygen. Can be produced from the electrolysis of water, requires energy which is normally generated from fossil fuels so not carbon neutral. Also hydrogen highly flammable so needs to be handled carefully when stored or transported
What is a reducing agent
A species that donates electrons
What is an oxidising agent
An agent that accepts electrons
Why are hydrogen fuel cells not as ‘green’ as they appear
Most hydrogen is made from crude oil. Could be made by electrolysis of water but most electricity is made by burning fossil fuels which emit carbon dioxide. Expensive to store and transport hydrogen as specialist equipment needed
The Roman numerals in a chemical names represent…
The oxidation number
The oxidation state for an atom will ……. by 1 for each electron lost
Increase
The oxidation state will ……. by 1 for each electron gained
Decrease
Identify a solution that could be used in an electrochemical cell to connect the half cells. Give 2 reasons why this solution is suitable for its purpose
Potassium nitrate
It allows ions to flow to balance charges
AND
It doesn’t react with the solutions
Explain environmental advantages of rechargeable cells compared with non-rechargeable cells
The compounds are reused, the supplies don’t deplete
Suggest why ethanol is seen as a carbon neutral fuel
CO2 released by combustion is the same amount taken in through photosynthesis when the plant was growing
