Unit 5 Section7 Electrochemistry Flashcards
What is reduction?
Gain of electrons
What do oxidising agents do?
Accept electrons and get reduced
What do reducing agents do?
Lose electrons and get oxidised.
What is the oxidation number of Flurine?
-1
What is the oxidation no. Of oxygen?
-2,except with peroxides where it is -1 and when with Flurine.
What is the oxidation number of hydrogen?
+1. Apart when with metal hydrides (NaH) where it is -1
If you see a Roman numeral in a compound, what does it indicate?
The oxidation number of the entity it id next to. It is positive
If an atom loses an electron, what happens to the oxidation number?
Increase by 1
If an atom gains an electron, what happens to the oxidation number?
Fall by 1
What does an electrochemical cell contain?
Two different metals in two different containers of water with the salt solutions of their own ions, connected by a wire.
What two metals could you use in an electrochemical cell?
zinc and copper. One is more reactive than the other. One will lose electrons easer than the other.(the more reactive one)
What happens in a zinc, copper electrochemical cell?
The zinc metal will lose electrons and Zn2+ ions will go into the water and electrons are released to the external circuit.
In the other container, Cu 2+ ions will go from the water to the metal as it gets electrons from the external circut.
What is oxidation?
Loss of electrons
What is a salt bridge?
Completes the circuit in electrochemical cell
Made from filter paper soaked in KNO₃. The K⁺ and NO₃⁻ ions flow through the salt bridge and balance out the charges.
What is the standard electrode potential?
The standard electrode potential of a half cell is the volatge measured under standard conditions when the half cell is connected to a standard hydrogen electrode.
What is a standard electrode?
A piece of platinum foil submerged in a 1 mol dm⁻³ solution of H⁺ ions(acid) H₂ gas is passed through at 1 atm of pressure. The platinum will absorb hydrogen gas
H₂⇄2H⁺ + 2e⁻
The electrode potential of this half cell is defined as zero. It is the reference cell. All other standard electrodes are measured against this.
How are standard electrode potentials measured?
The standard hydrogen electrode is on the left with the element to be measured in a container on the right in a solution of ions, connected to the external circuit with a salt bridge.
The whole cell potential is E(right) -E(left) and as the left is 0, it is just the measured volatge.
What is standard conditions?
Solutions with concentration of 1 mol dm⁻³
Temperature of 298 K
Pressure of 101 kPa
What does E ^small London Underground sign mean?
The standard electrode potential of a half cell.
What does plantinized mean?
The platinum electrode is plantinized meaning that the surface is covered with finely powdered platinum to increase the surface area.
What is the whole cell potential equal to?
Potential of the right hand side - the potential of the left hand side.
The most negative one is the on the left.
Always be positive
When writing electrochemical cell reactions, which state goes first?
The oxidised substance goes first. This the is ion with the electrons.
If a electrochemical reaction has a negative potential, will it react?
It will not go or it will go in the reverse direction.
If a metal is more reactive, how is this shown in the standard electro potential ?
It will be more negative.
If a non-metal is more reactive, how is this shown in the electropotential?
It will be more positive as it wants to gain electrons and become negative.
What is the anticlockwise rule?
Write the two half equations, with the more negative one on top.
Draw anti-clockwise arrows giving the direction of each and combine them.
Cell potential is that from the bottom- from the top.
How do you predict the feasibility of electrochemical reactions?
If above +0.4 v it will go to completion
If between 0 and 0.4 it will be reversible.
If below 0 it won’t go
Why might the prediction from cell potentials be wrong?
The conditions might not be standard. (You could change the concentrations of ions in the equilibrium half equations or the temp)
Unfavourable reaction kinetics (it might be so slow it doesn’t look like it’s happening or a high activation energy might stop it)
What is the relationship between cell potential , entropy and equilibrium constant?
E∝∆S total and E∝ ln(k)
How do you do a redox titration?
Measure out reducing agent and put in flask (pipette)
Add 20 cm³ of dilute sulphuric acid(in excess)
Add oxidising agent will burette and swirl.
Stop when the colour just starts to change and do more accurate ones until the difference is 0.1
What are the two main oxidising agents used?
Manganate (VII) ions (MnO4⁻) in aqueous potassium manganate (VII) (KMnO4) purple
Dichromate (VI) (Cr₂O₇²⁻) in aqueous potassium dichromate (VI) (K₂Cr₂O₇). These are orange.
Why is acid added in redox titrations?
To give plenty of H⁺ ions to allow the oxidising agent to be reduced.
What is a disadvantage of using the hydrogen electrode as a reference?
Difficult to set up and the gas sent in is under pressure.
when doing an iodine-sodium thioshulphate titration, what some of the sources of error?
If the starch indicator is added at the wrong point, the blue colour will take too long to disappear.
The starch needs to be freshly made.
The precipitate of copper iodine makes seeing the colour change hard.
The iodine can evaporate.
What is the fundamental of fuel cells?
A fuel (hydrogen) is reacted with an oxidant(oxygen)
How does a hydrogen fuel cell work?
At the anode, platinum catalyst splits H2 to protons and electrons.
Polymer electrode membraine lets H+ through but not electrons which have to go around a circuit and produce a current. v=0.6
At the cathode, oxygen, H+ and electrons combine to give water.
How can you produce hydrogen gas?
React natural gas with steam, also giving CO2
Electrolysis hydrogen, though this needs a lot electricity.
What are the other types of fuel cells?
Cells with a reformer that can turn hydrogen rich fuel cells into hydrogen.
Some can use alcohol directly.
Alcohol is oxidised at the anode with water, to give CO2, electrons and H+.
H+ ions are later oxidised with electrons and oxygen to give water.
What are some of the advantages of using alcohol in a fuel cell?
Higher hydrogen density than liquefied hydrogen
Already a large infrastructure for making methanol and ethanol and they can both be produced from renewable biomass.
They don’t need refrigeration
Methanol can be made from CO2.
What are the advantages of fuel cells over petrol?
They produce a lot less CO2 or none at all.
At least twice as efficient at converting fuel to energy as petrol.
What are some of the disadvantages of hydrogen fuel cells?
Expensive to make and toxic chemicals are needed.
They have a limited life span.
What are some of the other uses of fuel cells?
Used in space where normal batteries would weigh too much. They also produce drinking water.
They can be used in breathalysers.
What is the ratio between alcohol in blood and breath?
Same alcohol in 2100 cm cubed of breath as they is in 1 cm cubed of blood
How did old breathalysers work?
Potassium dichromate would react with ethanol and get reduced to green chromate from red as ethanol got oxidised to ethanoic acid. A photo cell would be used to measure the colour change.
How do the breathalysers in police stations work?
Infrared spectrometry to detect ethanol. Very accurate but not portable.
How do current portable breathalysers work?
It uses a small ethanol fuel cell. The breath is sent to the anode and the current is proportional to the ethanol in the breath. It is less susceptible to give false readings due to other chemicals.
Equation of iodine reacting with thiosulphate ?
I₂ + 2S₂O₃²⁻ ➲ 2I⁻ + S₄O₆²⁻
