Unit 5 Section7 Electrochemistry

Question 1

What is reduction?

Answer 1

Gain of electrons

Question 2

What do oxidising agents do?

Answer 2

Accept electrons and get reduced

Question 3

What do reducing agents do?

Answer 3

Lose electrons and get oxidised.

Question 4

What is the oxidation number of Flurine?

Answer 4

-1

Question 5

What is the oxidation no. Of oxygen?

Answer 5

-2,except with peroxides where it is -1 and when with Flurine.

Question 6

What is the oxidation number of hydrogen?

Answer 6

+1. Apart when with metal hydrides (NaH) where it is -1

Question 7

If you see a Roman numeral in a compound, what does it indicate?

Answer 7

The oxidation number of the entity it id next to. It is positive

Question 8

If an atom loses an electron, what happens to the oxidation number?

Answer 8

Increase by 1

Question 9

If an atom gains an electron, what happens to the oxidation number?

Answer 9

Fall by 1

Question 10

What does an electrochemical cell contain?

Answer 10

Two different metals in two different containers of water with the salt solutions of their own ions, connected by a wire.

Question 11

What two metals could you use in an electrochemical cell?

Answer 11

zinc and copper. One is more reactive than the other. One will lose electrons easer than the other.(the more reactive one)

Question 12

What happens in a zinc, copper electrochemical cell?

Answer 12

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.

Question 13

What is oxidation?

Answer 13

Loss of electrons

Question 14

What is a salt bridge?

Answer 14

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.

Question 15

What is the standard electrode potential?

Answer 15

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.

Question 16

What is a standard electrode?

Answer 16

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.

Question 17

How are standard electrode potentials measured?

Answer 17

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.

Question 18

What is standard conditions?

Answer 18

Solutions with concentration of 1 mol dm⁻³
Temperature of 298 K
Pressure of 101 kPa

Question 19

What does E ^small London Underground sign mean?

Answer 19

The standard electrode potential of a half cell.

Question 20

What does plantinized mean?

Answer 20

The platinum electrode is plantinized meaning that the surface is covered with finely powdered platinum to increase the surface area.

Question 21

What is the whole cell potential equal to?

Answer 21

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

Question 22

When writing electrochemical cell reactions, which state goes first?

Answer 22

The oxidised substance goes first. This the is ion with the electrons.

Question 23

If a electrochemical reaction has a negative potential, will it react?

Answer 23

It will not go or it will go in the reverse direction.

Question 24

If a metal is more reactive, how is this shown in the standard electro potential ?

Answer 24

It will be more negative.

Question 25

If a non-metal is more reactive, how is this shown in the electropotential?

Answer 25

It will be more positive as it wants to gain electrons and become negative.

Question 26

What is the anticlockwise rule?

Answer 26

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.

Question 27

How do you predict the feasibility of electrochemical reactions?

Answer 27

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

Question 28

Why might the prediction from cell potentials be wrong?

Answer 28

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)

Question 29

What is the relationship between cell potential , entropy and equilibrium constant?

Answer 29

E∝∆S total and E∝ ln(k)

Question 30

How do you do a redox titration?

Answer 30

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

Question 31

What are the two main oxidising agents used?

Answer 31

Manganate (VII) ions (MnO4⁻) in aqueous potassium manganate (VII) (KMnO4) purple Dichromate (VI) (Cr₂O₇²⁻) in aqueous potassium dichromate (VI) (K₂Cr₂O₇). These are orange.

Question 32

Why is acid added in redox titrations?

Answer 32

To give plenty of H⁺ ions to allow the oxidising agent to be reduced.

Question 33

What is a disadvantage of using the hydrogen electrode as a reference?

Answer 33

Difficult to set up and the gas sent in is under pressure.

Question 34

when doing an iodine-sodium thioshulphate titration, what some of the sources of error?

Answer 34

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.

Question 35

What is the fundamental of fuel cells?

Answer 35

A fuel (hydrogen) is reacted with an oxidant(oxygen)

Question 36

How does a hydrogen fuel cell work?

Answer 36

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.

Question 37

How can you produce hydrogen gas?

Answer 37

React natural gas with steam, also giving CO2 | Electrolysis hydrogen, though this needs a lot electricity.

Question 38

What are the other types of fuel cells?

Answer 38

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.

Question 39

What are some of the advantages of using alcohol in a fuel cell?

Answer 39

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.

Question 40

What are the advantages of fuel cells over petrol?

Answer 40

They produce a lot less CO2 or none at all. | At least twice as efficient at converting fuel to energy as petrol.

Question 41

What are some of the disadvantages of hydrogen fuel cells?

Answer 41

Expensive to make and toxic chemicals are needed. | They have a limited life span.

Question 42

What are some of the other uses of fuel cells?

Answer 42

Used in space where normal batteries would weigh too much. They also produce drinking water. They can be used in breathalysers.

Question 43

What is the ratio between alcohol in blood and breath?

Answer 43

Same alcohol in 2100 cm cubed of breath as they is in 1 cm cubed of blood

Question 44

How did old breathalysers work?

Answer 44

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.

Question 45

How do the breathalysers in police stations work?

Answer 45

Infrared spectrometry to detect ethanol. Very accurate but not portable.

Question 46

How do current portable breathalysers work?

Answer 46

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.

Question 47

Equation of iodine reacting with thiosulphate ?

Answer 47

I₂ + 2S₂O₃²⁻ ➲ 2I⁻ + S₄O₆²⁻