(Blue) redox equilibria III

Question 1

what happens when you mix :
zinc sulfate with iron

Answer 1

no change

Question 2

what happens when you mix :
zinc sulfate with lead

Answer 2

no change

Question 3

what happens when you mix :
zinc sulfate with magnesium

possibly oxidised decreasing reactivity

Answer 3

effervescence
black ppt
+10oC

Question 4

what happens when you mix :
copper sulphate with iron

Answer 4

Pale blue soln to Red /black ppt
+6oC

Question 5

what happens when you mix :
copper sulphate with lead

Answer 5

no change

Question 6

what happens when you mix :
copper sulphate with magnesium

possibly oxidised decreasing reactivity

Answer 6

solution —> colourless pink ppt.
+ 35oC

Question 7

write these metals in reactivity
zn
Fe
Cu
Mg

Answer 7

Magnesium> zinc> iron >copper >lead.

The magnitudes of the temperature increases should show a similar pattern.

Question 8

list in order the reactivity series

Answer 8

compare with the internet

Question 9

what does negative/positive electrode potential mean

Answer 9

positive electrical potential means that it tends to gain electrons

negative electrical potential means that it tends to loose electrons

Question 10

what is the definition of standard electrode potential

Answer 10

Standard electrode potential E○
of a standard half cell is the potential of that half cell relative to a standard hydrogen half cell under standard conditions, 100 kPa, 298K 1 mol dm-3.

Question 11

it is important that you relise the need for a standard i.e all solutions are at 1M , espescially the H+ ions MUST be 1M,

why is H2SO4 of concentration 1 Moledm-3 unsutible ?

Answer 11

because it is di protic therefore 2 moles of [H+] will be released

Question 12

how does a standard hydrogen electrode work

Answer 12

where hydrogen is pumped into a tube it reacts on the surface of a platnum electrode to form 2 molecues of H+ and release 2 electrons

on the other hand H+ from the HCL solution react with the platnum surface electrode to produce hydrogen (H2) gas

Question 13

what is a very common salt bridge

Answer 13

KNO3

Question 14

There is no voltage measured in the circuit until the ’salt bridge’ is connected. Why not?

for this example

zinc is being oxidised

and copper is being reduced

Answer 14

So the half cells are no longer equilibria and Zn —> Zn2+ + 2e-.

The [Zn2+] would increase and the reaction would stop unless ionic neutrality was restored i.e. [+ve ions] = [-ve ions] ( remember there in solution which allows them to get ionised)

This is achieved by ions migrating along the salt bridge.

Question 15

what is a salt bridge

Answer 15

A salt bridge contains a salt such as potassium nitrate, and is used to connect the two half cells. It enables ions to move in and out of the solution keeping both cells ionically neutral (the total –ve charges = the total +ve charges). The salt is chosen so that it will not react chemically with the ions in either half cell. From solubility studies last year you know that all group 1 ions and all nitrate(V) ions are soluble; therefore potassium nitrate makes a good salt bridge as the ions are always soluble. Other ionic salts can be used for example, KCl.

Question 16

for this example

zinc is being oxidised

and copper is being reduced

What happens to the concentration of the zinc solution as the reaction proceeds?

What happens to the mass of the zinc electrode?

What happens to the electrons released by the zinc?

What happens to the concentration of the copper solution as the reaction proceeds?

What happens to the mass of the copper electrode?

Answer 16

What happens to the concentration of the zinc solution as the reaction proceeds? increases

What happens to the mass of the zinc electrode? decreases

What happens to the electrons released by the zinc? Along the wire, round the external circuit.

What happens to the concentration of the copper solution as the reaction proceeds? decreases

What happens to the mass of the copper electrode? increases

Question 17

have a look at core practical 10

Answer 17

https://qualifications.pearson.com/content/dam/pdf/A%20Level/Chemistry/2015/teaching-and-learning-materials/A_level_Chemistry_Core_Practical_10_-_Electrochemical_Cells.pdf

Question 18

what is the function of the salt bridge

Answer 18

To complete the circuit

The cation concentration would increase around the negative electrode and the reaction would stop unless ionic neutrality was restored i.e. [+ve ions] = [-ve ions]

This is achieved by ions migrating along the salt bridge.

Question 19

3 Write the order of reactivity of the three metals involved in this experiment.

zinc
copper
silver

4 Explain why you decided this was the order.

Answer 19

Zn > Cu > Ag

Combinations of ½ cells, Zn à Zn2+ + 2e- with both Cu and Ag, Zn was the negative electrode readily oxidised.

Cu à Cu2+ + 2e- with Ag only, Cu was the negative electrode, it was more reactive

Question 21

how do you calculate emf

Answer 21

it is the diffrence between the two E cell values

Question 22

in the data book there are values for E cell , what reaction are they all undergoing

reduction or oxidation

Answer 22

reduction

the more positive the more / beter it gets reduced

Question 23

what is the analoge for writing out a cell diagram

Answer 23

ROOR

Question 24

on the cell diagram which way is the +ve if you obay the ROOR

Answer 24

left to right

Question 25

Pt [H2(g)] │ H+(aq) ║ [Fe3+(aq) , Fe2+ (aq)] │ Pt

look above there is a pt with a line seperating it and one that does not have that what does tha mean

Answer 25

the iron pt means that the iron is reacting on the surface of the platnum

while the hydrogen is reacting withing hte platnium

Question 26

how do you seperate somthing in the same phase in a cell diagram

Answer 26

with a comma

Question 27

waht is the equation for Ecell

using lhs and rhs

Answer 27

Ecell = RHS - LHS

Question 28

is the left / right hand side being oxidised or reduced

Answer 28

the left hand side is being oxidised

the right hand side is being reduced