I'm having a hard time with chemistry salts

Question

A voltaic cell is constructed using an aluminum electrode and a silver electrode.  

(a) In the image below, fill in all text boxes to label the cathode, anode and to identify suitable salts for the salt bridge and in each half cell. Add arrows above the wire to show the direction of electron flow.

(b) Write the balanced reaction that would occur in the cell and calculate the standard cell potential. 

(c) Which electrode would increase in mass? Explain.

(d) Explain why an electrolytic cell is not needed to carry out this reaction.

Answer 1

Look at the standard reduction potentials for Al and Ag:

Al³⁺ + 3e- ==> Al(s) Eº = -1.66 V

Ag⁺ + e- ==> Ag(s) Eº = +0.80 V

From this we can tell that Ag will be the cathode (reduction) and Al will be the anode.

a) I cannot see the diagram to which you refer, but you can use any salt for the salt bridge as long as it doesn't interfere with the current flow, so don't use anything that can form a gas or a precipitate. I safe choice is usually nitrate salts since they are soluble. You could use KNO3 for example. Direction of electron flow is from anode to cathode.

b) Ag⁺ e- ==> Ag cathode

Al ===> Al³⁺ + 3e- anode

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3Ag⁺(aq) + Al(s) ==> Ag(s) + Al³⁺(aq) overall equation

Eº_cell = 0.80 + 1.66 = +2.46 V

c) The Ag electrode will increase in mass because Ag⁺ ions are being reduced to solid Ag which add to the mass of the Ag electrode.

d) An electrolytic cell is not needed because the Eº_cell is positive meaning the reaction will occur spontaneously so no external source of energy is necessary.