An aluminium foil with a surface area of 500cm2 was anodized. Gow many coulombs of electricity were required to increase the thickness of the layer by 1*10^-3 cm (density of aluminium oxide=4.0g cm-3)?

Anodising is a process by which the protective oxide layer on the aluminium is thickened by an electrolytic process.

Sodium hydroxide solution is used as the electrolyte. At the anode #sf(OH^-)# are oxidised:

#sf(4OH^(-)rarrH_2O+O_2+4e)#

The oxygen reacts with the aluminium:

#sf(2Al+3/2O_2rarrAl_2O_3)#

The 1/2 equations are:

#sf(2Alrarr2Al^(3+)+6e)#

#sf(3/2O_2+6erarr3O^(2-))#

In reality you can use a.c current and coat 2 pieces of aluminium.

You can see that to form 1 mole of #sf(Al_2O_3)# you need 6 moles of electrons.

The charge carried by 1 mole of electrons is referred to as The Faraday Constant F which is #sf(9.65xx10^(4)"C/mol")#

We can use the date given to find the actual no. of moles deposited.

#sf(m=rhoxxv)#

#sf(v=500xx10^(-3)=0.5color(white)(x)cm^3)#

#:.##sf(m=4.0xx0.5=2.0color(white)(x)g)#

#sf(M_r[Al_2O_3]=102)#

#:.##sf(n_(Al_2O_3)=m/M_r=2.0/102=0.0196)#

We know that:

1 mole requires 6F Coulomb.

#:.# 0.0196 mole requires 0.0196 x 6F = 0.1176 F Coulomb

#sf(=0.1176xx9.65xx10^(4)=11348color(white)(x)C)#

#sf(~~11,000color(white)(x)C)#