Question

Is the following equation spontaneous or non spontaneous? Al(s) + Ca2+(aq) -> Al3+(aq) + Ca(s)

Answer 1

Non - spontaneous.

Explanation:

To predict the outcome of a redox reaction under standard conditions consult a table of standard electrode potentials `sf((E^@))`.

`sf(Ca^(2+)+2erightleftharpoonsCacolor(white)(xxxxxxxx)E^@=-2.87color(white)(x)V)`

`sf(Al^(3+)+3erightleftharpoonsAlcolor(white)(xxxxxxxxx)E^@=-1.66color(white)(x)V)`

The calcium 1/2 cell has the most -ve value so this will shift right to left and give out electrons.

This means the aluminium 1/2 cell will shift left to right and take in those electrons.

If you have heard of "The Anti-Clockwise Rule" this tells us that "top right will reduce bottom left".

To get the electrons to balance we multiply the 1st 1/2 cell by 3 and the 2nd 1/2 cell by 2:

`sf(3Ca+2Al^(3+)rarr3Ca^(2+)+2Al)`

`sf(E_(cell)^@=-1.66-(-2.87)=+1.21color(white)(x)V)`

The +ve value means that the reaction is spontaneous as it is written.

The question gives the reverse reaction, so this cannot be spontaneous.