Why does a voltaic cell's operating voltage differ from the predicted voltage?

[Physics197]

Homework Statement

State two reasons for the difference between the predicted voltage and the operating voltage of a voltaic cell.

 

[Physics197]

Sorry, but nevermind

Read the previous page and it said the standard conditions for the reduction potential are:
25 degrees celsius
1M solution
1atm pressure

So obviously 2 of those result in the difference

 

[chemisttree]

Don't forget the internal resistance of the cell...