The table on the left can be used to determine if a given reaction will take place. For example:
Zn(s) + Cu²⁺(aq) --> Zn²⁺(aq) + Cu(s)

1. Left-click on the reaction's two half equations.
Zn²⁺(aq) --> Zn(s) + 2e
Cu²⁺(aq) --> Cu(s) + 2e

2. Click on the 'Reverse' button of the equation you need to reverse in order to make the overall equation have a positive E⁰ value.
Zn²⁺(aq) --> Zn(s) + 2e