Explain why the following reaction occurs:
zn + 2hcl → zncl2 + h,

The reaction is spontaneous (under standard conditions) because the cell potential is positive.

Explanation:

In this reaction:

The anode is where the oxidation half-reaction takes place. In this case, the anode half-reaction is .

The cathode is where the reduction half-reaction takes place. In this case, the cathode half-reaction is .

Look up the standard reduction potential for these two half-reactions:

The standard cell potential is equal to the standard reduction potential at the cathode minus that at the anode:

.

The electrochemical reaction in the cell is spontaneous if and only if the cell potential is positive. Therefore, the reaction   is spontaneous under standard conditions.

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