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In a battery, I don't understand what makes the element in the anode separate from the anode to become a positive charge in the electrolyte and leave some electrons inside the anode.

Also, why does the molecule separated from the anode leave electrons on the anode?

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    $\begingroup$ What do you mean by ‘separate from the anode’? $\endgroup$ – Jan Dec 22 '16 at 19:43
  • $\begingroup$ So you’re basically asking why they react? The diffusion is because one can dissolve the other can’t. There would be no diffusion if there was no reaction. $\endgroup$ – Jan Dec 22 '16 at 21:25
  • $\begingroup$ For example in a zinc-copper battery, zinc^2+ ions separate from the zinc electrode and go in the solution. This leaves a negative charge in the zinc electrode which gives it a potential $\endgroup$ – gigi Dec 22 '16 at 21:25
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    $\begingroup$ Again, the ions don’t ‘separate’. The metal reacts giving away electrons to become an ion which is then free to dissolve and diffuse. $\endgroup$ – Jan Dec 22 '16 at 21:27
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We define anode to be the electrode where oxidation takes place.

Oxidation signifies loss of electrons and Reduction signifies gain of electrons.

For example, in this electrochemical cell,

enter image description here

Copper has more reduction potential than Zinc. So it will be easy for copper to reduce (or gain electrons) and if copper is reducing (or gaining electrons) then there must a source of these electrons. These electrons are supplied by Zinc (which gets oxidized).

At anode, $Zinc(s)$ in losing electrons and converting into $Zn^{+2}(aq)$ ions and at cathode, $Cu^{+2}(aq)$ ions are gaining these electrons lost by $Zinc(s)$ and converting into $Cu(s)$.

These reactions occur because they are spontaneous $(\Delta G =-ve)$. Spontaeous reactions occur in nature by themselves without any external driving force. A system aims for least energy $(\Delta H = -ve)$ and maximum entropy $(\Delta S = +ve)$ and its the combination of both these factors that result in spontaneous processes. $(\Delta G = \Delta H - T\Delta S)$.

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The driving force is the transfer of electrons from one substance to another. This is favored by thermodynamics. In a galvanic cell or battery, the completion of the circuit provides a path for the substance at the anode (oxidation) to give electrons through the circuit to the substance at the cathode (reduction).

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