Batteries
What is a battery?We all have seen and used batteries. See the simple circuit Figure below. Batteries allow us to store energy in chemical form, which can be converted into electromagnetic energy. Batteries are one of the two common methods (the other being the capacitor) by which electromagnetic energy can be stored.
How does the chemical reaction produce electromagnetic energy?
The chemical reaction results in electron charge flow; the moving charges enable the transfer of electromagnetic energy.
Does the electrons get accumulated in the +ve terminal?
No. If it does then incoming electrons will face repulsion, and voltage or current cannot be maintained.
What happens to the electrons?
The electrons participate in a chemical reaction whereby the charge is carried to the other terminal by ions, thus continuing the current flow.
Why does the battery gets used up? What exactly gets depleted through this process?
One part of Cu-Zn battery chemical reaction is as follows:
Zn -> Zn 2+(aq) + 2 e-
The above reaction can not continue forever, because Zn depletes over time.
Is this the chemical reaction that takes place in the battery?
Not exactly. Different type of batteries use different type of chemical reactions. One spontaneous redox reaction that can be found in batteries is as follows:
Zn(s) + Cu2+(aq) + SO2-4 (aq) -->Cu(s) + Zn2+(aq) + SO2-4 (aq)
Zn + Cu cations (aq) + Sulphate anions(aq) -->Cu + Zn cations (aq) + Sulphate anions (aq)
Zinc reacts with copper cations in an aqueous solution spontaneously, and produces solid copper and Zinc cations. Note that Zinc will be depleted over time. The reaction is a spontaneous redox reaction.
You are using too many new jargons. What is a cation, and how does it get formed?
An ion is formed when a neural atom gains or losses electrons in a chemical reaction or formaing a molecule. Cations is an ion which is positively charged. Cu2+ is a cation, it has lost 2 electrons and is positively charged. When you mix CuSO4 with water, Cu2+(aq) get formed.
What is a anion?
Anion is a negatively charged atom or moleclue such as SO42-(aq).
What is an aqueous solution?
Aqueous solution is when a compound or solute dissolved in water. Recall that a solute is the substance that gets dissolved in the solvent to make the solution. In an aqueous solution, the solvent is the water.
What is a reduction-oxidation reaction?
The above chemical reaction can be viewed as sum of two half-reaction for analytical purposes.
Zn(s)--> Zn2+(aq) + 2e-
Cu2+(aq)+ 2e--->Cu(s)
The first reaction where the Zinc losses two electrons is called the oxidation or oxidation half reaction. When an atom losses electrons and becomes positively charged the reaction is called oxidation reaction. Why the name oxidation? I don't know. It is the conventional name.
The second reaction where Cu cation gains electrons and becomes solid copper is called the reduction or half-reduction reaction.
Does these two reactions occur separately?
No. The redox reaction is a single reaction, even though electrodes are physically separated.
What are electrodes?
In the set up shown, the Zinc metal strip, and Copper metal strip are called electrodes; they are connected to the external circuit.
Also note that the Zinc metal where the oxidation occurs is the anode and the Copper metal where the reduction occurs is the cathode.
I am aware that electric current is a result of continuous flow of electric charge. But, here it appears that Zinc gets depleted of charges and Copper seem to use up all the electrons in reduction. Where is the continuous electron or charge flow?
If there was no "salt bridge", you will be right. The reaction will not last long. The excess SO2-4 (aq) will repel incoming electrons, and vice versa. But, note that there is a salt bridge.