BATTERIES

A battery is a galvanic cell, or a set of cells connected in series, that provides a direct current of consistent strength for a useful period of tie. Two redox half reactions occur at two electrodes. The nature of the reactions depends on the chemicals that make up the cell.

Primary cell – Dry cell A primary cell cannot be recharged. Once it has run down, it must be thrown away. The primary cell is also called a dry cell, because the electrolyte in the cell is not a liquid but a paste or gel. Secondary cells – Wet cell  A secondary cell can be recharged. It is also called a wet cell, because the electrolyte is a liquid.

Carbon-zinc dry cell A – top, non-conducing seal The cathode - a graphite (carbon) rod A – top, non-conducing seal C - the anode – a cylindrical zinc casing The battery is filled with a mixture of manganese dioxide (MnO2) as oxidant, ammonium chloride (NH4Cl) as a source of H+ ions, and zinc chloride (ZnCl2) .These two salts serve as electrolytes.

The zinc is oxidised according to the following half-equation (anode): Zn(s) → Zn 2+(aq) + 2 e- The cathode (reduction) reaction is as follows: 2MnO2 (s) + 2H2O (l) (aq) +2e- →Mn2O3 (s) + 2OH-(aq) In this half-reaction, the manganese is reduced from an oxidation state of (+4) to (+3). The manganese(IV) oxide paste also contains ammonium chloride and zinc chloride, which act as the electrolyte for the cell. The overall reaction in a dry cell is as follows: Zn(s) + 2MnO2(s) +2H2O (l) → Zn2+ (aq) + Mn2O3(s) + 2OH-(aq) The battery has an e.m.f. of about 1,5 V.

The lead – acid battery. This type of battery is used in car engines. It delivers a large amount of current for a short space of time. A large current is needed to provide the spark across the spark plug, which starts the car. This type of cell is called a secondary cell because it stores energy when the chemical reaction proceeds in one direction and releases energy when it proceeds in the opposite direction.

Construction of lead-acid batteries The container (battery case) houses the separate cells. Most containers are hard rubber, plastic or some other material that is resistant to the electrolyte and mechanical shock. The container is aired through vent plugs to allow the gases, which form within the cells, to escape. The plates in the battery are the cathodes and anodes. Each cell consists of two electrodes – one made of lead (Pb), the other of lead oxide (PbO2) – and an electrolyte solution of concentrated sulfuric acid (H2SO4).

It is rechargeable. A number of cells are connected in series to form the lead-acid battery.

The chemical reactions in the lead-acid battery – LEARN REACTIONS OFF BY HEART The chemical reactions are (charged to discharged): Anode (oxidation): Pb (s) + SO42- (aq)  PbSO4(s) + 2e- E0 = 0,356V Cathode (reduction): PbO2 (s) + SO42- (aq) + 4H+ + 2e-  PbSO4 (s) + 2H2O (l) E0 = 1,685V

The overall reaction: Pb(s) + PbO2(s) + 2H2SO4 (aq)  PbSO4 (s) + 2 H2O (l) Solid PbSO4 forms on both the anode and cathode. As the battery discharges, the H2SO4 is used to supply the SO42- ion and water is produced.

Reduction half reaction – occurs at the cathode. Oxidation half reaction – occurs at the anode. Reduction half reaction – occurs at the cathode.

Environmental and human impact Overcharging - Danger Because of the open cells with liquid electrolyte, overcharging will generate oxygen and hydrogen gas by electrolysis of water, which forms an extremely explosive mix Environmental and human impact Lead batteries must not be dumped on landfill sites. Pd can ‘dissolve’ in rain and get in the ground water. Pb is poisonous. Plastic covers do not biodegrade and can fill up the landfill site The acid electrolyte is corrosive and if comes into contact with a persons skin it can ‘burn’ the skin

What to do Don’t dump lead-acid batteries Recycle depots -Remove the plastic cover and recycle the plastic. - Reuse the lead that is left over in new batteries

Energy stored in batteries W = V x q W = work or energy in joules (J) V = potential difference in volts (V) q = charge in coulombs (C) A cell can do 300 J of work when delivering 50 C of charge. What is the potential difference of the cell?

Cell Capacity- NB Always in exam The cell capacity is measured in ampere-hours (Ah), or milliamp-hours (mAh). It indicates the number of hours that the cell can supply a certain amount of current before its voltage drops below an acceptable threshold value. We can use this to determine how long a battery will deliver the correct current to a certain apparatus and thus for how long it will be able to run.

q = It q = cell capacity (Ah) t = time (hrs) I = current (A) Example Amp-hours and coulombs measure the same quantity–charge. One amp-hour is 3 600 coulombs, but it is easier to use amp-hours in battery design Example How long will a motor last if a it is drawing 20 A, using a 0,5 Ah cell

(The charge on one electron is -1,6 x 10-19 C.) Example If the cell capacity of such a cell is 5 A∙h, calculate the number of electrons that flow through the cell in 120 minutes. Assume the cell discharges completely during the 120 minutes. (The charge on one electron is -1,6 x 10-19 C.)

NOVEMBER 2009 Some cells, such as the nickel-cadmium cell used in calculators and electric shavers, can be recharged. Others, such as those used in watches and torches, cannot be recharged. Are rechargeable cells primary or secondary cells? The half-reactions occurring in a nickel-cadmium cell are shown below: Cd(s) + 2OH-(aq) → Cd(OH)2(s) + 2e- ……………………(I)   NiO2(s) + 2H2O( ) + 2e- → Ni(OH)2(s) + 2OH-(aq)…..…(II)  The emf of the nickel-cadmium cell is 1,4 V. 2. Which ONE of these half-reactions occurs at the cathode? Give a reason for your answer 3. Write down the balanced equation for the overall cell reaction 4. Calculate the maximum work done by the cell under standard conditions as 1 mol of Cd is used up. (NOTE: 1 mole of electrons has a charge of 9,65 x 104 C.)

NOVEMBER 2010 Lead-acid batteries have been used in cars for the past 85 years. The equations of the half-reactions that take place in each cell of such batteries are shown below Pb(s) + HSO4-(aq) → PbSO4(s) + H+(aq) + 2e- PbO2(s) + 3H+(aq) + HSO4-(aq) + 2e-→ PbSO4(s) + 2H2O(ℓ) 1. Write down the oxidation number of lead (Pb) in PbSO4(s). 2. Write down the balanced equation for the net (overall) cell reaction. 3. Which ONE of the reactants is the reducing agent in this cell reaction? Give a reason for the answer.

One of the safety concerns related to the lead-acid battery is the dangers associated with recharging (that is reversing the net reaction) of a flat battery. Water in the battery can be electrolysed to produce hydrogen and oxygen gas during recharging 4. Use the Table of Standard Reduction Potentials and write down the half- reaction which explains the formation of oxygen gas. Why is the recharging of flat batteries a safety concern? If the cell capacity of such a cell is 3,5 A∙h, calculate the number of electrons that flow through the cell in 30 minutes. Assume the cell discharges completely during the 30 minutes. (The charge on one electron is -1,6 x 10-19 C.)