La idea de justicia en el documento de Medellín

Galvanic Cells:

Remember the Daniel Cell from Unit 2? This requires a Zinc anode(in ZnNO3 solution) and Copper cathode (In Copper (II) solution). Refer to the diagram below:

 A current passes through the circuit to the globe. This part of the cell is the external circuit. The globe converts electrical energy to light and heat energy.

 The current flows because of the chemical reaction taking place in the cell (of which there is little indication of happening initially.)

In the above example, if left long enough we observe the following:

 The zinc metal corrodes.

 The copper metal becomes covered with a furry brown-black deposit.

 The blue copper (II) sulfate solution loses some of it’s colour.

These changes provide evidence of a chemical reaction.

 There is also a current flowing from the zinc electrode, through the wire, to the copper electrode.

 Current flows only if a salt bridge is present. (Made of some type of salt).

These findings help us decide what is happening inside the cell.

 The reaction in the cell is redox, electrons are being produced and consumed.

 The zinc electrode is eaten away, forming zinc ions in solution.

 The oxidation of the zinc metal releases electrons, these flow through the wire to the copper electrode.

 Electrons are accepted by copper ions in the solution when ions collide with the copper electrode.

 Copper atoms are insoluble and deposit on the electrode producing brown-black coating.

Purpose of the Salt Bridge:

 The salt bridge is an essential component of the cell, It allows charges to balance.

Without it, the cell will be polarized and electrons will accumulate on one half of

the cell. This would prevent any further current passing through, however the salt bridge stops this.

 The salt bridge contains ions that can migrate to either half cell so that a buildup of electrons (and charge) is prevented. The cation of the salt goes to the Cathode, and the anion goes to the anode.

Below is a summary of the processes occurring in the cell:

 The overall reaction in the cell is found by adding the two half equations that occur in each cell.

Half Cells:

In a half cell, an electrode is in a solution. The species present in each half cell (the elctrode and solution) form a conjugate redox pair.

 Generally, the metal in the conjugate redox pair is used as the electrode. The other is used in the solution.

 If there isnt a metal, graphite or platinum is used as the electrode.

 When a gas is involved as one of the conjugates, a special “gas electrode” is used.

 Sometimes spectator ions are present (not involved in the reaction.)

 The oxidation reaction is always at the anode. (Copper is the oxidant)

 The reduction reaction is always at the cathode. (Zinc is the reductant)

 Remember to combine the two half cell equations for the overall reaction.

Summary: (Daniel Cell)- Salt Bridge is KNO3:

Why is electrical energy released?

 By separating the two half cells, a current is allowed to produce between them.

 This allows the chemical energy of the bonds in the substances, to convert to electrical energy (for use in batteries, etc.)

 If the cells were not separated then there is no current, and instead of the chemical energy transforming to electrical energy, in forms into thermal energy.

Writing Half Cell Equations?

 Half cell equations involving a redox pair contains an atom and a simple ion. It can also be taken from the electrochemical series. (For example, If Zn is the electrode, we know it forms Zn+2 as a redox pair- so this will be in the solution.)

 Where it may get tough is when we have to work it out for polyatomic ions half cell equations. In which case, just adopt KOHES (from Unit 3 Redox) and find the

corresponding redox pair molecule.

The Electrochemical Series:

Even though substances involved in redox reactions may both want to lose electrons, they will differ in their tendencies to do so. This will mean one loses its electrons (oxidized) while the other is reduced.

For example, Zinc loses its electrons more readily than copper (we say it is more reactive.)

 The stronger reductant will be at the anode (it is in an oxidation reaction.)

 The stronger oxidant will be at the cathode (it is in a reduction reaction.)

How do we know what substance between the half cells will be stronger oxidants or reductants? This is done using an electro chemical series.

 The left side of the of the series increases upwards to the strongest oxidant.

 The right side of the series increases downwards to the strongest reductant.

This means that if we have to half cells and the substances making them up, we can identify the reactions taking place. The half cell with the stronger oxidant out of the two will undergo reduction (at the cathode), the other has the strongest reductant so will

undergo oxidation (at the anode.)

For a direct redox reaction to occur in the cell, a chemical on the left must be higher and react with a lower chemical on the right side. A “Z” shape should be formed. Likewise, the reduction reaction (higher) occurs forward ways, with the oxidation reaction occurring reverse ways as read from the electro chemical series.

 The electrochemical series is shown below.

 This is for conditions at a temperature of 25 degrees Celcius, pressure of 1 atm and 1M concentration of solutions:

Order of equations can change under varying conditions- Experimentation should occur.

Potential Difference:

 Also called the EMF.

 There is a potential difference between the two half cells. (As one half cell has a greater tendency to push electrons intro the external circuit than the other half cell.)

 It is measured in Volts (V) using a voltmeter.

 The Daniel Cell has a potential difference of about 1V.

Cell potentials are given in the electrochemical series for the listed half cell equations. This is given in E⁰ values.

 The hydrogen half cell is used as a standard (it’s cell potential is 0.00V).

 The cell potential values may vary from the electrochemical series if not under the same, standard conditions.

 The standard half cell potential is a numerical indication of the tendency of the cell’s reaction to occur as a reduction reaction. (Giving away electrons.)

 The high E⁰ values (most of the time) mean it is a reduction reaction. Lower E⁰ mainly indicate an oxidation reaction. (This is consistent with the E.C series.)

The potential difference of the overall cell is given by its two half cells:

Note: The lower word says “oxidant” not “oxidation”. This means the OXIDANT comes first (the reduction reaction.) The equation can similarly be written as:

This is because the oxidant is usually the higher value, and the reductant usually lower.

Limitations of Predictions:

 If the cells are not in standard conditions, the series does not hold as true.

 Electrochemical series gives no indication of the rates of reaction. Some reactions may actually be too slow to occur- the electrochemical series won’t show this! It only gives indication of extent of reaction (using E⁰ values).

 Experimentation should be done always to ensure accurate and reliable results.