Electricity from chemical reactionsGalvanic CellsChapter 14

Study Design• use of the electrochemical series in predicting the products of

redox reactions and deducing overall equations from redox half equations

• limitations of predictions made using the electrochemical series, including the determination of maximum cell voltage under standard conditions

• the chemical principles, half-equations and overall equations of simple primary and secondary galvanic cells

Redox Reactions• Redox reactions involve the transfer of electrons.• A redox reaction involves two reactions occurring

simultaneously: a reduction reaction and an oxidation reaction.

• A substance is reduced when it gains electrons. It is said to have undergone reduction.

• A substance is oxidised when it loses electrons. It is said to have undergone oxidation.

Reductants and Oxidants• A reductant causes another compound to undergo reduction.• An oxidant causes another compound to undergo oxidation.

• A reductant will undergo oxidation.• An oxidant will undergo reduction.

Oxidation Numbers• A compound that undergoes reduction gains electrons. It will

therefore have a decrease in its oxidation number.

• A compound that undergoes oxidation loses electrons. It will therefore have a increase in its oxidation number.

An example redox reaction.

Oxidation No.Zero

Oxidation No.Zero

Oxidation No.+2

Oxidation No.-2

An example redox reaction.

• In this example, the oxidation number of magnesium has increased.

• This means it has undergone oxidation. It has been oxidised.• Therefore magnesium is the reductant.

• In this example, the oxidation number of oxygen has decreased.

• This means it has undergone reduction. It has been reduced.• Therefore oxygen is the oxidant.

Redox half equations and electrochemical cells.• What occurs when a zinc strip is placed in copper(II) sulphate

solution?

Redox half equations and electrochemical cells.• What occurs when a zinc strip is placed in copper(II) sulphate

solution?

Redox half equations and electrochemical cells.• The net ionic equation of this reaction is

• This redox reaction occurs spontaneously. No external force or energy is being supplied.

• Redox reactions can be represented by two half equations.• In this example,

Redox half equations and electrochemical cells.• Since electric current can be the flow of electrons, and redox

reactions involve electron transfer, it possible to use redox reactions to produce electricity.

• This can be achieved by separating the half equations to form half cells.

• The transferred electrons can then be made to pass through an external circuit, usually a conductor.

• The apparatus than can be used to generate electricity from a chemical reaction is called a galvanic cell.

• It can also be called a voltaic or electrochemical cell.

Key Features of a Galvanic Cell

• Two Half Cells• An anode half cell where oxidation occurs. • A cathode half cell where reduction occurs.• Both half cells usually contain a metal electrode with the metal

solution.• A conducting path, usually a metal wire, that connects the anode

and the cathode.• A salt bridge that connects the two solutions in each half cell.• In a galvanic cell, electrons flow from the anode to the cathode.

The Daniell Cell

Types of Half Cells• The Metal Ion-Metal Half Cell

Types of Half Cells• The Solution Half Cells

Types of Half Cells• The Gas Non Metal Ion Half Cell

Standard Electrode Potentials

• The electrical potential of a galvanic cell is the ability of the cell to produce an electric current.

• The reduction potential of a half cell is a measure of the tendency of the oxidant to accept electrons and so undergo reduction.

• The difference between the reduction potentials of the two half cells I called the cell potential difference.

Standard Reference Cell

• The standard cell potential difference is the measured cell potential difference, under standard conditions, when the concentration of each species in solution is 1 mol/L , the pressure of the gas is 101325 Pa and the temperature is 298K.

• To obtain a comparative measure of the reduction potentials of different half cells, the standard hydrogen half cell is used as a standard reference electrode. It has been assigned an arbitrary value of 0.00V.

Standard Reference Cell

• The standard cell potential difference is the measured cell potential difference, under standard conditions, when the concentration of each species in solution is 1 mol/L , the pressure of the gas is 101325 Pa and the temperature is 298K.

• To obtain a comparative measure of the reduction potentials of different half cells, the standard hydrogen half cell is used as a standard reference electrode. It has been assigned an arbitrary value of 0.00V.

Standard Reference Cell E.g.

• When the Cu/ half cell is connected to the reference half cell, a voltage of 0.34V is recorded.

• Electrons flow toward the Copper electrode and so this means it is the cathode.

Electrochemical Series

Predicting Spontaneous Reactions in Galvanic Cells.• The electrochemical series can be used to predict what

reactions will be spontaneous.• If an arrow can be drawn from the oxidant to the reductant so

that the start of the arrow is at the top left and the tip of the arrow is at the bottom right, you will obtain a spontaneous reaction.

Predicting Spontaneous Reactions in Galvanic Cells.

• The two half equations that will occur are as follows.

• The reduction half equation will be the top equation and is written out as it is shown on the electrochemical series.

• The oxidation half equation will be the bottom equations and is written out in the reverse manner to which it is shown on the electrochemical series.

Predicting Spontaneous Reactions in Galvanic Cells.

𝐶𝑢 (𝑠 )↔𝐶𝑢2+¿+ 2𝑒−¿

Predicting Spontaneous Reactions in Galvanic Cells.

• To write the full equation, the two half equations must have the same number of electrons. If they do not, then the relevant half equations must be altered by multiply the co-efficient of each compound by the same number until they do.

• If they do have the same number of electrons, then the reactants of both half equations are written down as reactants and the combined products of both half equations are written down as products.

• You do not write down the electrons.

Predicting Spontaneous Reactions in Galvanic Cells.

𝐶𝑢 (𝑠 )↔𝐶𝑢2+¿+ 2𝑒−¿

𝑪𝒖 (𝒔 )+𝑶𝟐 (𝒈 )+𝟐𝑯+¿ (𝒂𝒒)↔𝑪𝒖𝟐+¿(𝒂𝒒)+𝑯𝟐𝑶

𝟐( 𝒍) ¿ ¿

Unpredictability of the rate of electrochemical reactions.• Although the electrochemical series allows us to predict whether a

spontaneous reaction will occur, it does not provide any information about the rate.

• Although it may be predicted that a reaction may occur, no reaction may be observed if they rate is very slow.

Questions

• Q1-Q2 pg 338• Q3-Q4 pg 339• Q5-Q7pg 340