# 4.2 Standard potentials  (Page 4/4)

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Calculate the cell potential of the electrochemical cell in which the following reaction takes place, and represent the cell using standard notation.

$Mg\left(s\right)+2{H}^{+}\left(aq\right)\to Mg2+\left(aq\right)+{H}_{2}\left(g\right)$

1. $M{g}^{2+}+2{e}^{-}⇌Mg$ (E ${}^{o}$ V = -2.37)

$2{H}^{+}+2{e}^{-}⇌{H}_{2}$ (E ${}^{o}$ V = 0.00)

2. From the overall equation, it is clear that magnesium is oxidised and hydrogen ions are reduced in this reaction. Magnesium is therefore the anode reaction and hydrogen is the cathode reaction.

3. $Mg|M{g}^{2+}||{H}^{+}|{H}_{2}$
4. E ${}_{\left(cell\right)}^{0}$ = E ${}^{0}$ (cathode) - E ${}^{0}$ (anode)

= 0.00 - (-2.37)

= +2.37 V

## Predicting whether a reaction will take place spontaneously

Look at the following example to help you to understand how to predict whether a reaction will take place spontaneously or not.

In the reaction,

$P{b}^{2+}\left(aq\right)+2B{r}^{-}\left(aq\right)\to B{r}_{2}\left(l\right)+Pb\left(s\right)$

the two half reactions are as follows:

$P{b}^{2+}+2{e}^{-}⇌Pb$ (-0.13 V)

$B{r}_{2}+2{e}^{-}⇌2B{r}^{-}$ (+1.06 V)

Half cell reactions

You will see that the half reactions are written as they appear in the table of standard electrode potentials. It may be useful to highlight the reacting substance in each half reaction. In this case, the reactants are Pb ${}^{2+}$ and Br ${}^{-}$ ions.

Look at the electrode potential for the first half reaction. The negative value shows that lead loses electrons easily, in other words it is easily oxidised. The reaction would normally proceed from right to left (i.e. the equilibrium lies to the left), but in the original equation, the opposite is happening. It is the Pb ${}^{2+}$ ions that are being reduced to lead. This part of the reaction is therefore not spontaneous. The positive electrode potential value for the bromine half-reaction shows that bromine is more easily reduced, in other words the equilibrium lies to the right. The spontaneous reaction proceeds from left to right. This is not what is happening in the original equation and therefore this is also not spontaneous. Overall it is clear then that the reaction will not proceed spontaneously.

Will copper react with dilute sulfuric acid (H ${}_{2}$ SO ${}_{4}$ )? You are given the following half reactions:

$C{u}^{2+}\left(aq\right)+2{e}^{-}⇌Cu\left(s\right)$ (E ${}^{0}$ = +0.34 V)

$2{H}^{+}\left(aq\right)+2{e}^{-}⇌{H}_{2}\left(g\right)$ (E ${}^{0}$ = 0 V)

1. In the first half reaction, the positive electrode potential means that copper does not lose electrons easily, in other words it is more easily reduced and the equilibrium position lies to the right. Another way of saying this is that the spontaneous reaction is the one that proceeds from left to right, when copper ions are reduced to copper metal.

In the second half reaction, the spontaneous reaction is from right to left.

2. What you should notice is that in the original reaction, the reactants are copper (Cu) and sulfuric acid (2H ${}^{+}$ ). During the reaction, the copper is oxidised and the hydrogen ions are reduced. But from an earlier step, we know that neither of these half reactions will proceed spontaneously in the direction indicated by the original reaction. The reaction is therefore not spontaneous.

A second method for predicting whether a reaction is spontaneous

Another way of predicting whether a reaction occurs spontaneously, is to look at the sign of the emf value for the cell. If the emf is positive then the reaction is spontaneous . If the emf is negative , then the reaction is not spontaneous .

## Balancing redox reactions

We will look at this in more detail in the next section.

## Predicting whether a reaction will take place spontaneously

1. Predict whether the following reaction will take place spontaneously or not. Show all your working.
$2Ag\left(s\right)+C{u}^{2+}\left(aq\right)\to Cu\left(s\right)+2A{g}^{+}\left(aq\right)$
2. Zinc metal reacts with an acid, H ${}^{+}$ (aq) to produce hydrogen gas.
1. Write an equation for the reaction, using the table of electrode potentials.
2. Predict whether the reaction will take place spontaneously. Show your working.
3. Four beakers are set up, each of which contains one of the following solutions:
1. Mg(NO ${}_{3}$ ) ${}_{2}$
2. Ba(NO ${}_{3}$ ) ${}_{2}$
3. Cu(NO ${}_{3}$ ) ${}_{2}$
4. Al(NO ${}_{3}$ ) ${}_{2}$
Iron is added to each of the beakers. In which beaker will a spontaneous reaction take place?
4. Which one of the following solutions can be stored in an aluminium container?
1. Cu(SO) ${}_{4}$
2. Zn(SO) ${}_{4}$
3. NaCl
4. Pb(NO ${}_{3}$ ) ${}_{2}$

## Electrochemical cells and standard electrode potentials

1. An electrochemical cell is made up of a copper electrode in contact with a copper nitrate solution and an electrode made of an unknown metal M in contact with a solution of MNO ${}_{3}$ . A salt bridge containing a KNO ${}_{3}$ solution joins the two half cells. A voltmeter is connected across the electrodes. Under standard conditions the reading on the voltmeter is 0.46V. The reaction in the copper half cell is given by: $\mathrm{Cu}\to {\mathrm{Cu}}^{2+}+2{\mathrm{e}}^{-}$
1. Write down the standard conditions which apply to this electrochemical cell.
2. Identify the metal M. Show calculations.
3. Use the standard electrode potentials to write down equations for the:
1. cathode half-reaction
2. anode half-reaction
3. overall cell reaction
4. What is the purpose of the salt bridge?
5. Explain why a KCl solution would not be suitable for use in the salt bridge in this cell.
(IEB Paper 2, 2004)
2. Calculate the emf for each of the following standard electrochemical cells:
1. $Mg|M{g}^{2+}||{H}^{+}|{H}_{2}$
2. $Fe|F{e}^{3+}||F{e}^{2+}|Fe$
3. $Cr|Cr2+||C{u}^{2+}|Cu$
4. $Pb|P{b}^{2+}||H{g}^{2+}|Hg$
3. Given the following two half-reactions:
• $F{e}^{3+}\left(aq\right)+{e}^{-}⇌F{e}^{2+}\left(aq\right)$
• $Mn{O}_{4}^{-}\left(aq\right)+8{H}^{+}\left(aq\right)+5{e}^{-}⇌M{n}^{2+}\left(aq\right)+4{H}_{2}O\left(l\right)$
1. Give the standard electrode potential for each half-reaction.
2. Which reaction takes place at the cathode and which reaction takes place at the anode?
3. Represent the electrochemical cell using standard notation.
4. Calculate the emf of the cell

what is different between work and power in details
When a force is exerted to an object and causes it to move in the direction of the force the work is done whereas power is the rate at which work is done.
ISRAEL
Why is alcohol flameable?
What is interconversion reaction for? ... under substitution reaction
Moloto
how can u tell whether the compound is an alkane, alkene or alkyne
alkane compounds are saturated, consists of single bonds between atoms and alkenes are unsaturated consists of one double bond between two adjacent carbon atoms .alkynes are also unsaturated but they consists of a triple bond between 2 adjacent carbon atoms.
Andile
you can tell simple by looking at the bonds which exist between the carbon-carbon atoms of the compound, so if a compound consists of double bond between its carbon-carbon atoms it is an alkene etc.
Tatyana
can it say it cause I understand it better
Hey, what is momentum?
the product of mass n its velocity
Albert
And what's simple harmonic?
Inocent
What is organic compounds
are compounds which consists of carbon atoms
Mkhize
Thanks bro
Gaba
How many industrial processes are there?
A homologous group is a group with the same amount of electrons in the outermost shell.
What are hydrocarbons
Its a compound of hydrogen and carbon atoms
Luyanda
do hydrocarbons get unsaturated?
yes
Thembi
they are saturated
Sinoty
Yes
Gugu
what is monomer
what is polymerism
Tobeka
I think a polymer is a chemical compound with molecules bonded together in long, repeating chains. Because of their structure, polymers have unique properties that can be tailored for different uses.
Kagiso
Louis
which essays are expected in march exams
what do you look at when you want to successfully compare the boiling points of two|three organic molecules
1-check the homologous group 2-number of carbons or chain length n their intermolecular forces between the bonds
Lesego
why are alkanes none as primary
meant to say known as primary alchols
Babulele
because there is only 1 OH attached to carbon and carbon
Sinoty
intermoleculer forces