Category: Chemical Engineering Math
"Published in Newark, California, USA"
For the reaction
Calculate the potential or voltage if the [Cu+2] is 1 x 10-4 and the [Al+3] is 1 x 10-3.
Solution:
From the given chemical equation above
Aluminum lost six electrons and copper gained six electrons. The chemical equation is already balanced including the ions.
From Table of Common Standard Reduction Potentials for Al, E0 = 1.662 V since aluminum lost three electrons and oxidation takes place in the reaction. If you multiply any of half-reactions with a coefficient, then the value of reduction potential is still the same. We don't multiply the reduction potential with any coefficients and we change only the sign if the half-reaction is reversed.
Again, from Table of Common Standard Reduction Potentials for Cu, E0 = 0.3419 V since copper gained two electrons and reduction takes place in the reaction.
The reduction potential of the reaction is the sum of the reduction potential of two half-reactions. Hence, the reduction potential is
If the concentration of metal ions are given, then we can calculate the voltage of the reaction by using Nernst Equation as follows
where n is the number of electrons lost or gained in the reaction. Hence, the voltage for the reaction is
For any whole or pure metals in half-reactions, their concentration is 1 M. In this case, the concentration of Al and Cu is 1 M. Therefore, the voltage for the reaction is
Category: Chemical Engineering Math
"Published in Newark, California, USA"
Calculate the voltage of the cell Fe; Fe+2 ║H+; H2 if the iron half cell is at standard conditions but the H+ ion concentration is 0.001 M.
Solution:
The half-cell reaction for Fe is
and it is an oxidation reaction because iron lost two electrons. From Table of Common Standard Reduction Potentials, E0 = 0.447 V. If you reverse the half-cell reaction, the sign of reduction potential will change. Since Fe+2 is at standard condition, then the concentration is 1 M. The concentration of any whole or pure metal in the half-cell reaction is 1 M.
If the concentration of any metal ion is given, then we can calculate the voltage of half-cell reaction by using Nernst Equation as follows
where n is the number of electrons lost or gained in half-cell reaction. Hence, the voltage for Fe is
The half-cell reaction for H is
and it is a reduction reaction because hydrogen gained two electrons. From Table of Common Standard Reduction Potentials, E0 = 0 V. If you reverse the half-cell reaction, the reduction potential is still the same because it is 0 V. If the concentration of H+ is 0.001M, then the voltage for H by Nernst Equation is
Therefore, the voltage of the cell is
Category: Chemical Engineering Math
"Published in Newark, California, USA"
The freezing point constant of toluene is 3.33°C per mole per 1000 g. Calculate the freezing point of a solution prepared by dissolving 0.4 mole of solute in 500 g of toluene. The freezing point of toluene is -95.0°C.
Solution:
The freezing point constant is defined as the number of degrees the freezing point will be lowered per mole of solute per 1000 g or 1 kg of solvent present. This can be written as
where m is the molality of a solution.
From the given problem, the molality of a solute dissolved in toluene is
The freezing point depression is defined as the product of the freezing point constant of a solvent and the molality of a solution.
Hence, the freezing point depression of a solution is
Therefore, the freezing point of a solution is
Note: The good application of freezing point depression is that the ice will keep longer in the container at the lower temperature. Few amount of table salt is added to the crushed ice so that the temperature will be lower than the freezing point of ice which is 0°C. If the temperature is lower than the freezing point, then the ice will not melt in the closed container. The ice cream vendors, food track vendors, and street food vendors are using ice with table salt so that their foods and drinks will keep colder in their container. If you will plan for a picnic and camping, then you can use also ice with table salt especially at the warmer or hotter areas. If the container is left open, then the ice with table salt will melt quite or slowly. Please take note that the ice with table salt will melt for a few days even though the container is closed. Table salt inhibits the melting of ice.
