electrochemistry Flashcards
Describe the activity of ions (forces) and the deviation from ideal behaviour.
Strong inter-ionic forces, due to electrostatics, dictate deviations from ideality. Deviation from ideal behaviour can be expected to occur rapidly, therefore activity can be replaced by concentration/molality only in the limit of very diluted solutions.
How do ions in a solution arrange?
Opposite charged ions are more likely to
be found in proximity to each other than
farther away. This leads to the organisation of ions into shells, each ion (pos or neg) being at the centre of a “cloud” and part of another cloud. The overall energy, and the chemical potential all-together, are lowered due to local relaxaMon of ionic clouds around central ions.size of shell can be found as the Debye length
What is the Debye-Huckel limiting law?
It describes the activity coefficients of ions in dilute electrolyte solutions. It states that the logarithm of the mean ionic activity coefficient is proportional to the square root of the ionic strength of the solution.
Describe the extended model of the Debye-Huckel law.
It is applicable when ionic strength is higher, outside of the expected validity range for the linear D-H relation. Referred to as Extended Debye-Hückel law.
What are the components of an electrochemical cell?
Electrodes, electrolyte, salt bridges
What is a Galvanic cell?
A prototypical battery. In a Galvanic cell, current is generated in a spontaneous way. As such, Galvanic cells can be used as batteries. Composed of two half cells, joined by a salt bridge. Energy released by spontaneous redox reaction is converted to electrical energy. The redox reaction is spontaneous.
What is an electrolytic cell?
external potential
The direction of the reaction is determined by an external potential. Such cells can be used for electrolysis, for example in the
production of hydrogen or aluminium. The use of electrolytic cells to split water can be used to store (renewable) energy. Electrical energy is used to drive nonspontaneous redox reaction.
Where does oxidaion and reduction occur?
Reduction occurs at the cathode, oxidation occurs at the anode.The applied potential will charge the cathode negatively, the anode
positively.
What are the types of electrodes?
Gas electrode (Pt, H2, H+), insoluble salt electrode (a metal electrode,covered with a layer of an insoluble salt MX, immersed in a solution of X-), redox electrode (two different oxidations states in solution, same element, inert electrode)
What is the equation relating cell potential with Gibbs free energy?
ΔrGm = − νeFE
v is the number of (hidden) electrons in the balanced equation.
F is the Faraday constant, 96485.3 C mol-1, which is the charge of 1 mol electrons.
It follows that, if E is positive, ΔrGm is negative, and therefore the reaction is spontaneous.
What elements are in a Daniel cell?
Copper and Zinc.
ZnSO4, CuSO4
What does the Nernst equation give?
The Nernst equation gives the potential for the total reaction, i.e. the one that is actually taking place. This imply that any two cells with the same total reaction and the same number of electrons , will have the same potential.
How can industrial electrolysis be used for the extraction of metals from ores?
Metal extraction typically requires a reduction (metals are in oxidised form).
Iron is extracted from the oxide ore by means ofcarbon, which produced carbon dioxide. In some cases, the extreme stability of the oxide ore complicates the process.
Bauxite, the main source of Al. Al2O3 can be extracted from bauxite.
How can industrial electrolysis be used for chlorine production?
Competing reactions: reduction of sodium ions at the cathode. In practice, this reaction does not occur, as its potential is more
negative (-2.71 V) compared to -0.41 V for hydrogen reduction.
* At the anode, oxygen production is possible: the reduction potential is +0.825 V compared to +1.37 V for chlorine - therefore this reaction is actually competitive. While oxygen reduction is in principle favoured, hydroxide concentration is lower than chloride, therefore oxygen will just be
a contaminant at the end of the reaction, which can be separated.
- chlorine is the main product despite higher potential.
Properties of batteries.
- Battery material contain a readily reducible/oxidisable element, normally a
transition metal. - The reaction with lithium is reversible.
- High free energy of reaction
- high capacity - means that the number of lithiums per transition metal
needs to be high (at least one!). - high energy storage (voltage x capacity)
- Rapid reaction with lithium.
- Good electronic conductor.
- Stable.
- no degradation, no over-dischage/over-charge.
