The silversilver chloride electrode
The silver-silver chloride electrode SSCE is another secondary reference electrode. A schematic of its half-cell is The AgCl layer has a pale beige colour immediately it is made, but soon afterwards it assumes a pale mauve and then a dark purple aspect. The colour changes reflect chemical changes within the film, caused as a result of photolytic breakdown AgCl hv electron donor- gt Ag0 Cl- 7.46 The purple colour is caused by colloidal silver, formed in a similar manner to the image on a...
Why is milk cloudy
Particle suspensions and phase dispersal Milk contains both organic and inorganic components. A majority of the milk is water based, and contains water-soluble solutes such as calcium compounds. But milk also contains as much as 15 per cent by mass of water-insoluble, fat-based compounds. These two extremes of oil and water do not mix, and remain as separate phases see Chapter 5 . Mixing these oil- and water-based components causes two distinct layers to form in a process called phase...
Thermodynamics of phase changes
Rotary evaporators are a common feature in most undergraduate laboratories. Their primary purpose is to remove solvent following a reflux, perhaps before crystallization of a reaction product. To operate the evaporator, we place the reaction solution in a round-bottomed flask while the pressure inside the evaporator is decreased to about is then rotated. The solvent evaporates more easily at this low pressure than at pO. The solvent removed under vacuum is trapped by a condenser and collected...
Thermodynamics of concentration cells
A concentration cell contains the same electroactive material in both half-cells, but in different concentration strictly, with different activities . The emf forms in response to differences in chemical potential x between the two half-cells. Note that such a concentration cell does not usually involve different electrode reactions other than, of course, that shorting causes one half-cell to undergo reduction while the other undergoes oxidation . Worked Example 7.20 Consider the simple cell Cu...
The virial equation
An alternative approach to quantifying the interactions and deviations from the idealgas equation is to write Equation 1.13 in terms of 'virial coefficients' RT l B'p C'p2 2.3 The word 'virial' comes from the Latin for force or powerful. where the V n term is often rewritten as Vm and called the molar volume. Equation 2.3 is clearly similar to the ideal-gas equation, Equation 1.13 , except that we introduce additional terms, each expressed as powers of pressure. We call the constants, B', C...
The standard hydrogen electrode the primary reference
The internationally accepted primary reference is the standard hydrogen electrode SHE . The potential of the SHE half-cell is defined as 0.000 V at all temperatures. We say the schematic for the half-cell is A reference electrode is a constant-potential device. We need such a reference to determine an unknown electrode potential. We define the value of E sHe gt as zero at all temperatures. The SHE is depicted in Figure 7.12, and shows the electrode immersed in a solution of hydrogen ions at...
The Nernst equation
A new torch battery has a voltage of about 1.5 V, but the emf decreases with usage until it becomes too small to operate the torch for which we bought it. We say the battery has 'gone flat', and throw it away. We need to realize from Faraday's laws that chemicals within a battery are consumed every time the torch is switched on, and others are generated, causing the composition within the torch to change with use. Specifically, we alter the relative amounts of oxidized and reduced forms within...
The mean ionic activity coefficient y
The value of y depends on the solute employed. The extent of ionic association depends on the ions we add to the solution. And the extent of association will effect the extent of screening, itself dictating how extreme the difference is between perceived and real concentration. For these reasons, the value of Y a c depends on the choice of solute as well as its concentration, so we ought to cite the solute whenever we cite an activity coefficient. The value of y is even more difficult to...
The Eyring approach to kinetic theory
The Arrhenius theory above was wholly empirical in terms of it derivation. A more rigorous, but related, form of the theory is that of Eyring also called the theory of absolute reaction rates . The Eyring equation is where kB is the Boltzmann constant and h is the Planck constant. AH is the enthalpy change associated with forming the activated complex and AS is the change in entropy. A plot of ln k T as 'y' against 1 T as 'x' should be linear, of gradient -AH R and intercept 'AS R ln kB h '....
Motion of particles in the gas phase
The question, 'What is pressure ' is another odd question, but is not too difficult to answer. The constituent particles of a substance each have energy. In practice, the energy is manifested as kinetic energy - the energy of movement - and explains why all molecules and atoms move continually as an expression of that kinetic energy. This energy decreases as the temperature decreases. The particles only stop moving when cooled to a temperature of absolute zero 0 K or -273.15 C. The particles...
Variations in the value of Ka
Carbonic acid, H2CO3, is naturally occurring, and forms when carbon dioxide from the air dissolves in water. From its formula, we expect it to be a di-protic acid, but it is generally classed as mono-protic. Why In water at 298 K, the ionization reaction follows the equation H2CO3 aq H2O - HCO3- aq H3O aq 6.37 The value of Ka for the reaction in Equation 6.37 is 4.3 x 10-7, so carbonic acid is certainly a very weak acid. The hydrogen carbonate anion HCO3- could dissociate further, according to...
The Maxwell Boltzmann distribution of energies
If no heat was distributed, then our faces and those parts closest to the fire would quickly become unbearably hot, while the remainder of our flesh would continue to feel cold. Heat conducts through the body principally by the fire warming the blood on the surface of the skin, which is then pumped to other parts of the body through the circulatory system. The energy in the warmed blood is distributed within cooler, internal tissues. It is important to note how the heat energy is distributed...
The photochemistry of vision
We have mentioned microwave ovens a few times already. Although we might employ such an oven regularly, no one has actually seen microwave radiation. The photons are wholly invisible. The phenomenon of 'sight' can be simplified to the photo-effected transformation of a pigment related to retinal within the retina at the back of the eye. Retinal is derived from vitamin A. The outermost layer of the retina is the pigment epithelium with supporting cells for the neural portion of the retina . The...
The SCE
By far the most common secondary reference electrode is the SCE Hg Hg2Cl2 KCl sat'd Note that the 'S' of 'SCE' here does NOT mean 'standard', but 'saturated'. Platinum contact Saturated KCl solution Figure 7.13 Schematic representation of the saturated calomel electrode SCE The potential of the SCE is 0.242 V at 298 K relative to the SHE. At the 'heart' of the SCE is a paste of liquid mercury and mercurous chloride Hg2Cl2 , which has the old-fashioned name 'calomel'. Figure 7.13 depicts a...
Effect of temperature on a phase change sublimation
Horror films commonly show scenes depicting smoke or fog billowing about the screen during the 'spooky' bits. Similarly, smoke is also popular during pop concerts, perhaps to distract the fans from something occurring on or off stage. In both cases, it is the adding of dry ice to water that produces the 'smoke'. Dry ice is carbon dioxide CO2 in its solid phase. We call it 'dry' because it is wholly liquid-free at pO such solid CO2 looks similar to normal ice solid water , but it 'melts' without...