Free expansion Throttling Electricity flow through a resistance Heat transfer Plastic deformation. Irreversibilities are of two types: External irreversibilities. These are associated with dissipating effects outside the working fluid. Example: Mechanical friction occurring during a process due to some external source. 2. Internal irreversibilities. These are associated with dissipating effects within the working fluid. Example: Unrestricted expansion of gas, viscosity and inertia of the gas. Lecture 30: Spontaneous Process: Spontaneous processes do not require energy input to proceed, whereas nonspontaneous processes do. A spontaneous process is capable of proceeding in a given direction without needing to be driven by an outside source of energy. Spontaneous changes, also called natural processes, proceed when left to themselves, and in the absence of any attempt to drive them in reverse. The sign convention of changes in free energy follows the general convention for thermodynamic measurements. This means a release of free energy from the system corresponds to a negative change in free energy, but to a positive change for the surroundings. The laws of thermodynamics govern the direction of a spontaneous process, ensuring that if a sufficiently number of individual interactions (like atoms colliding) are involved, then the direction will always be in the direction of increased entropy.
53. Unusual Properties of Water: With 70% of our earth being ocean water and 65% of our bodies being water, it is hard to not be aware of how important it is in our lives. There are 3 different forms of water, or H2O: solid (ice), liquid (water), and gas (steam). Because water seems so ubiquitous, many people are unaware of the unusual and unique properties of water, including: Boiling Point and Freezing Point Surface Tension, Heat of Vaporization, and Vapor Pressure Viscosity and Cohesion Solid State Liquid State Gas State Boiling Point and Freezing Point: we all know that the boiling point of water is 100°C. So despite its small molecular weight, water has an incredibly big boiling point. This is because water requires more energy to break its hydrogen bonds before it can then begin to boil. The same concept is applied to freezing point as well. The boiling and freezing points of water enable the molecules to be very slow to boil or freeze, this is important to the ecosystems living in water. If water was very easy to freeze or boil, drastic changes in the environment and so in oceans or lakes would cause all the organisms living in water to die. This is also why sweat is able to cool our bodies. Surface Tension, Heat of Vaporization, and Pressure: Besides mercury, water has the highest surface tension for all liquids. Water's high surface tension is due to the hydrogen bonding in water molecules. Water also has an exceptionally high heat of vaporization. Vaporization occurs when a liquid changes to a gas, which makes it an endothermic reaction.
Whether it is through the collision theory, transition theory, or just common sense, chemical reactions are typically expected to proceed faster at higher temperatures and slower at lower temperatures. By 1890 it was common knowledge that higher temperatures speed up reactions, often doubling the rate for a 10 degree rise, but the reasons for this were not clear. Finally, in 1899, the Swedish chemist Svante Arrhenius (1859-1927) combined the concepts of activation energy and the Boltzmann distribution law into one of the most important relationships in physical chemistry. The "Arrhenius Equation" was physical justification and interpretation. Arrhenius performed experiments that correlated chemical reaction rate constants with temperature. After observing that many chemical reaction rates depended on the temperature, Arrhenius developed this equation to characterize the temperature dependent reactions: Lecture114: Arrhenius Equation 2: Arrhenius Equation Where: k: Chemical reaction rate constant In unit of s-1(for 1st order rate constant) or M-1s-1(for 2nd order rate constant) A: The pre-exponential factor or frequency factor Specifically relates to molecular collision Deals with the frequency of molecules that collide in the correct orientation and with enough energy to initiate a reaction. It is a factor that is determined experimentally, as it varies with different reactions. In unit of L mol-1s-1 or M-1s-1(for 2nd rate constant) and s-1(for 1st order rate constant) Because frequency factor A is related to molecular collision, it is temperature dependent
Usually, a free radical is marked by a dot beside the symbol which represents an odd electron exists on the species. This odd electron makes the intermediate very reactive. For example, the oxygen, chlorine and ethyl radicals are represented by and respectively. The radicals can be formed by the homolytic photodissociation reaction: Lecture123: Mechanism of Chain Reactions 1: The elementary steps used for mechanisms of chain reactions can be grouped into the following categories: Initiation step Chain propagation steps branching steps Chain inhibition steps Chain termination steps For example, the chlorination of ethane is a reaction, and its mechanism is explained in the following way. If we mix chlorine, Cl2, and ethane, CH3CH3, together at room temperature, there is no detectable reaction. However, when the mixture is exposed to the reaction suddenly initiates, and explodes. To explain this, the following mechanism is proposed. Initiation Step: Light (hν) can often be used to initiate chain reactions since they can generate free radical intermediates via a photodissociation reaction. The initiation step can be written as:
Lecture 131: Measuring Reaction Rate: To measure a reaction rate, we usually monitor either a product or a reactant for its change. Any physical characteristic related to the quantity or concentration of a product or reactant can be monitored. Some of the characteristics to be monitored are: change in pressure, change in color (spectroscopic measurement), temperature for exothermic or endothermic reaction, and presence of certain key substance The change can be plotted on a graph, and from the graph, we can get the average rate or the instantaneous rate by either graphical methods or using a computer for the data analysis. Rate Constants and the Orders: Usually, the rate of a reaction is a function of the concentrations of reactants. For example, the rate of the reaction 2NO+O2→2NO2 has the form: Rate=k[O2][NO]2 The rate is proportional to the concentration of O2, usually written as [O2] and is proportional to the square of [NO], or [NO]2. The orders of 1 and 2 for [O2] and [NO]respectively have been determined by experiment, NOT from the chemical equation. The total order of this reaction is 3 (=2+1). Note that orders are NOT determined from the stoichiometry of the reaction equation. Lecture132:
Once the reaction has been stopped, the mixture is withdrawn and analyzed in an appropriate manner. The quenched flow technique works something like the stopped flow method described above, with a slightly altered plumbing arrangement. The reactants A and B are mixed and fed directly through the diverter valve to the measuring cell, which is not shown in this diagram. After a set interval that can vary from a few milliseconds to 200 sec or more, the controller activates the quenching syringe and diverter valve, flooding the cell with the quenching solution.