|Lecture Notes: 29 September||
Last time we looked at a limiting sandwich problem. Today let's try a chemistry problem.
What is the maximum mass of Fe which could be made from 115.0 g Fe3O4 of and 24.00 g C?
Another frequent question arising in chemical processes is the percent yield. This deals with the question of how effective was a given process in producing a product. Its an important consideration because chemical reactions rarely go completley to products. The maximum possible yield for a reaction is known as the Theoretical Yield, as we saw earlier. How do we determine how close to theoretical a reaction proceeds- th epercent yield.
Looking at the reaction above with 0.50 g of Zn reacting with an excess of acid, the maximum amount of hydrogen generated would be the theoretical yield:
For Zn limiting: (1 mole H2/1 mole Zn)(0.50 g Zn) / (65.39 g Zn/mol) = 7.646 x 10-3 mol H2
Now if the actual yield for a particular experiment turned out to be 6.75 x 10-3 mol, the percent yield would be calculated to be:
It is useful to remember some simple "rules" (really more like guidelines) to help in predicting reactions. For common compounds such as we see in general chemistry we can use the following rules:
The elements listed below are listed by activity, those with higher activity will displace those of lower activity. That is the more active element gives its electrons to the less active, e.g. copper metal gives its electrons to silver ion resulting in the release of copper ion and the plating of silver as we discussed in class.
The ions are listed from most active (left) to least active (a table may be found in your text on p 177).
K, Ba, Sr, Ca, Na, Mg, Al, Mn, Zn, Cr, Fe, Cd, Co, Ni, Sn, Pb, H2, Sb, Bi, Cu, Ag, Hg, Pd, Pt, Au.
H2 released in cold water
H2 released in steam
H2 released in acids
H2 not released
Note that thte elements more active than hydrogen will react with acids, while those less active do not (at least not with the protons - some acids have redox properties as well and so can react with low activity metals).
Energy is neither created nor destroyed in chemical processes. The problem here of course is - What is energy? Energy is the capacity to do work. So what's work? Work occurs when an object (mass) is moved against a force. Some common forms of energy important to our study include:
Another form of energy we need to be familiar with is:
Note that these forms of energy are readily interconverted.
Kinetic energy, potential energy, pressure, and volume are all examples of State Functions. They are all properties that depend only on the current state - they are all independent of the path used to reach this state.
The First Law of Thermodynamics says that the energy of the Universe is constant. Thus it is another name for the law of conservation of energy. Symbolically it is written:
Note that according to this law we can still do things with energy, its just that they are always compensated. (Thus as the Universe expands, work is done against gravity and the heat in the Universe decreases as manifested by a decreasing average temperature.)
Generally in thermodynamics we refer to systems. A system is simply a portion of the universe we wish to work with. For the expression
E = q + w
where E is the internal energy (the total KE and PE) of the system.
q = the quantity of heat exchanged by the system:
Notice in each case endo- and exo- are in respect to the system, not the surroundings. For example, a fire is exothermic, because heat comes out of the fire - the fuel loses heat, even though you (part of the surroundings) may gain some of it.
Keep in mind that heat always flows naturally from hotter to cooler systems. Energy must be used up to move heat in the opposite direction, as in a refrigerator.
© R A Paselk
Last modified 28 September 2009