To understand the difference between thermodynamic and kinetic properties of a c
ID: 1004603 • Letter: T
Question
To understand the difference between thermodynamic and kinetic properties of a chemical process.
Thermodynamics quantifies the state functions of a system. The state functions are independent of the path followed by the reaction, and depend only on the difference between the beginning and the final points of the reaction. The general thermodynamic formula that is used to predict the spontaneity of a chemical reaction is the Gibbs free energy, defined as follows:
G=HTS, whereG is the standard change in free energy from the reaction, H is the standard change of enthalpy (heat flow) in the reaction, Tis the temperature in kelvins, and S is the standard change in entropy (disorder) during the reaction.
If G is negative for a given reaction, it means that there will be a net flow of free energy from the reacting system into the environment, and the reaction is favored thermodynamically. Such a reaction is said to bespontaneous.
Be aware, however, that spontaneous does not meaninstantaneous, and that thermodynamically favored reactions may still occur very slowly. For example, the hydrogenation of ethene, H2C=CH2, to form ethane, H3CCH3, is a thermodynamically favored reaction for which G=125 kJ/mol. However, when hydrogen gas, H2, and ethene are mixed together at room temperature, they react spontaneously, but at a rate so slow as to be undetectable.
From this example you can see that thermodynamics can predict whether a reaction is favored, but it says nothing about the rate at which the reaction will occur. To understand rates of reactions, we must turn to chemical kinetics.
The reason that the above reaction occurs so slowly is that bonds must be broken before anything else can proceed. The bond-breaking event requires a certain amount of energy. This energy barrier is known as theactivation energy of the reaction.
Adding platinum to this reaction will increase the reaction rate because the platinum acts as a catalyst. A catalyst speeds up a chemical reaction by lowering the activation energy but is not used up in the reaction. In this case, the platinum helps break the hydrogen-hydrogen bonds in H2, thus lowering the energy barrier.
It is important to note that catalysts do not change the thermodynamic properties of a reaction. A catalyst cannot change the equilibrium constant of a reaction.
Some other common metal catalysts, in addition to platinum, are copper, iron, nickel, palladium, rhodium, and ruthenium. To increase the accessible surface area, they can sometimes be mixed with an inert substance such as charcoal.
1) The reaction between carbon tetrachloride,
CCl4, and water, H2O, to form carbon dioxide, CO2, and hydrogen chloride, HCl, has a G value of 232 kJ/mole, and so is thermodynamically favored. But when you mix carbon tetrachloride with water, no change is observed. What is a possible explanation for this?
The reaction between carbon tetrachloride, , and water, , to form carbon dioxide, , and hydrogen chloride, , has a value of , and so is thermodynamically favored. But when you mix carbon tetrachloride with water, no change is observed. What is a possible explanation for this?
2) You have some propene ,
H2C=CHCH3, in a compressed gas tank that you need to convert to propane, H3CCH2CH3, to run your cabin heater and fridge. You have another tank of compressed hydrogen gas, H2, sitting out in your shed. Your first idea is to fill up a third tank with a mixture of the two gases, but when you try that, nothing happens. You look under your kitchen sink and find some powdered palladium mixed in inert charcoal. You take the palladium and put it into the flowpath from the hydrogen and propene tanks. As you begin flowing the mixture of propene and hydrogen, you notice with glee that by the time the gas reaches the receiving tank, it is propane gas. After all the propene has been converted to propane, you pour the palladium-charcoal mixture into a crucible. After analyzing the palladium, you notice that its chemical state has not changed.
Part B
What word best describes the role that the palladium plays in the reaction between propene and hydrogen above?
What word best describes the role that the palladium plays in the reaction between propene and hydrogen above?
3)What did the palladium do to to increase the rate of the reaction between the propene and hydrogen?
What did the palladium do to to increase the rate of the reaction between the propene and hydrogen?
The reaction is not favored thermodynamically. It is a slow reaction kinetically. The activation energy of the reaction is too small. The reaction is not spontaneous.Explanation / Answer
Q.1: Since deltaG is negative, the reaction is spontaneous and thermodynamically stable. Since the reaction occurs very slowly the activation energy is also high. Hence the best possible explanation is it is a slow reaction kinetically.
Hence second option is correct.
Q.2: During hydrogenation of ethene, Palladium acts as a catalyst and decreases the activation energy without itself underging any chemical change. Hence first option is correct.
Q.3: Palladium lowers the activation energy of the reaction between propene and hydrogen.Hence 3rd option is correct.
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