Directions: For each question, provide your work and final answer below the question. Without work, no credit will be given. Check significant figures and units.
A(aq) + 2 B(aq) 3 C(aq) + D(aq)
- For the reaction above, carried out in solution of 30C, the following kinetic data were obtained:
Experiment / Initial Conc. of Reactants
(mole.liter-1) / Initial Rate of Reaction
(mole.liter-1.hr-1)
Ao / Bo
1 / 0.240 / 0.480 / 8.00
2 / 0.240 / 0.120 / 2.00
3 / 0.360 / 0.240 / 9.00
4 / 0.120 / 0.120 / 0.500
5 / 0.240 / 0.0600 / 1.00
6 / 0.0140 / 1.35 / ?
(a)Write the rate-law expression for this reaction.
(b)Calculate the value of the specific rate constant k at 30C and specify its units.
(c)Calculate the value of the initial rate of this reaction at 30C for the initial concentrations shown in experiment 6.
(d)Assume that the reaction goes to completion. Under the conditions specified for experiment 2, what would be the final molar concentration of C?
- Consider the following general equation for a chemical reaction.
A(g)+ B(g) C(g)+ D(g) H reaction = -10 kJ
(a)Describe the two factors that determine whether a collision between molecules of A and B results in a reaction.
(b)How would a decrease in temperature affect the rate of the reaction shown above? Explain your answer.
(c)Write the rate law expression that would result if the reaction proceeded by the mechanism shown below.
A + B [AB](fast)
[AB] + B C + D(slow)
(d)Explain why a catalyst increases the rate of a reaction but does not change the value of the equilibrium constant for that reaction.
(I)A2 + B2 2 AB
(II)X2 + Y2 2 XY
- Two reactions are represented above. The potential-energy diagram for reaction I is shown below. The potential energy of the reactants in reaction II is also indicated on the diagram. Reaction II is endothermic, and the activation energy of reaction I is greater than that of reaction II.
(a)Complete the potential-energy diagram for reaction II on the graph above.
(b)For reaction I, predict how each of the following is affected as the temperature is increased by 20C. Explain the basis for each prediction.
(i) Rate of reaction
(ii) Heat of reaction
(c) For reaction II, the form of the rate law is rate = k[X2]m[Y2]n. Briefly describe an experiment that can be conducted in order to determine the values of m and n in the rate law for the reaction.
(d)From the information given, determine which reaction initially proceeds at the faster rate under the same conditions of concentration and temperature. Justify your answer.
Directions: Read the passage below and answer the summary questions that follow in complete sentences.
Reactions and Catalysts
Because of the key role of activation energy in deciding whether a collision will result in a reaction, it would obviously be useful to know what sort of proportion of the particles present have high enough energies to react when they collide. In any system, the particles present will have a very wide range of energies. For gases, this can be shown on a graph called the Maxwell-Boltzmann Distribution which is a plot of the number of particles having each a particular energy.
Remember that for a reaction to happen, particles must collide with energies equal to or greater than the activation energy for the reaction. We can mark the activation energy on the Maxwell-Boltzmann distribution:
Notice that the large majority of the particles don't have enough energy to react when they collide. To enable them to react we either have to change the shape of the curve, or move the activation energy further to the left.
A catalyst is a substance which speeds up a reaction, but is chemically unchanged at the end of the reaction. When the reaction has finished, you would have exactly the same mass of catalyst as you had at the beginning. Some common examples which you may need for other parts of your syllabus include: MnO2, concentrated sufuric acid, iron, V2O5, nickel.
Only those particles represented by the area to the right of the activation energy will react when they collide. The great majorities don’t have enough energy, and will simply bounce apart. To increase the rate of a reaction you need to increase the number of successful collisions. One possible way of doing this is to provide an alternative way for the reaction to happen which has a lower activation energy.
Adding a catalyst has exactly this effect on activation energy. A catalyst provides an alternative route for the reaction. That alternative route has lower activation energy. Showing this on an energy profile:
Be very careful if you are asked about this in an exam. The correct form of words is "A catalyst provides an alternative route for the reaction with lower activation energy." It does not "lower the activation energy of the reaction".
Surface catalysts work by increasing the percentage of collisions that have correct orientation. Simply put, if 10% of collisions have correct orientation initially and, by adding something, I can increase that to 30% , the rate will triple (assuming that all other conditions stay the same).We can use an example to explain the process. In the engine of a car, oxygen and gasoline are mixed, ignited and then shoved out into the exhaust system. This happens incredibly quickly, so quickly in fact, that there is often not enough time for the reaction to go to completion. This can result in production of carbon monoxide, a toxic gas. This carbon monoxide moves through the tailpipe along with unreacted oxygen. These molecules bump together and are certainly hot enough to react, but most collisions that occur do not result in a further reaction (producing carbon dioxide) because correctly oriented collisions are extremely rare. Now, however, the gases pass through the catalytic converter on the way to the tailpipe. This is a container in which a massive amount of surface area is covered in platinum and palladium. Remember that metals (like everything else) are made of atoms, so that a seemingly smooth surface of metal is actually quite bumpy at the atomic scale. Platinum and palladium were chosen because the spacing of the bumps in these metals matches the spacing of the atoms in carbon monoxide quite well. So, when a CO bumps into the platinum and palladium surface it may stick. The result is that, with the carbon monoxide no longer moving freely, the likelihood of a well-oriented collision with oxygen goes up dramatically. Carbon dioxide does not fit into the metal surface as well, so the product is released leaving the metal surface unchanged and able to help more reactions occur. This is the classic demonstration of a catalyst: it helps a reaction occur, but is unchanged itself in the process. In this way, the catalytic converter (and the catalysts it contains—the Pt and Pd) help to produce more carbon dioxide and less carbon monoxide, making the exhaust fumes less toxic. As a side note, the introduction of the catalytic converter is one reason that leaded fuels were phased out. Certain lead compounds had been added to gasoline to make them burn more efficiently, but it was found that the lead coated the surfaces in the catalytic converter rendering them useless. All gasoline sold in the United States are now unleaded, but for a number of years, the choices at the gas station were not regular and premium, but rather regular or unleaded.
Acid Base Catalysis,acceleration of a chemical reaction by the addition of an acid or a base, the acid or base itself not being consumed in the reaction. The catalytic reaction may be acid-specific (acid catalysis), as in the case of decomposition of the sugar sucrose into glucose and fructose in sulfuric acid; or base-specific (base catalysis), as in the addition of hydrogen cyanide to aldehydes and ketones in the presence of sodium hydroxide. Many reactions are catalyzed by both acids and bases.
Enzymesare highly selective catalysts, greatly accelerating both the rate and specificity of metabolic chemical reactions, from the digestion of food to the synthesis of DNA.Enzymes act by converting starting molecules (substrates) into different molecules (products). Almost all chemical reactions in a biological cell need enzymes in order to occur at rates sufficient for life. Since enzymes are selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which metabolic pathways occur in that cell, tissue and organ. Organelles are also differentially enriched in sets of enzymes to compartmentalise function within the cell.
Questions:
- Explain why not all collisions result in a reaction taking place.
- How does adding a catalyst affect the Maxwell-Boltzmann distribution? Sketch a diagram:
- Explain how increasing the temperature of a reaction increases the rate using the distribution below:
- How does adding a catalyst affect the potential energy graph? Sketch a diagram:
- Explain how surface catalysts work briefly.
- What do enzymes do?