aboratory 3 Factors Which Influence the Rate of Reaction DATE. SECTION: Post-Lab
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Question
aboratory 3 Factors Which Influence the Rate of Reaction DATE. SECTION: Post-Lab Questions urn in to your instructor with the post laboratory report for this experiment.) 1. When observing the effect of surface area on the rate of a heterogeneous reaction is it common to observe that the mossy zinc is reacting much faster than the zinc should (at least at the onset of the initial reaction). This is exactly opposite what more be observed for this reaction. What would cause the zinc powder to react slowly than the mossy zinc. (Hint: remember what you did to the zinc prior to the experiment.) 2. You may have clearly been able to distinguish the difference in the time required for the initial reaction of HCl with zinc in the temperature study. However, after a brief the three test tubes are reacting at the same rate. Explain why C very period this was (was not) expected. (Hint: your observation of the test tubes mayExplanation / Answer
Ans. #1. Mossy zinc is prepared by rapidly cooling molten zinc in water. It is highly porous and has greater surface area than Zn-stripes and granules.
Owing to larger surface area than granules, mossy Zn reacts faster than Zn-granules.
#2. When Zn is placed in tubes with HCl at different temperatures (say, 40C, 200C and 350C), the initial rate of reaction is different- higher the temperature of the tube, greater is the rate of reaction.
Reaction between Zn and HCl is a highly exothermic reaction.
When the reaction is allowed to run for a brief moment, the temperature of all tubes may become nearly equal due to absorption of heat released during the reaction. Therefore, all tubes attain almost the same temperature after certain time. Being at nearly the same temperature, all tubes exhibit nearly the same rate of reaction.
#3. Catalysts increase the rate of reaction by providing an alternate route/mechanism of reaction with lower activation energy. They are required in relatively very less concentration and are NOT consumed during the reaction. That is, the quantity of a catalysts remains the same before and after the reaction.
The best way to use precious metals as catalyst is to use them as a fine film/ layer in the reaction vessel (say, a fine layer of platinum on the walls of reaction vessel). It provides following two advantages-
I. Relatively very less quantity of the precious metal is required (say, to forms few micron thick layer on the wall of reaction vessel) for efficient catalysis. If the catalyst is used in form of granules or powder, much greater amount of the catalyst would be required- it further would increase the cost of metal.
II. Using the fine layer increases the active surface area of catalysis to several orders than the same when granules or powder form of catalyst is used. Moreover, the reactants are in direct contact with the catalyst surface and maximum catalysis efficiency is observed.
When granular or powder form of catalyst is used, the reactants act only on the surface because their diffusion inside the catalyst sphere is limited and inefficient.
Therefore, to minimize the amount of catalyst required (thus, minimize the cost) and maximize the active surface area of catalysts, a thin film of the catalyst is usually preferred.
#4. Everyday examples of ‘effect of temperature on reaction rates’-
I. Spoilage of food: Higher is the temperature (in general, upto 450C), higher is the rate of enzymatic reaction, so food spoils more rapidly due to enzymatic degradation of biomolecules in it. That why, we preserve food (milk, curry, etc.) in refrigerator.
II. Fermentation: The rate of fermentation (also enzymatic process, requires anaerobic condition) of bread dough (observed as dough rising) is proportion to temperature- it’s fast at higher temperatures and lower at lower temperatures.
III. Browning of apple slices: The slices of apple turn brown due to activity of polyphenol oxidase (PPO) on phenolic compounds (colorless) to form O-quinones (brown in color). Higher is the temperature, higher is the rate of browning.
IV. Rusting of metal (Fe3+ + 3 OH- ---------> Fe(OH)3 -------> Fe2O3. nH2O) s the reaction that forms hydrated forms of Fe(III) oxides when iron metal comes in contact with atmospheric water (moisture). Rate of rust formation increases with increase in temperature.
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