Which of the following organic compounds is the least soluble in water? a) CH3CO

Question

Which of the following organic compounds is the least soluble in water?

a) CH3COOH, ethanoic acid

b)C6H12O6, glucose

c)CH3CH2CH2NH2, propyl amine

d)C5H12, pentane

e)CH3OH, methanol

Reaction A has a high activation energy whereas Reacton B has a low activation energy. Which of the statements about Reaction A and Reaction B are true? Reaction B is likely to occur at a faster rate than Reaction A. Reaction B is more likely to occur at all than Reaction A. Reaction A is likely to occur at a faster rate than Reaction B Reaction A is more likely to occur at all than Reaction B.

Explanation / Answer

Ans.

. Part A. Polar liquids (ethanoic acid, propylamine, methanol) are soluble in water to good extent.

Glucose, also polar due to several -OH groups, is also highly soluble in water.

Pentane, being non-polar gas, is least soluble in water.

Correct option. D. Pentane

Part B. Activation energy of the minimum amount of energy required (input) by the specific reaction to begin.

A reaction with lower activation energy may begin and complete at low temperature.

Consider the hypothetical reactions A and B mentioned in the question. Both the reaction conditions are all identical (say, have same value of temperature, frequency factor, concentration and all other variables) except the activation energy.

Suppose activation energy of reaction A is 100 units and that of reaction be is 25 units.

So, owing to low activation energy, reaction B begins and completes at room temperature. But reaction A, only begins at 1000C because it requires higher energy input (note that heat is form of energy, increase in temperature of reaction mixture means increase in the energy being supplied to reaction mixture).

So, at 250C, reaction B completes but reaction A does not even start because activation energy is not yet provided to it.

When both reactions are incubated at 1000C- reaction A starts because its activation energy is supplied, reaction B also begins because its activation energy (equivalent to 250C) has also been met. So, both the reactions occur at 1000C

It is noteworthy that activation energy can’t be directly related to rate of reaction unless more information is provided. For example, enzymatic hydrolysis of starch, which occurs at 250C due to low activation energy requires incubation of 30 minutes or more for completion. However, burning a paper sheet requires higher energy input (ignition point around 2500-3000 C) but completes as soon as it begins.

Conclusion:

I. Rate of reaction of two different reactions can’t directly be related to their activation energy.

II. AT 250C, reaction B begins and compete, but A does not even start. The same is not true at 100C – both reactions start.

So, correct option is- B. Reaction B is more likely to occurs (at lower temperature or lower energy input) at all than reaction A (may not even start).

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