Entropy

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AP Chemistry › Entropy

Questions 1 - 10
1

Which of the following does not represent an increase in entropy?

Boiling water

NaCl dissolving in water

Sublimation of CO2

Condensation of water on glass

Melting of mercury

Explanation

Of phases, from greatest to least entropy is gas>liquid>solid. Dissolving a salt into solution is an increase in entropy because the salt has greater space to move. The only decrease in entropy is condensation of water on glass (gas to liquid)

2

Which of the following reactions represents a negative change in entropy?

Water heating

Ice melting

Salt dissolving

Sublimation of dry ice

None of these represent a negative change in entropy

Explanation

Entropy for phases are Gas>Liquid>Solid. A salt dissolving into solution has a positive entropy change (the solution becomes more chaotic). Therefore all of the listed reactions are a positive change in entropy, making E the correct answer.

3

A reaction will reach equilibrium when the system's entropy __________.

is at a maximum

is at a minimum

is zero

entropy does not influence equilibrium at all

Explanation

Equilibrium is the basis of entropy, which can essentially be thought of as the distribution of energy in the system. For a reaction at a given temperature, the system's entropy will be at a maximum when it reaches equilbrium because this is the ideal energy state, and the forward and reverse reactions are occurring at a steady rate.

4

Which of the following scenarios describes a process with a negative entropy change?

Building a skyscraper

An igloo melting

Osmosis

A clean room becomes cluttered

Explanation

Entropy can be thought of as the tendency for a system to become disordered, or random. Osmosis, melting ice, and a clean room becoming dirty are all examples of entropy at work. Each of these processes will occur spontaneously without the input of outside energy or work.

If you make a skyscraper, you are incorporating a series of compounds and building products into a single, distinct formation. Think of it this way: what are the odds that mother nature would spontaneously build a building? This process is very ordered, which means that entropy of the universe has decreased.

5

What is the change in entropy for the following reaction?

Explanation

The change in entropy is calculated by:

When cannot be measured, it can be calculated from known entropies of formation.

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

6

What is the change in entropy for the following reaction?

Explanation

The change in entropy is calculated by:

When cannot be measured, it can be calculated from known entropies of formation.

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

7

What is the change in entropy for the following reaction?

Explanation

The change in entropy is calculated by:

When cannot be measured, it can be calculated from known entropies of formation.

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

8

What is the change in entropy for the following reaction?

Explanation

The change in entropy is calculated by:

When cannot be measured, it can be calculated from known entropies of formation.

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

9

What is the change in entropy for the following equation?

Explanation

The change in entropy is calculated by:

When cannot be measured, it can be calculated from known entropies of formation.

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

10

What is the entropy for the following reaction?

Explanation

The change in entropy is calculated by:

When cannot be measured, it can be calculated from known entropies of formation.

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in entropy of a reaction.

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