MU #1 Entropy is a measure of the randomness or disorder of a system. A system with greater disorder has greater entropy.

Example

In a chemical reaction, if the products have more entropy (S) than the reactants than the change in entropy (ΔS) is positive.

ΔS = S_products - S_reactants

Example

H₂O(l) --> H₂O(g)

In a chemical reaction, if the products have less entropy (s) than the reactants than the change in entropy (ΔS) is negative.

ΔS = S_products - S_reactants

Example

H₂O(g) --> H₂O(s)

Problem

Describe the change in entropy for all the reactions in table I

MU #2 Systems in nature tend to undergo changes toward lower energy and higher entropy. 

Therefore,  in general ΔH must be negative (exothermic) and ΔS must be positive (more disorder)

Example

The burning of wood.

TEST YOUR UNDERSTANDING

1/07

27 In terms of energy and entropy, systems in nature tend to undergo changes toward

(1) higher energy and higher entropy (2) higher energy and lower entropy  (3) lower energy and higher entropy (4) lower energy and lower entropy

6/02

Which process is accompanied by a decrease in entropy?

        (1) boiling of water  (2) condensing of water vapor  (3) subliming of iodine (4) melting of ice

8/02

Which sample has the lowest entropy? (1) 1 mole of KNO₃(l)  (2) 1 mole of KNO₃(s)  (3) 1 mole of H₂O(l)  (4) 1 mole of H₂O(g)

1/03

Which phase change represents a decrease in entropy?   (1) solid to liquid    (2) gas to liquid    (3) liquid to gas (4) solid to gas

6/03

50. As carbon dioxide sublimes, its entropy  (1) decreases  (2) increases  (3) remains the same

8/03

Base your answers to questions 67 through 69 on the information below. Given the equation for the dissolving of sodium chloride in water:

              H₂O

NaCl(s) -----> Na ⁺ (aq) + Cl ^- (aq)
67 Describe what happens to entropy during this dissolving process.

68 Explain, in terms of particles, why NaCl(s) does not conduct electricity.
69 When NaCl(s) is added to water in a 250-milliliter beaker, the temperature of the mixture is lower than the original temperature of the water. Describe this observation in terms of heat flow.

1/04

50 Given the equation: KNO₃ (s) ---H₂O(l)---> KNO₃ (aq)  As H₂O(l) is added to KNO₃ (s) to form KNO₃ (aq), the entropy of the system

        (1) decreases (2) increases (3) remains the same

6/04

44 Which 10-milliliter sample of water has the greatest degree of disorder?

        (1) H₂O(g) at 120°C (2) H₂O(l) at 80°C (3) H₂O(l) at 20°C (4) H₂O(s) at 0°C

8/04

21 Even though the process is endothermic, snow can sublime. Which tendency in nature accounts for this phase change?

                (1) a tendency toward greater entropy (2) a tendency toward greater energy

                (3) a tendency toward less entropy (4) a tendency toward less energy

1/05

43 Which of these changes produces the greatest increase in entropy?

                (1) CaCO₃(s) → CaO(s) + CO₂ (g)       (2) 2 Mg(s) + O₂(g) → 2 MgO(s)

                (3) H₂O(g) → H₂O(l)           (4) CO₂(g) → CO₂(s)

6/05

20 Systems in nature tend to undergo changes toward 

                    (1) lower energy and lower entropy (2) lower energy and higher entropy 

                    (3) higher energy and lower entropy (4) higher energy and higher entropy 

44 Given the balanced equation: 

Which statement best describes this process? 

                    (1) It is endothermic and entropy increases. (2) It is endothermic and entropy decreases. 

                    (3) It is exothermic and entropy increases. (4) It is exothermic and entropy decreases. 

1/06

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