Reactivity 1.4—Entropy and spontaneity HL

Question 1

Entropy

Answer 1

A measure of the amount of disorder of the particles in a system.

Question 2

Standard entropy of a substance

Answer 2

Entropy change from heating the substance from absolute zero (0 K) to the thermodynamic standard temperature of 298 K.

Question 3

How can you predict if a physical or chemical change will increase or decrease the entropy of a system?

Answer 3

Consider the change in disorder and energy distribution. Increases in entropy are associated with more spread-out particles and energy (more disorder). E.g., melting increases entropy, while freezing decreases it.

Question 4

How do you calculate standard entropy changes (ΔS°) from standard entropy values (S°)?

Answer 4

Use ΔS° = ΣS°(products) - ΣS°(reactants). This requires the entropy values of all reactants and products under standard conditions. A positive ΔS° indicates an increase in entropy, while a negative ΔS° indicates a decrease.

Question 5

Spontaneous reaction

Answer 5

A chemical reaction that favors product formation once the activation energy has been provided to start the reaction.

Question 6

Activation energy

Answer 6

The minimum energy required to cause a process (such as a chemical reaction) to occur.

Question 7

Gibbs energy

Answer 7

The energy linked with a chemical reaction capable of performing useful work under standard conditions of temperature and pressure (273.15 K and 100 kPa).

Question 8

What is the ultimate deciding factor
that tells us whether a reaction is spontaneous or not?

Answer 8

Gibbs energy change (ΔG)

Question 9

How does entropy change with the states of matter?

Answer 9

Entropy increases as the state changes from solid to liquid to gas, reflecting a move towards greater disorder (solid < liquid < gas).

Question 10

What do positive (+ΔS) and negative (−ΔS) signs indicate about entropy change?

Answer 10

A positive sign (+ΔS) indicates an increase in entropy, while a negative sign (−ΔS) signifies a decrease in entropy.

Question 11

How can the change in Gibbs energy (ΔG) of a reaction be calculated?

Answer 11

ΔG can be calculated based on its change in enthalpy (ΔH), change in entropy (ΔS), and the absolute temperature (T).

Question 12

Under what condition is a chemical reaction at constant pressure spontaneous?

Answer 12

A reaction is spontaneous if the change in Gibbs energy (ΔG) is negative.

Question 13

Are exothermic reactions with a positive change in entropy (ΔS) always spontaneous?

Answer 13

Yes, exothermic reactions with a positive change in entropy (ΔS) are always spontaneous.

Question 14

Are endothermic reactions with a negative change in entropy (ΔS) ever spontaneous?

Answer 14

No, endothermic reactions with a negative change in entropy (ΔS) are never spontaneous.

Question 15

What determines the spontaneity of exothermic reactions with a negative ΔS and endothermic reactions with a positive ΔS?

Answer 15

The spontaneity of these reactions depends on the temperature.

Question 16

When do reversible reactions reach equilibrium?

Answer 16

When the ratio of particles of products and reactants (Q) equals the equilibrium constant (K) for that reaction at a specific temperature.

Question 17

What happens to the change in Gibbs energy (ΔG) as a reversible reaction approaches equilibrium?

Answer 17

As a reversible reaction approaches equilibrium, ΔG becomes less negative and finally reaches zero at equilibrium.