5b Chemical Energetics II Flashcards
Define entropy.
Entropy S of a system is a measure of the disorder in the system. The more disorder the system is, the larger its entropy.
It is related to the number of ways the particles in the system can be arranged or distributed and also the number of ways the energy in the system can be dispersed or spread out.
Define a spontaneous process. If the process is spontaneous, will the reverse process be spontaneous too? Can enthalpy change account for the direction of spontaneous change? What is required then?
A process that once started will continue without any external assistance. If the process is spontaneous, the reverse process will not be spontaneous. Enthalpy change alone cannot account for the direction of spontaneous change as spontaneous reactions can be exothermic or endothermic. A second thermodynamic factor termed entropy (S) must be used.
What are the two natural tendencies behind spontaneous processes?
Tendency to achieve a lower energy state
Tendency toward a state of greater entropy
How does entropy change as the substance change from solid to gas?
For any substance, the entropy increases as the substance changes from solid to liquid to gas (Entropy increases as the system becomes more disordered and has more energy dispersed in it). (Pg 3 of notes)
Define entropy change.
Entropy change for a reaction or process is a measure of the change in disorder in a system (where the change in entropy = entropy of the final state - entropy in the initial state)
What does the sign of entropy change indicate?
When entropy change is positive, the final state is more disordered than the initial state. When entropy change is negative, the final state is less disordered than the initial state.
Explain the 4 factors that affect the entropy of a chemical system.
(i) Change in temperature: When the temperature of a system is increased, the increase in entropy comes about because, in the broadening of the Boltzmann energy distribution, there are more ways of arranging energy quanta in the hotter system. OR There is a broadening of the energy distribution of the particles. Thus there are more possible energy states in which the particles can adopt at a higher temperature, leading to an increase in entropy.
(ii) Change in phase: When a solid melts into a liquid, the order in the solid is destroyed. Particles in a liquid are more randomly arranged and more disordered than those in the solid, resulting in a (gradual) increase in entropy. When the liquid changes into a gas or when a solid becomes a gas, there is an even larger (abrupt) increase in entropy as the gaseous state is the most disordered since the gas particles can move freely and are the most randomly arranged. (This change in phase is accompanied by a large increase in volume.)
(iii) Change in the number of particles: With more particles, there are more ways to arrange the particles and more ways to distribute the energy in the system, hence creating greater disorder in the system. (For a process that results in an increase in the number of moles of gaseous particles in a system, the entropy increases even more.)
(iv) Mixing of particles
- Gas particles: Upon mixing, each gas expands to occupy the whole container, more ways to arrange in larger volume, greater entropy
- Liquid particles: Liquids of similar polarities / miscible liquids mix together to form a solution in which there are more ways for the molecules to arrange themselves in the larger volume, greater disorder and increased entropy.
- Dissolution of an ionic solid in water: This process can lead to a net increase or decrease in disorder depending on the constituent ions of the ionic solid. This is because the dissolution process involves the disruption of the crystal structure of the ionic solid (ions previously rigidly held in the lattice can move freely) which increases disorder and the hydration process (hydrating water molecules are put into an orderly arrangement about the ions) which decreases disorder.
(State whether the decrease in entropy in the hydration process outweighs the increase in entropy due to the disruption of the crystal lattice or vice versa, leading to a decrease/increase in entropy)
What is the equation that gives the Standard Gibbs Free energy change?
Delta G = Delta H - Temp (Delta S)
What does the sign of Standard Gibbs Free energy change indicate?
When the Standard Gibbs Free energy change is negative (reaction is exergonic), the forward reaction is spontaneous. When the Standard Gibbs Free energy change is positive (reaction is endergonic), the forward reaction is not spontaneous. This can also be used to deduce whether a process or reaction is spontaneous under non-standard conditions (i.e. at a particular temperature and pressure).
What is a limitation of using the (standard) Gibbs Free energy change?
It indicates thermodynamic feasibility (spontaneity) of a reaction (i.e. whether it can occur under standard conditions etc) but it gives no information about the kinetic feasibility of the reaction (i.e. whether the reaction can proceed at an observable rate) as the kinetic feasibility depends on the activation energy of the reaction. Due to very high activation energies, some exergonic reactions may not occur.
What are the steps involved to find the effect of temperature on the spontaneity of the reaction?
(i) Determine the sign of enthalpy change
(ii) Determine the sign of entropy change
(iii) Determine the sign of -T(Entropy change)
If both (ii) and (iii) are negative, Gibbs Free energy change will be less than 0 at all temperatures.
If both (ii) and (iii) are positive, Gibbs Free energy change will be more than 0 at all temperatures (reaction has to be driven continuously with external existence).
If (i) is positive and (iii) is negative, Gibbs Free energy change will be less than 0 at high temperatures as the negative (iii) outweighs positive (ii).
If (iii) is positive and (i) is negative, Gibbs Free energy change will be less than 0 at low temperatures as the negative (i) outweighs the positive (iii).
