CH301 Thermodynamics

have ∆G < 0

All systems undergoing spontaneous reaction at constant T and P

positive, negative, positive The process is spontaneous, which means ∆Suniverse > 0 according to the Second Law of Thermodynamics, energy is removed from the surroundings to get the water to evaporate so ∆Ssurroundings is negative, and the system is H2O(ℓ) → H2O(g) so ∆Ssystem > 0.

For the evaporation of water from an open pan at 25C, the values of ∆S for the water, the surroundings, and the universe must be, respectively

may either increase or decrease, depending on the relative amounts of heat absorbed and work done (∆E = q + w).

If a system absorbs heat and also does work on its surroundings, its energy...

III or IV only

In which example does entropy increase? I. A piece of fruit freezes in liquid nitrogen. II. Librarians put library books back onto shelves at the end of the day. III. Liquid nitrogen boils in a beaker at room temperature. IV. Dry ice sublimes. V. A balloon collapses as its contents cool in liquid nitrogen.

The surroundings are undergoing changes in the direction of greater disorder

Liquid water freezing to crystalline ice is an example of a system going to a state with a higher degree of order. Which of the following is the best explanation of how this process can be compatible with the Second Law of Thermodynamics?

The reaction occurs at any temperature. ∆G = ∆H − T ∆S = (−) − T (+) < 0 so the reaction is spontaneous.

Six moles of gas react exothermically to yield seven moles of gas products. What can be said about how the temperature of the reaction effects the free energy of the reaction?

negative, postive, postive

The sublimation of solid carbon dioxide is a spontaneous process. Predict the sign (+, −, or 0) of ∆Gr , ∆Hr , and ∆Sr , respectively.

In spontaneous changes, the entropy of the universe increases: ∆Suniverse = ∆Ssystem + ∆Ssurroundings > 0

What is the second law of thermodynamics?

∆H > 0 (A C H bond is being broken. Bond breaking requires energy input, which is endothermic. For endothermic reactions, ∆H is positive.)

What would be the sign of ∆H for the reaction CH4(g) → CH3(g) + H(g) ?

-, +, -

When water condenses, what are the signs for q, w, and ∆Ssys, respectively?

4. Ca(OH)2; weak base

Which acid or base is INCORRECTLY identified as to type of compound? 1. HClO3; strong acid 2. H3PO2; weak acid 3. CsOH; strong base 4. Ca(OH)2; weak base 5. HF; weak acid

3. Br2(ℓ) because liquids always have a much lower entropy than gases. If there were no liquids, the substance with the lowest entropy would be Kr(g), because it is the lightest

Which of the following has the smallest entropy at 298 K? 1. Kr(g) 2. Xe(g) 3. Br2(ℓ) 4. Br2(g) 5. Cl2(g)

3. HNO2

Which one of the following is a weak electrolyte? 1. Ba(OH)2 2. Ca(NO3)2 3. HNO2 4. NH4Cl 5. HClO3

increases

Without performing any calculations, determine whether the entropy of the system increases or decreases in the dissolution of table salt: NaCl(s) → NaCl(aq)

3. conducts an electric current in an aqueous solution.

An electrolyte 1. All of these 2. never comes from molecular compounds. 3. conducts an electric current in an aqueous solution. 4. is always derived from ionic compounds.

w, q, ∆E, and ∆H (P = 0 so w = −P ∆V = 0. There is no change in molecular kinetic energy or potential energy due to intermolecular attraction (which is zero for ideal gases). Therefore; ∆E = 0 ∆E = q + w 0 = q + 0; q = 0 ∆H = q = 0

An ideal gas is allowed to expand isothermally from 2 liters at 5 atm to 5 liters at 2 atm against a vacuum (no outside pressure). Which of the quantities (q, w, ∆E, and ∆H) are zero for this process?

II and III

Consider the following processes. (Treat all gases as ideal.) I) The pressure of one mole of oxygen gas is allowed to double isothermally. II) Carbon dioxide is allowed to expand isothermally to 10 times its original volume. III) The temperature of one mole of helium is increased 25C at constant pressure. IV) Nitrogen gas is compressed isothermally to one half its original volume. V) A glass of water loses 100 J of energy reversibly at 30C. Which of these processes leads to an increase in entropy?

Na(s) and F2(g) (the heat of formation = 0 for elements in their standard states)

Consider the following substances: HCl(g), F2(g), HCl(aq), and Na(s) Which response includes ALL of the substances listed that have a heat of formation equal to 0?

The increase of the entropy of the system.

The second law of thermodynamics requires that spontaneous processes generate entropy, either in the system or in the surroundings. What is the thermodynamic driving force for dissolving a solid in a liquid if it is an endothermic process (which reduces the entropy of the surroundings)?

Increasing the temperature

If you have an endothermic process in which the change in entropy is positive, how can you make it spontaneous?

greater; greater; high. Energy input (positive ∆H, endothermic) is required to break bonds. Because you inevitably end up with more individual particles than what you start with, the entropy of the system increases. Processes are spontaneous when ∆G is negative. ∆G = ∆H − T ∆S

The bond breaking process has ∆H (greater, less) than zero and ∆S (greater, less) than zero; thus it is favorable at (high, low) temperatures.

Endothermic process with increasing system disorder

The melting of ice is an example of a physical process that can be non-spontaneous but is increasingly likely to occur as the temperature of the system increases. What characteristics of ∆H and ∆S for the process would correspond to this behavior?

It is not a spontaneous chemical reaction; it is driven by an external agency - light.

The oxidation of sugar to carbon dioxide and water is a spontaneous chemical reaction. Since we know that reactions that occur spontaneously in one direction cannot occur spontaneously in the reverse direction, how can we understand photosynthesis?

4. that the compound ionizes or dissociates completely or almost completely in solution.

The term strong in strong electrolyte means 1. All of these 2. that the compound is very soluble. 3. that the compound is very concentrated. 4. that the compound ionizes or dissociates completely or almost completely in solution.

∆H < 0, ∆S > 0 (Burning a house involves a release of heat, which means ∆H is negative, or exothermic. Also, a burning house goes from ordered to disordered. More disorder means that ∆S is positive.)

What best describes the burning of a house from a thermodynamical standpoint?

The entropy of a pure, perfect crystalline substance at 0K is zero.

What is the 3rd law of thermodynamics?

Entropy is a state function such that a change in entropy is given by the flow of heat into a system in a reversible process divided by the temperature at which it occurs.

What is the best definition of entropy?

the energy of the universe is constant; energy cannot be created nor destroyed.

What is the first law of thermodynamics?

1. All of these compounds are strong electrolytes. correct

Which of these compounds would you expect to be a weak electrolyte? 1. All of these compounds are strong electrolytes. 2. NH4Br 3. All of these compounds are nonelectrolytes. 4. HClO4 5. NaF 6. RbOH

4. When ∆H for a reaction is negative the reaction can never occur spontaneously

Which one of the following statements is NOT correct? 1. When ∆G for a reaction is zero the system is at equilibrium 2. When ∆G for a reaction is negative the reaction can occur spontaneously 3. When ∆H for a reaction is very positive the reaction will probably not occur spontaneously at lower temperatures. 4. When ∆H for a reaction is negative the reaction can never occur spontaneously 5. When ∆G for a reaction is positive the reaction cannot occur spontaneously

The reaction does not occur spontaneously at any temperature. Combustion reactions are exothermic, so the reverse of a combustion reaction is endothermic (so ∆H is positive) Counting the moles of gas on each side, we have 18 mol gas → 15 mol gas. Therefore the overall entropy will decrease (∆S is negative) and ∆G = ∆H − T ∆S = (+) − T (−) = +

Written below is a reaction in which butane is formed from carbon dioxide and water. Note that this is the reverse of the butane combustion reaction. What would you say about the temperature dependence of this reaction to form butane? 8 CO2(g) + 10 H2O(g) → 2 C4H10(g) + 13 O2(g)

1. decomposition, spontaneous, unstable

s Ethane is C2H6. Consider the reaction: 2 C(s) + 3 H2(g) ←→ C2H6(g) ∆Grxn = 7.86 1 The (formation/decomposition) of ethane is (spontaneous/non-spontaneous) and therefore ethane is (stable/unstable)?

The reaction is not spontaneous at any temperature For the reaction 2 mol solid + 1 mol gas → 3 mol solid. There is no net change in the number of moles, but the gas has become a solid so order has increased. ∆S is negative, so ∆G = (+)−T(−). For ∆G to be negative, T would have to be negative, which is not possible.

s The following reaction is endothermic. 2 Cu(s) + CO2(g) → 2 CuO(s) + C(s) Which statement is true? 1. The reaction is spontaneous at all temperatures. 2. It is impossible to determine if the reaction is spontaneous without calculations 3. The reaction will only be spontaneous at high temperatures. 4. The reaction will only be spontaneous at low temperatures. 5. The reaction is not spontaneous at any temperature