CHEM113 EXAM 1

When is a reaction always spontaneous?

∆H is negative. ∆S is positive. ∆G is always less than zero. Both heat and matter are being dispersed.

When is a reaction never spontaneous?

∆H is positive. ∆S is negative. ∆G is always greater than zero. Both heat and matter are being constrained.

Which of the following statements regarding spontaneous processes is not true? A. Spontaneous processes are exothermic. B. Spontaneous processes proceed without outside intervention once initiated. C. Spontaneous reactions are not always rapid. D. The motional freedom of particles tends to increase during spontaneous change. E. Particles tend to become more spread out during spontaneous change.

A. Spontaneous processes are exothermic. This is false because both endothermic and exothermic processes can be spontaneous.

Suppose a system contains 1 mole of ice melting at room temperature. Which of the following is not valid: A. ∆Ssys=-∆Ssurr B. ∆Hsys=-∆Hsurr C. ∆Ssys+∆Ssurr>0 D. ∆Ssurr=-∆Hsys/Tsurr E. ∆Ssys=∆Hsys/Tsys

A. ∆Ssys=-∆Ssurr In a spontaneous reaction the entropy change for the universe must be greater than zero.

At 0 K, the entropy of a perfect crystal is: A. > 0 B. = 0 C. < 0 D. > 0 or < 0, depending on the chemical structure of the crystal. E. ≥ 0, depending on the chemical structure of the crystal.

B. =0 Third law of thermodynamics.

The synthesis of ammonia is shown below. Under standard conditions, ΔH° = -92.2 kJ and ΔS° = -198.3 J/K for this reaction. Which of the following best describes the spontaneity of this reaction at 25° C? N2 (g) + 3 H2 (g) → 2 NH3 (g) A. The reaction is spontaneous at all temperatures (including 25°C) because both enthalpy and entropy decrease in the reaction. B. The reaction is spontaneous at 25°C because free energy decreases at this temperature. C. The reaction is nonspontaneous at 25°C because free energy increases at this temperature. D. The reaction is neither spontaneous nor nonspontaneous at 25°C because it is at equilibrium. E. The spontaneity of this reaction cannot be determined without the free energies of formation for the reactants and products.

B. The reaction is spontaneous at 25°C because free energy decreases at this temperature.

Which of the following is always true for a spontaneous exothermic process? A. ΔSsys < 0 B. ΔSsurr > 0 C. ΔSuniverse = 0 D. ΔSsys > 0 E. ΔSsurr < 0

B. ΔSsurr > 0 Exothermic processes release heat into the surroundings.

Determine the sign for entropy change for the system and surroundings for the spontaneous reaction: 2A(g)+B(g)→2C(g)

Because the reaction is spontaneous, the ∆Suniverse must be positive. ∆Ssystem is negative because the number of particles decreases. ∆Ssurroundings is positive

During which of the following processes is ΔS°sys < 0? A. Salt crystals dissolve in water. B. Air escapes from a hole in a balloon. C. Elemental iron and oxygen react to form rust (Fe₂O₃). D. Ice melts in your hand. E. Water boils in a pan.

C. Elemental iron and oxygen react to form rust (Fe₂O₃). In order forΔS°sys to be negative, the amount of dispersal must decrease. When you have two substances reacting to produce one substance there are less particles.

The _____ Law of Thermodynamics states that the entropy change in an isolated system during a spontaneous process must be greater than zero: A. Zeroth B. First C. Second D. Third E. None is correct.

C. Second

Determine the conditions under which the reaction below would be spontaneous. N₂O₄(g) → 2NO₂(g) A. The reaction is spontaneous under all conditions. B. The reaction is never spontaneous under any conditions. C. The reaction is spontaneous at high temperatures. D. The reaction is spontaneous at low temperatures. E. There is not enough information given to predict the spontaneity of this process.

C. The reaction is spontaneous at high temperatures. This equation requires energy to break the bonds of N₂O₄(g) which means it is endothermic and ∆H is +. Matter is also being dispersed because you are reacting 1 mole of N₂O₄(g) to produce 2 moles of NO₂(g) which means ∆S is positive.

A new cooler is advertised to have a perfectly tight seal and to hold ice for eternity. What kind of system does the cooler represent? A. This is an open system. B. This is a closed system. C. This is an isolated system. D. This is an exothermic process. E. All of the above are true.

C. This is an isolated system. Neither matter nor energy is exchanged in this system.

Why does the conversion of the ATP to ADP (Reaction 1) often occur simultaneously with other biochemical reactions such as the production of sucrose shown in Reaction 2? Reaction 1: ATP → ADP + H2PO4 -30 kJ/mol=ΔG Reaction 2: glucose + fructose → sucrose +27 kJ/mol=ΔG A. Reaction 1 is exothermic, so it can provide the energy to fuel endothermic reactions, such as Reaction 2. B. Entropy increases in Reaction 1 but decreases in Reaction 2. Reactions where entropy decreases cannot happen without being coupled to a reaction where entropy increases. C. The product of Reaction 1, ADP, is a reactant in the production of sucrose shown in Reaction 2. D. Reaction 1 is a spontaneous reaction in which free energy is released. This released free energy can be used as the driving force for a nonspontaneous reaction, such as Reaction 2. E. The conversion of ATP to ADP (Reaction 1) cannot happen alone because it requires the input of work, which can be provided from the free energy change in Reaction 2.

D. Reaction 1 is a spontaneous reaction in which free energy is released. This released free energy can be used as the driving force for a nonspontaneous reaction, such as Reaction 2. Think about coupled reactions.

Which of the following does not demonstrate a valid equality for the reaction shown below: 4NH₃(g)+5O₂(g)→4NO(g)+6H₂O(g) A.(1/4)(∆[NH₃]/∆t)=(1/5)(∆[O₂]/∆t) B.(-1/4)(∆[NH₃]/∆t)=(1/6)(∆[H₂O]/∆t) C. rate=(1/6)(∆[H₂O]/∆t) D. rate=(1/4)(∆[NH₃]/∆t) E. (-∆[NH₃]/∆t)=(∆[NO]/∆t)

D. rate=(1/4)(∆[NH₃]/∆t) This should be negative because NH₃ is a reactant in the equation

Gibbs free energy:

Describes the spontaneity of a process at a constant temperature.

What are the signs of ΔS, ΔH, and ΔG for the condensation of water on the sides of a glass of ice tea at room temperature? A. +, +, - B. -, +, - C. +, -, - D. -, +, + E. -, -, -

E. -,-,- Water is going from a gas to a liquid, so ∆S must be negative. When water molecules are condensed, energy is released meaning it is an exothermic reaction so ∆H is negative. This is also a spontaneous reaction so ∆G is negative.

Which of the following does not accurately describe reaction rates? A. Rates of reactions generally decrease over the course of the reaction. B. Overall rate of a reaction is always positive and has units of concentration over time. C. The average rate of a reaction represents the relative change in concentration of a reactant or product over a specific time period. D. The instantaneous rate of a reaction is a more accurate description of the actual rate of a reaction than the average rate of a reaction at a given point in time. E. The instantaneous rate of a reaction represents the initial rate of a reaction (at t =0.)

E. The instantaneous rate of a reaction represents the initial rate of a reaction (at t =0.) You don't have to have the initial rate of a reaction start at time=0.

First Law of Thermodynamics:

Energy is conserved. Energy cannot be created or destroyed. The total energy of the universe is constant.

If the rate of formation of the product of the following equation were 0.16M/s, how fast would F₂ be disappearing? 2NO₂(g)+F₂(g)→2NO₂F

For every 1 mole of F₂ that is reacted, 2 moles of NO₂F are produced, therefore F₂ would disappear at 1/2 the reaction rate.

Open System:

Matter and energy can be exchanged between the surroundings and the system.

Isolated System:

Neither matter nor energy can be exchanged.

Closed System:

Only energy can be exchanged between the surroundings and the system.

What is a spontaneous process?

Process that has no outside intervention. In any spontaneous process either matter or heat(energy) must be dispersed. Spontaneous processes can only occur in one direction.

Determine the sign for entropy change for the system and surroundings of the sublimation of dry ice at room temperature.

Sublimation of dry is a spontaneous reaction so ∆Ssystem must be positive. The reaction is endothermic(heat is entering the system) so ∆Hsurroundings is losing heat, therefore ∆Ssurroundings must be negative because the particles in the surroundings are slowing down due to the loss of heat.

Relate the change in enthalpy of a reaction to the change in entropy of the surroundings:

The entropy change of the surroundings will be related to the amount of energy that enters the surroundings in the form of heat divided by the temperature of the surroundings.

Third Law of Thermodynamics:

The entropy of a pure crystalline substance at absolute 0°(0K) is zero. entropy=0.

Second Law of Thermodynamics:

The entropy of the universe increases for all spontaneous processes.

Entropy:

The measure of how dispersed the energy of a system is. S=klnW k=Boltzmann Constant ln=natural log W=Number of possible microstates

Predict the conditions under which the following reaction is spontaneous: SF₆(g)→SF₆(l)

∆H is negative because this is an exothermic reaction. ∆S is negative because the particles are becoming more ordered. So this reaction is spontaneous at lower temperatures.

When is a reaction spontaneous at low temperatures only?

∆H is negative. ∆S is negative. ∆G is negative at lower temperature. Heat is being dispersed, matter is being constrained.

When is a reaction spontaneous at high temperatures only?

∆H is positive. ∆S is positive. ∆G is negative at higher temperature. Heat is being constrained, matter is being dispersed.