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Exam 6: Thermochemistry: Energy Flow and Chemical Change

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Question 31

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Two solutions (the system) , each of 25.0 mL volume and at 25.0 $\degree$ C, are mixed in a beaker. A reaction occurs between them, and the temperature rises to 35.0 $\degree$ C. After the products have equilibrated with the surroundings, the temperature is again 25.0 $\degree$ C and the total volume is 50.0 mL. No gases are involved in the reaction. Which one of the following relationships concerning the change from initial to final states (both at 25.0 $\degree$ C) is correct?

A) ( $\Delta$ E = 0)
B) ( $\Delta$ H = 0)
C) ( $\Delta$ E > 0)
D) q = 0
E) w = 0

F) D) and E)
G) A) and D)

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Question 32

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Which one of the following statements about standard states is incorrect?

A) The standard state of a solid compound is the pure solid.
B) The standard state of a liquid compound is the pure liquid.
C) The standard state of a gaseous compound is the gas at a pressure of 1 atmosphere.
D) The standard state of an aqueous solute is a saturated solution in water.
E) The standard state of an element is the form in which it is stable at 1 atm and a specified temperature, usually 25 $\degree$ C.

F) A) and C)
G) A) and D)

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Question 33

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Calculate q when 28.6 g of water is heated from 22.0 $\degree$ C to 78.3 $\degree$ C.

Which one of the following equations represents the formation reaction of CH₃OH(l) ?

An important step in the synthesis of nitric acid is the conversion of ammonia to nitric oxide. Calculate $\Delta$ H $\degree$ _rxn for this reaction. $\Delta$ H $\degree$ _f [NH₃(g) ] = -45.9 kJ/mol; $\Delta$ H $\degree$ _f [NO(g) ] = 90.3 kJ/mol; $\Delta$ H $\degree$ _f [H₂O(g) ] = -241.8 kJ/mol.

A backpacker collects snow at 0 $\degree$ C, and places it in a cooking pot on a camp stove. It takes 643 kJ of heat energy to melt the snow and bring the water to boiling. Assuming no heat loss, and neglecting the specific heat capacity of the pot, calculate the mass of snow that the backpacker collected. (Data: specific heat capacity of liquid water, c = 4.18 J/g ? K; and H₂O(s) $\rightarrow$ H₂O(l) $\Delta$ H = $\Delta$ H_fusion = 6.02 kJ/mol)

A) Starting from the equation H = E + PV, show how the relationship $\Delta$ H = q_p is derived. Clearly indicate any necessary assumptions or conditions. B) In one sentence, state in full what is meant by the equation: $\Delta$ H = q_p.

The dissolution of barium hydroxide in water is an exothermic process. Which of the following statements is correct?

What is the final temperature when 20.0 g of water at 25 $\degree$ C is mixed with 30.0 g of water at 80 $\degree$ C?

In which one of the following reactions would you expect $\Delta$ H to be substantially greater than $\Delta$ E (i.e., $\Delta$ H > $\Delta$ E) ?

The enthalpy (H) of liquid water is greater than that of the same quantity of ice at the same temperature.

Ethylene glycol, used as a coolant in automotive engines, has a specific heat capacity of 2.42 J/(g-K) . Calculate q when 3.65 kg of ethylene glycol is cooled from 132 $\degree$ C to 85 $\degree$ C.

Calculate the $\Delta$ H $\degree$ _rxn for the following reaction. ( $\Delta$ H $\degree$ _f [SiO₂(s) ] = -910.9 kJ/mol; $\Delta$ H $\degree$ _f [SiCl₄(g) ] = -657.0 kJ/mol; $\Delta$ H $\degree$ _f [HCl(g) ] = -92.3 kJ/mol; $\Delta$ H $\degree$ _f [H₂O (g) ] = -241.8 kJ/mol)

The specific heat capacity c of a metal is approximately related to its molar mass ? as follows: c * ? = 3R, where R is the universal gas constant, 8.314 J/mol-K. Use this relationship to identify the metal which has a specific heat capacity of 0.900 J/g ? K.

A Snickers ^{$\rightarrow$} candy bar contains 280 Calories, of which the fat content accounts for 120 Calories. What is the energy of the fat content, in kJ?

Galena is the ore from which elemental lead is extracted. In the first step of the extraction process, galena is heated in air to form lead(II) oxide. What mass of galena is converted to lead oxide if 975 kJ of heat are liberated?

40.0 g of ice cubes at 0.0 $\degree$ C are combined with 150. g of liquid water at 20.0 $\degree$ C in a coffee cup calorimeter. Calculate the final temperature reached, assuming no heat loss or gain from the surroundings. (Data: specific heat capacity of H₂O(l) , c = 4.18 J/g- $\degree$ C; H₂O(s) $\rightarrow$ H₂O(l) $\Delta$ H = 6.02 kJ/mol)

For all processes, both q and w will have the same sign.

Nitric acid, which is among the top 15 chemicals produced in the United States, was first prepared over 1200 years ago by heating naturally occurring sodium nitrate (called saltpeter) with sulfuric acid and collecting the vapors produced. Calculate $\Delta$ H $\degree$ _ffor this reaction. $\Delta$ H $\degree$ _f [NaNO₃(s) ] = -467.8 kJ/mol; $\Delta$ H $\degree$ _f[NaHSO₄(s) ] = -1125.5 kJ/mol; $\Delta$ H $\degree$ _f[H₂SO₄(l) = -814.0 kJ/mol; $\Delta$ H $\degree$ _f[HNO₃(g) ] = -135.1 kJ/mol. NaNO₃(s) + H₂SO₄(l) $\rightarrow$ NaHSO₄(s) + HNO₃(g)

and has work done on it by the surroundings has: