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Exam 6: Thermochemistry: Energy Flow and Chemical Change

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Question 1

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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°C.

F) A) and B)
G) A) and C)

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Question 2

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A Snickers® candy bar contains 280 Calories,of which the fat content accounts for 120 Calories.What is the energy of the fat content,in kJ?

A) 5.0 $\times$ 10^-1 kJ
B) 29 kJ
C) 5.0 $\times$ 10² kJ
D) 1.2 $\times$ 10³ kJ
E) 5.0 $\times$ 10⁵ kJ

F) B) and C)
G) A) and B)

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Question 3

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A system delivers 225 J of heat to the surroundings while delivering 645 J of work.Calculate the change in the internal energy, $\Delta$ E,of the system.

Diborane (B₂H₆)has been considered as a possible rocket fuel.Calculate $\Delta$ H° for the reaction B₂H₆(g) $\to$ 2B(s)+ 3H₂(g) using the following data:

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°_rxn for this reaction. $\Delta$ H°_f [NaNO₃(s) ] = -467.8 kJ/mol; $\Delta$ H°_f [NaHSO₄(s) ] = -1125.5 kJ/mol; $\Delta$ H°_f [H₂SO₄(l) = -814.0 kJ/mol; $\Delta$ H°_f [HNO₃(g) ] = -135.1 kJ/mol NaNO₃(s) + H₂SO₄(l) $\to$ NaHSO₄(s) + HNO₃(g)

A system that does no work but which transfers heat to the surroundings has

Benzene is a starting material in the synthesis of nylon fibers and polystyrene (styrofoam) .Its specific heat capacity is 1.74 J/(g·K) .If 16.7 kJ of energy is absorbed by a 225-g sample of benzene at 20.0°C,what is its final temperature?

A mass of 1.250 g of benzoic acid (C₇H₆O₂)was completely combusted in a bomb calorimeter.If the heat capacity of the calorimeter was 10.134 kJ/K and the heat of combustion of benzoic acid is -3226 kJ/mol,calculate (to three decimal places)the temperature increase that should have occurred in the apparatus.

The standard state of a substance in aqueous solution is a 1 M solution.

Natural gas,or methane,is an important fuel.Combustion of one mole of methane releases 802.3 kilojoules of energy.How much energy does that represent in kilocalories?

A system receives 575 J of heat and delivers 425 J of work.Calculate the change in the internal energy, $\Delta$ E,of the system.

A system expands from a volume of 1.00 L to 2.00 L against a constant external pressure of 1.00 atm. The work (w) done by the system,in J,is

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.

Sand is converted to pure silicon in a three step process.The third step is SiCl₄(g) + 2Mg(s) $\to$ 2MgCl₂(s) + Si(s) $\Delta$ H = -625.6 kJ What is the enthalpy change when 25.0 mol of silicon tetrachloride is converted to elemental silicon?

A system contracts from an initial volume of 15.0 L to a final volume of 10.0 L under a constant external pressure of 0.800 atm.The value of w,in J,is

A system which undergoes an adiabatic change and has work done on it by the surroundings has

Consider the equation $\Delta$ E = q + w Explain fully the meaning of all three terms in the equation,and also the implied sign convention for q and w.

A common laboratory reaction is the neutralization of an acid with a base.When 50.0 mL of 0.500 M HCl at 25.0°C is added to 50.0 mL of 0.500 M NaOH at 25.0°C in a coffee cup calorimeter,the temperature of the mixture rises to 28.2°C.What is the heat of reaction per mole of acid? Assume the mixture has a specific heat capacity of 4.18 J/(g·K) and that the densities of the reactant solutions are both 1.00 g/mL.

The Starship Enterprise is caught in a time warp and Spock is forced to use the primitive techniques of the 20th century to determine the specific heat capacity of an unknown mineral.The 307-g sample was heated to 98.7°C and placed into a calorimeter containing 72.4 g of water at 23.6°C.The heat capacity of the calorimeter was 15.7 J/K.The final temperature in the calorimeter was 32.4°C.What is the specific heat capacity of the mineral?

Two solutions (the system) ,each of 25.0 mL volume and at 25.0°C,are mixed in a beaker.A reaction occurs between them,and the temperature rises to 35.0°C.After the products have equilibrated with the surroundings,the temperature is again 25.0°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°C) is correct?