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Exam 5: Thermochemistry

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Question 41

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The highly exothermic thermite reaction, in which aluminum reduces iron(III) oxide to elemental iron, has been used by railroad repair crews to weld rails together. 2Al(s) + Fe₂O₃(s) → 2Fe(s) + Al₂O₃(s) ΔH°_rxn= -847.6 kJ/mol What mass of iron is formed if 725 kJ of heat are released?

A) 23.9 g
B) 47.8 g
C) 65.3 g
D) 95.5 g
E) 112 g

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Question 42

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What is the standard enthalpy of formation of liquid methanol, CH₃OH(l) ? C(graphite) + O₂(g) → CO₂(g) ΔH°_rxn = -393.5 kJ/mol H₂(g) + ½O₂→ H₂O(l) ΔH°_rxn = -285.8 kJ/mol CH₃OH(l) + 3/2O₂(g) → CO₂(g) + 2H₂O(l) ΔH°_rxn = -726.4 kJ/mol

A) -1691.5 kJ/mol
B) -238.7 kJ/mol
C) 1691.5 kJ/mol
D) 47.1 kJ/mol
E) -47.1 kJ/mol

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Question 43

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Which is not a state function?

Your favorite candy bar, Gummy Beakers, contains 1.2 × 10⁶ J of energy while your favorite soft drink, Bolt, contains 6.7 × 10⁵ J. If you eat two packs of Gummy Beakers a day and drink 3 cans of Bolt, what percent of your 2000 Calorie daily food intake is left for broccoli, beans, beef, etc.? (1 cal = 4.184 J)

A piece of copper metal is initially at 100.0°C. It is dropped into a coffee cup calorimeter containing 50.0 g of water at a temperature of 20.0°C. After stirring, the final temperature of both copper and water is 25.0°C. Assuming no heat losses, and that the specific heat (capacity) of water is 4.18 J/g·°C, what is the heat capacity of the copper in J/°C?

A(n) ________ ________ is a process where heat is released to the surroundings.

ΔH does not depend on the path of a reaction, but ΔU does.

If, as a pioneer, you wished to warm your room by taking an object heated on top of a pot-bellied stove to it, which of the following 15-pound objects, each heated to 100°C, would be the best choice? The specific heat capacity (in J/g·°C) for each substance is given in parentheses. Iron (0.450) , copper (0.387) , granite (0.79) , gold (0.129) , water (4.184) .

The sign of w when work is done on the system by the surroundings is ________.

A system delivers 222 J of heat to the surroundings while delivering 645 J of work. What is the change in the internal energy, ΔU, of the system?

A 1.00-g sample of octane (C₈H₁₈) is burned in a bomb calorimeter that has a heat capacity of 5.80 kJ/°C. The temperature of the calorimeter rises from 25.00^oC to 33.20^oC. What is ΔU per mole for the combustion of octane?

An endothermic reaction causes the surroundings to

Aluminum oxide can be reduced to aluminum metal using carbon, the other reaction product being carbon monoxide. What is the enthalpy change if 12.50 g of aluminum is produced by this method?

When 0.7521 g of benzoic acid was burned in a calorimeter containing 1000. g of water, a temperature increase of 3.60°C was observed. What is the heat capacity of the bomb calorimeter, excluding the water? The heat of combustion of benzoic acid is -26.42 kJ/g.

The statement, "Energy can neither be destroyed nor created," is the ________.

A piece of copper with a mass of 218 g has a heat capacity of 83.9 J/°C. What is the specific heat of copper?

The enthalpy of formation of a liquid is always larger than the enthalpy of formation of the gas of the same compound.

How much heat is released if 7.15 g CaO(s) is added to 152 g of H₂O(l) ? CaO(s) + H₂O(l) → Ca(OH) ₂(s) ΔH°_rxn = -64.8 kJ/mol

Ozone (O₃) in the atmosphere can be converted to oxygen gas by reaction with nitric oxide (NO) . (Nitrogen dioxide is also produced in the reaction.) What is the enthalpy change when 8.50 L of ozone at a pressure of 1.00 atm and 25°C reacts with 12.00 L of nitric oxide at the same initial pressure and temperature (R = 0.0821 L atm/mol K) ? [ΔH°_f(NO) = 90.4 kJ/mol; ΔH°_f(NO₂) = 33.85 kJ/mol; ΔH°_f(O₃) = 142.2 kJ/mol]

What law states that energy can be converted from one form to another, but cannot be created or destroyed?