Question

1 multiple choice

1. select the best answer.

(1) (5 points) heat is best defined as ...
o a substance that increases the temperature and causes water to boil.
o a form of potential energy.
o energy transferred due to moving things.
o the total energy that a substance has.
o energy transferred as the result of a temperature difference.
(ii) (5 points) a student dissolving some ammonium nitrate in water notices that the
beaker gets colder as the solid dissolves (assume the ammonium nitrate is the sys-
tem). this is an example of
o an exothermic process. o an endothermic process.
o a combustion reaction. o a thermodynamic cycle.
(iii) (5 points) what is the change in internal energy (δe) when a system absorbs 35
kj of heat and does 15 kj of work?
o 50 kj o 20 kj o -20 kj o -50 kj o 35 k
(iv) (10 points) what is the volume of a gas that exerts a pressure of 457 mmhg if
it exerted a pressure of 2.50 atm when its volume was 25.0 ml? o 9.62 ml
o 1.80 l o 0.104 l o 6.01 ml o 25.0 l
(v) (10 points) a sample of gas at 4.0 atm and 25.0 ml is heated from 25°c to 40°c.
if the pressure remains constant, what is the final volume of the gas? o 26.2 ml
o 40.0 ml o 23.8 ml o 105 ml o 25.0 ml
page 3

Explanation:

(I)

Heat is defined as energy transferred as the result of a temperature difference.

• A substance that increases temperature is not the definition of heat.
• Heat is not potential energy.
• Energy transferred due to moving things is work (related to mechanical energy transfer, not heat).
• The total energy a substance has is internal energy, not heat.

(II)

When ammonium nitrate dissolves and the beaker gets colder (system is ammonium nitrate), the system (ammonium nitrate) is absorbing heat from the surroundings (beaker and water). An endothermic process is one where the system absorbs heat.

• An exothermic process would release heat (beaker would get warmer).
• A combustion reaction is a specific type of exothermic reaction (usually rapid oxidation).
• A thermodynamic cycle is a sequence of thermodynamic processes.

(III)

The first law of thermodynamics is \(\Delta E=q + w\). Here, \(q = 35\space kJ\) (heat absorbed by the system, so positive) and \(w=- 15\space kJ\) (work done by the system, so negative).

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(IV)

We use Boyle's law \(P_1V_1 = P_2V_2\). First, convert \(P_1 = 2.50\space atm\) to \(mmHg\): \(P_1=2.50\times760 = 1900\space mmHg\), \(V_1 = 25.0\space mL\), \(P_2 = 457\space mmHg\).

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(V)

We use Charles's law \(\frac{V_1}{T_1}=\frac{V_2}{T_2}\). \(T_1=(25 + 273)K=298K\), \(V_1 = 25.0\space mL\), \(T_2=(40 + 273)K = 313K\).

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Answer:

(I) energy transferred as the result of a temperature difference.
(II) an endothermic process.
(III) \(20\space kJ\)
(IV) \(0.104\space L\)
(V) \(26.2\space mL\)