chem study guide ch 5&6

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1

Which of the following processes are exothermic, and which are endothermic?

(a) a candle burns;

(b) rubbing alcohol feels cold on the skin;

(c) a supersaturated solution crystallizes, causing the temperature of the solution to rise

1-exothermic

2-endothermic

3-exothermic

2

Which of the following processes are exothermic, and which are endothermic?

(a) frost forms on a car window in the winter;

(b) water condenses on a glass of ice water on a humid summer afternoon;

(c) adding ammonium nitrate to water causes the temperature of the solution to decrease

1. exothermic

2.exothermic

3.endothermic

3

How much P–V work does a gas system do on its surroundings at a constant pressure of 1.00 atm if the volume of gas triples from 250.0 mL to 750.0 mL? Express your answer in L · atm and joules (J).

W = PΔV.ZF

=1atm(0.750-0.250)L

=0.5atmL

=0.5 atm Lx101300Pa/atm x M^3/1000L

=50.7 J

w = −0.500 L ∙ atm = −50.7 J

4

Calculate ΔE for

a.

the combustion of a gas that releases 210.0 kJ of heat to its surroundings and does 65.5 kJ of work on its surroundings.

b.

a chemical reaction that produces 90.7 kJ of heat but does no work on its surroundings.

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5

Filling Air Bags Automobile air bags produce nitrogen gas from the reaction:

2 NaN3(s) → 2 Na(s) + 3 N2(g)

a.

If 2.25 g of NaN3 reacts to fill an air bag, how much P–V work will the N2 do against an external pressure of 1.00 atm given that the density of nitrogen is 1.165 g/L at 20°C?

b.

If the process releases 2.34 kJ of heat, what is ΔE for the system?

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6

5.39 Metallic Hydrogen A solid with metallic properties is formed when hydrogen gas is compressed under extremely high pressures. Predict the sign of the enthalpy change for the following reaction:

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7

The same quantity of energy is added to 10.00 g pieces of gold, magnesium, and platinum, all initially at 25°C. The molar heat capacities of these three metals are 25.41 J/(mol ∙ °C), 24.79 J/(mol ∙ °C), and 25.95 J/(mol ∙ °C), respectively. Which piece of metal has the highest final temperature?

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8

Exactly 10.0 mL of water at 25.0°C is added to a hot iron skillet. All the water is converted into steam at 100.0°C. The mass of the pan is 1.20 kg and the molar heat capacity of iron is 25.19 J/(mol ∙ °C). What is the temperature change of the skillet?

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9

The complete combustion of 1.200 g of cinnamaldehyde (C9H8O, one of the compounds in cinnamon) in a bomb calorimeter (Ccalorimeter = 3.640 kJ/°C) produced an increase in temperature of 12.79°C. Calculate the molar enthalpy of combustion of cinnamaldehyde (ΔHcomb) in kilojoules per mole of cinnamaldehyde.

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10

Hormone Mimics Phthalates, used to make plastics flexible, are among the most abundant industrial contaminants in the environment. Several have been shown to act as hormone mimics in humans by activating the receptors for estrogen, a female sex hormone. In characterizing the compounds completely, the value of ΔHcomb for dimethyl phthalate (C10H10O4) was determined to be −4685 kJ/mol. Assume that 1.00 g of dimethyl phthalate is combusted in a calorimeter whose heat capacity (Ccalorimeter) is 7.854 kJ/°C at 20.215°C. What is the final temperature of the calorimeter?

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11

Adding 2.00 g of Mg metal to 95.0 mL of 1.00 M HCl in a coffee-cup calorimeter leads to a temperature increase of 9.2°C.

a.

Write a balanced net ionic equation for the reaction.

b.

If the molar heat capacity of 1.00 M HCl is the same as that for water [cP = 75.3 J/(mol ∙ °C)], what is ΔHrxn?

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12

How much glucose must be metabolized to completely evaporate 1.00 g of water at 37°C, given ΔHcomb,glucose = −2803 kJ/mol?

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13

5.76 Cleansing the Atmosphere The atmosphere contains the highly reactive molecule OH, which acts to remove selected pollutants. Use the values for ΔHrxn given below to find the ΔHrxn for the formation of OH and H from water.

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14

5.78 Ozone Layer The destruction of the ozone layer by chlorofluorocarbons (CFCs) can be described by the following reactions:

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15

What is ΔHrxn for the reaction between H2S and O2 that yields SO2 and water,

given ΔHrxn for the following reactions?

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16

For which of the following reactions does represent an enthalpy of formation?

a.

C(s) + O2(g) → CO2(g)

b.

CO2(g) + C(s) → 2 CO(g)

c.

CO2(g) + H2(g) → H2O(g) + CO(g)

d.

2 H2(g) + C(s) → CH4(g)

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17

Use the following standard heats of formation to calculate the molar enthalpy of vaporization of liquid hydrogen peroxide: of H2O2(ℓ) is −188 kJ/mol and of H2O2(g) is −136 kJ/mol.

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18

Ammonium nitrate decomposes to N2O and water vapor at temperatures between 250°C and 300°C. Write a balanced chemical reaction describing the decomposition of ammonium nitrate, and calculate the enthalpy of reaction by using the appropriate enthalpies of formation from Appendix 4.

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19

Improvised Explosives Mixtures of fertilizer (ammonium nitrate) and fuel oil (a mixture of long-chain hydrocarbons similar to decane, C10H22) are the basis for powerful explosions. Determine the enthalpy change of the following explosive reaction by using the appropriate enthalpies of formation :

3 NH4NO3(s) + C10H22(ℓ) + 14 O2(g) → 3 N2(g) + 17 H2O(g) + 10 CO2(g)

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20

5.93 Use the following data to calculate the enthalpy of formation of NO2Cl from N2, O2, and Cl2:

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21

Carbon tetrachloride (CCl4) was at one time used as a fire-extinguishing agent. It has a molar heat capacity cP of 131.3 J/(mol ∙ °C). How much energy is required to raise the temperature of 275 g of CCl4 from room temperature (22°C) to its boiling point (77°C)?

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22

Sodium may be used as a heat-storage material in some devices. The specific heat (cs) of sodium metal is 1.23 J/(g ∙ °C). How many moles of sodium metal are required to absorb 1.00 × 103 kJ of energy?

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23

Chlorofluorocarbons (CFCs) such as CF2Cl2 are refrigerants whose use has been phased out because of their destructive effect on Earth’s ozone layer. The standard enthalpy of vaporization of CF2Cl2 is 17.4 kJ/mol, compared with for liquid water. How many grams of liquid CF2Cl2 are needed to cool 200.0 g of water from 50.0°C to 40.0°C? The specific heat of water is 4.184 J/(g ∙ °C).

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24

Balance the following chemical equation, name the reactants and products, and calculate the standard enthalpy change by using the data in Appendix 4.

FeO(s) + O2(g) → Fe2O3(s)

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25

Metabolism of Methanol Methanol is toxic because it is metabolized in a two-step process in vivo to formic acid (HCOOH). Consider the following overall reaction under standard conditions:

O2(g) + 2 CH3OH(ℓ) → 2 HCOOH(ℓ) + 2 H2O(ℓ) + 1019.6 kJ

a.

Is this reaction endothermic or exothermic?

b.

What is the value of for this reaction?

c.

How much energy would be absorbed or released if 60.0 g of methanol were metabolized in this reaction?

d.

In the first step of metabolism, methanol is converted into formaldehyde (CH2O), which is then converted into formic acid. Would you expect for the metabolism of 1 mole of CH3OH(ℓ) to give 1 mole of formaldehyde to be larger or smaller than 509.8 kJ?

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26

5.125 Use Hess’s law and the following data to calculate the standard enthalpy of formation of CH4(g).

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27

5.127 Calculate for the reaction

from the following information:

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28

5.134 Odor of Urine Urine odor gets worse with time because urine contains the metabolic product urea [CO(NH2)2], a compound that is slowly converted to ammonia, which has a sharp, unpleasant odor, and carbon dioxide:

CO(NH2)2(aq) + H2O(ℓ) → CO2(aq) + 2 NH3(aq)

This reaction is much too slow for the enthalpy change to be measured directly by using a temperature change. Instead, the enthalpy change for the reaction may be calculated from the following data:

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29

5.137 At high temperatures, such as those in the combustion chambers of automobile engines, nitrogen and oxygen form nitrogen monoxide:

Any NO released into the environment is oxidized to NO2:

Is the overall reaction

N2(g) + 2 O2(g) → 2 NO2(g)

exothermic or endothermic? What is for this reaction

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30

Smelting Iron Iron metal is obtained by reducing iron oxide with carbon. The balanced chemical equation for making iron from Fe2O3 is:

2 Fe2O3(s) + 3 C(s) → 4 Fe(s) + 3 CO2(g)

Iron melts at 1538°C with ΔHfus = 19.4 kJ/mol. The molar heat capacity of iron is cP,Fe = 25.1 J/(mol ∙ °C). Is the energy required to melt recycled iron less than that needed to reduce the iron in Fe2O3 to the free metal?

...