Thermochemistry Unit 12 Page 1 (Use with Notes Page 1)

Directions: Determine whether the following scenarios best describe an endothermic, or exothermic process. 1. On a sunny winter day, the snow on a rooftop begins to melt. As the melted water drips from the roof, it refreezes into icicles. Describe the direction of the heat flow as the water freezes. Is this process endothermic or exothermic? In general: melting, vaporization, sublimation = endothermic freezing, condensation, deposition = exothermic The snow gains heat to melt (endothermic), then the water releases heat to freeze (exothermic). 2. A container of melted wax stands at room temperature. What is the direction of heat flow as the liquid wax solidifies? Is the process endothermic or exothermic? Heat leaves the molten wax (exothermic), heat goes to the surroundings 3. When barium hydroxide octahydrate, Ba(OH)2•8H2O is mixed in a beaker with ammonium thiocyanate, NH4SCN, a reaction occurs. The beaker becomes very cold. Is the reaction endothermic or exothermic? Beaker feels cold because the reaction is absorbing heat from the beaker. Reaction is endothermic. 4. Two substances in a glass beaker chemically react, and the beaker becomes too hot to touch. Is the reaction endothermic or exothermic? Beaker becomes hot because the reaction is pushing heat out from itself onto the beaker. Reaction is exothermic. Directions: Make the following conversions between the different units for heat energy. 5. 8.50x102 cal to kilocalories .850 kcal 6. 444 cal to joules 1860 J

7. 1.8kJ to joules 1800 J 8. 4.5x10-1kJ to calories 110 cal

Directions: Use your understanding of the specific heat formula to answer the following questions. 9. The temperature of a 95.4g piece of copper increases from 25.0°C to 48.0°C when the copper absorbs 849J of heat. What is the specific heat of copper? 10. When 435J of heat is added to 3.4g of olive oil at 21°C, the temperature increases to 85°C. What is the specific heat of the olive oil? 11. How much heat is required to raise the temperature of 250.0g of mercury 52°C if the specific heat of mercury is 0.14J/(g•°C)? 12. How much heat is required to raise the temperature of 400.0g of silver 45°C if the specific heat of silver is 0.24J/(g•°C)?

Thermochemistry Unit 12 Page 2 (Use with Notes Page 1-2)

1. A piece of metal of mass 20 g at 100°C is placed in a calorimeter containing 50.7 g of water at 22°C. The final temperature of the mixture is 25.7°C. What is the specific heat capacity of the metal? Assume that there is no energy lost to the surroundings.

2. When 25 g of a metal at 90°C is added to 50 g of water at 25°C, the temperature of the water rises to 29.8°C. What is the specific heat capacity of the metal? Assume no heat was lost to the surroundings.

Thermochemistry Unit 12 Page 3 (Use with Notes Page 3)

1. Use the following thermochemical equation to calculate the energy change (in kJ) that occurs when 2.25mol of NaHCO3(s) decomposes. Is the reaction endothermic or exothermic?

2NaHCO3(s) + 85 kJ Na2CO3(s) + H2O(l) + CO2(g)

2. When liquid ethanol, C2H6O(l), burns, it reacts with O2(g) to produce CO2(g) and H2O(l) and 1368 kJ of heat. Use

the thermochemical equation to calculate the energy change that occurs when 12.5 g of ethanol burns. Is the

reaction endothermic or exothermic?

C2H6O(l) + 3O2(g) 2CO2(g) + 3H2O(l) ∆H = –1368 kJ

3. The heat of combustion of propane (C3H8), is −2220 kJ/mol. Write the balanced thermochemical equation for the combustion of propane.

a. What mass of propane must be burned to release 5,550kJ of heat? b. Is the reaction endothermic or exothermic?

9. What is the change in enthalpy (ΔH) for the reaction A → B? 10. Is the reaction endothermic or exothermic?

11. What is the change in enthalpy (ΔH) for the reaction A → B?

12. Is the reaction endothermic or exothermic?

Thermochemistry Unit 12 Page 4 (Use with Notes Page 4)

1. Using the bond energies from the reference chart above, estimate ΔH for the following reaction.

2. Does the reaction illustrate an endothermic or exothermic process?

H2(g) + Cl2(g) → 2HCl(g) 3. Estimate the carbon-fluorine bond energy given the remaining bond energies provided in the reference chart and the information provided in the equation below.

4. Does the reaction illustrate an endothermic or exothermic process?

(g) + F2(g) (g) ΔH = -549kJ →

5. Using the bond energies from the reference chart above, estimate ΔH for the following reaction.

6. Does the reaction illustrate an endothermic or exothermic process?

N2(g) + 3H2(g) → 2NH3(g) 7. Using the bond energies from the reference chart above, calculate ΔH for the reaction of methane with chlorine and fluorine to give Freon-12 (CF2Cl2). (You must use the calculated C-F bond energy from question 3). 8. Does the reaction illustrate an endothermic or exothermic process?

CH4(g) + 2Cl2(g) + 2F2(g) → CF2Cl2(g) + 2HF(g) + 2HCl(g)

Thermochemistry Unit 12 Page 6 (Use with Notes Page 5)

1. Find the enthalpy change for the formation of lead(IV) chloride by the reaction of lead(II) chloride with chlorine.

PbCl2(s) + Cl2(g) PbCl4(l) ∆H = ? Use the following thermochemical equations:

Pb(s) + 2Cl2(g) PbCl4(l) ∆H = –329.2 kJ Pb(s) + Cl2(g) PbCl2(s) ∆H = –359.4 kJ

2. Use standard heats of formation (

H f) to calculate the change in enthalpy for the following reaction:

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) 3. Find the enthalpy change for the formation of pentane, C5H12, by the reaction of carbon with hydrogen.

5C(s) + 6H2(g) C5H12(g) Use the following thermochemical equations: C(s) + O2(g) CO2(g) ∆H = -393.5kJ/mol H2(g) + ½ O2(g) H2O(l) ∆H = -285.8kJ/mol C5H12(g) + 8O2(g) 5CO2(g) + 6H2O(l) ∆H = -3535.6kJ/mol 4. Calculate the heat of formation for sulfur dioxide, SO2, from its elements sulfur and oxygen. Use the balanced chemical equation and the following information.

S(s) +

3

2O2(g) SO3(g) ∆H = -395.2kJ/mol

2SO2(g) + O2(g) 2SO3(g) ∆H = -198.2kJ/mol

Directions: Use standard heats of formation (

H f) to calculate the change in enthalpy for the following reactions.

5. C(s) + O2(g) CO2(g) endothermic or exothermic? 6. CH4(g) + 2O2(g) CO2(g) + 2H2O(l) endothermic or exothermic? 7. CaCO3(s) CaO(s) + CO2(g) endothermic or exothermic? 8. H2O(g) H2O(l) endothermic or exothermic? 9. 3CO(g) + Fe2O3(s) 2Fe(s) + 3CO2(g) endothermic or exothermic?

Thermochemistry Unit 12 Page 8 (Use with Notes Page 6)

Directions: Show all work, and express answers with significant figures and proper labels (units and substance). 1. Calculate the molar enthalpy of solidification (ΔHsolidification) when 10.00kJ of energy are lost as 30.00g of water

are frozen at 0ºC. Remember that this value will be negative, because energy is lost when water freezes. (Hint: start by finding out how many moles of water that are freezing)

2. Calculate the molar enthalpy of condensation (ΔHcondensation) for ammonia when 50.0g of NH3 gas turn into a

liquid at its boiling point. 68,500J of energy are released in the process. 3. Calculate the energy absorbed when 2.0x103g of dry ice (CO2) sublimate at the normal sublimation point. The

molar enthalpy of sublimation of CO2 is 8.647kJ/mol. 4. Methane (CH4) has a normal boiling point of -161.6 ºC. At this temperature, the ΔHcondensation = -8.17kJ/mol. If

16.5g of liquid methane vaporize, how much energy is absorbed?

5. How much energy is required to melt a 20.0 lb bag of ice at 0ºC? A pound (lb.) of ice is equivalent to 0.4536 kg.

The ΔHfusion of ice is +6.009kJ/mol.

6. When water vaporizes at its normal boiling point, its ΔHvaporization = +40.79kJ/mol. Calculate the number of moles of water that condense if 3456kJ of energy are released.

7. What mass of aluminum metal would absorb 250.0 kJ when it melted at its melting point? The molar enthalpy

of fusion for aluminum is +10.71kJ/mol.

Directions: The heating curve shown above is a plot of temperature vs time. It represents the heating of substance X at a constant rate of heat transfer. Answer the following questions using this heating curve:

1. In what part of the curve would substance X have a definite shape and definite volume? 2. In what part of the curve would substance X have a definite volume but no definite shape? 3. What part of the curve represents a mixed solid/liquid phase of substance X? 4. What part of the curve represents a mixed liquid/vapor phase of substance X? 5. What is the melting temperature of substance X? 6. What is the boiling temperature of substance X?

7. In what part(s) of the curve would increasing kinetic energy be displayed? 8. In what part(s) of the curve would increasing potential energy be displayed? 9. In what part of the curve would the molecules of substance X be farthest apart? 10. In what part of the curve would the molecules of X have the lowest kinetic energy? 11. In what part of the curve would the molecules of X have the greatest kinetic energy?