Chapter 5 Worksheet – Thermochemistry Calorimetry 1. A 27.825 g sample of nickel is heated to 99.85°C and placed in a coffee cup calorimeter

containing 150.0 g of water at 23.65°C. After the metal cools, the final temperature of metal and water is 25.15°C.

a. What released heat? _________________ What absorbed heat? _________________

b. Calculate the heat absorbed by the water. (Water’s specific heat is 4.184 J/g·°C.)

c. Calculate the specific heat of the metal assuming no heat is lost to the surroundings or the calorimeter—i.e., all the heat absorbed by the water had to be released by the metal.

2. When a solution consisting of 2.00 g of potassium hydroxide in 75.0 g of solution is added to

75.0 mL of 0.500M nitric acid at 24.9C in a calorimeter, the temperature of the resulting

solution increases to 28.0C. Assume the heat absorbed by the calorimeter is negligible.

HNO3(aq) + KOH(aq) H2O(l) + KNO3(aq)

a. What released heat? _________________ b. What absorbed heat? ______________ c. Calculate the amount of heat (in J) absorbed by the solution given the density of nitric acid is

1.03 g/mL. Assume the solution is sufficiently dilute that its specific heat is equal to water’s,

4.184 J/g·C.

d. Assuming the total amount of heat absorbed by the solution was released by the reaction,

calculate the enthalpy change (H) for the reaction in kJ/mol of H2O formed. 3. a. What is the calorimeter constant of a bomb calorimeter if burning 1.000 g of benzoic acid in it causes the temperature of the calorimeter to rise by 7.248 °C? The heat of combustion of benzoic acid is ∆Hcomb = -26.38 kJ/g. b. If 5.00 g of a mixture of hydrocarbons is burned in our bomb calorimeter and it causes the temperature to rise 6.76 °C, how much energy (in kJ) is released during combustion? Fuel Values and Food Values 4. Hostess Twinkies are one of the icons of American junk food, making them also among the most maligned, especially given their unnaturally long shelf life. But are Twinkies really so bad for you? a. Calculate the food value of a Twinkie (in Cal/g) if a 0.45 g Twinkie raises the temperature of a bomb calorimeter (Ccal=6.20 kJ/°C) by 1.06°C. (1 Cal = 4.184 kJ) b. If a typical Twinkie has a mass of 43 g, calculate the number of nutritional calories (Cal) in one Twinkie.

c. If a 12 fl. oz. can of Coke contains 140 Cal and a 16 oz. Starbucks Grande (latte with 2% milk contains 190 Cal, is the Twinkie really so much worse than these drinks based on calorie content? ENTHALPIES OF FORMATION AND ENTHALPIES OF REACTION

5. Calculate the mass of hydrogen that must burn in oxygen to produce 50.0 kJ of heat. 6. Consider the following thermochemical equation:

4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g) H = –904 kJ

a. Calculate the heat (in kJ) released when 50.0 g of ammonia react with excess oxygen.

b. Calculate the mass of steam produced when 675 kJ of heat are released.

7. Calculate the fuel value (as a positive value in kJ/g) given the thermochemical equations for the

combustion of methane and propane below:

a. CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) H = –803.3 kJ

b. C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g) H = –2043.9 kJ

8. If the fuel value for butane (C4H10) is 45.75 kJ/g, calculate the heat of combustion (H) in kJ per mole of butane.

Hess’s Law

9. Calculate the enthalpy change, H, for the following reaction,

Cgraphite(s) + 2 H2(g) CH4(g)

given that methane can be produced from the following series of steps:

(a) Cgraphite(s) + O2(g) CO2(g) H = –393.5 kJ

(b) 2 H2(g) + O2(g) 2 H2O(l) H = –571.6 kJ

(c) CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) H = –890.4 kJ

10. Rearrange the following data:

(a) Cgraphite(s) + O2(g) CO2(g) H = –393.5 kJ

(b) 2 CO(g) + O2(g) 2 CO2(g) H = 566.0 kJ

to calculate the enthalpy change for the reaction:

2 Cgraphite(s) + O2(g) 2 CO(g)

11. From the following data:

(a) N2(g) + 3 H2(g) 2 NH3(g) H = –92.6 kJ

(b) N2(g) + 2 O2(g) 2 NO2(g) H = 67.70 kJ

(c) 2 H2(g) + O2(g) 2 H2O(l) H = –571.6 kJ

Calculate the enthalpy change for the reaction:

4 NH3(g) + 7 O2(g) 4 NO2(g) + 6 H2O(l)

Enthalpies of Formation and Reaction

12. Indicate which of the following H˚ is a standard enthalpy of formation, Hf, and for

those that are not, write the equation for the correct reaction:

a. N2(g) + 3 H2(g) 2 NH3(g)

b. Cgraphite(s) + 2 H2(g) CH4(g)

c. Ca(s) + Br2(g) CaBr2(s)

13. When 50.0 g of nitrogen react with excess hydrogen to form ammonia gas, 164.5 kJ of heat

are liberated (released) at standard state conditions. Calculate the standard enthalpy of formation (in kJ/mol) for ammonia gas. (Hint: Use the balanced chemical equation in 1a.)

Table of Standard Enthalpies of Formation (Hf) at 25˚C

Substance

Hf (kJ/mol) Substance

Hf (kJ/mol) Substance

Hf (kJ/mol)

H2O(l) -285.8 CO(g) -110.5 NH3 -45.9

H2O(g) -241.8 CO2(g) -393.5 CH4 -74.9

CH3OH(l) -238.7 C2H5OH(l) -277.0

14. Use the table above to calculate H° for the reaction below:

Cgraphite(s) + O2(g) CO2(g)

H° = (1mol) Hf (CO2, g) – [(1mol) Hf

(Cgraphite, s) + (1mol) Hf

(O2, g)]

H° =

products the making with

associatedenergy Total –

reactants the making with

associatedenergy Total

15. Calculate the enthalpy change (H°) for the reaction below:

2 CH3OH(l) + 5 O2(g) 2 CO2(g) + 4 H2O(g)

products the making with

associatedenergy Total=

–

reactants the making with

associatedenergy Total=

16. Consider the reaction for the combustion of glucose:

C6H12O6(s) + 6 O2(g) 6 CO2(g) + 6 H2O(g) H°= –2537 kJ

Use the standard enthalpies of formation provided to calculate the enthalpy of formation for glucose.

17. Consider the following thermochemical equation:

4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g) H°= –904 kJ

Use the standard enthalpies of formation provided to calculate the enthalpy of formation for NO.