5) A sample of iron absorbs 81.0 J of heat, upon which the temperature of the sa

Question

5) A sample of iron absorbs 81.0 J of heat, upon which the temperature of the sample increases from 17.9 °C to 32.5 °C. If the specific heat capacity of iron is 0.450 J/gK, what is the mass (in grams) of the sample?

6) Consider the following reaction. (Molar mass of CH3OH = 32.04186 g/mol) 2 CH3OH(g) 2 CH4(g) + O2(g) H = +252 kJ

a) Is this reaction exothermic or endothermic?

b) Calculate the amount of heat transferred when 24.0g of CH3OH(g) is decomposed by this reaction at constant pressure.

c) For a given sample of CH3OH(g), the enthalpy change during the reaction is 82.1 kJ. How many grams of methane gas are produced?

d) How many kilojoules of heat are released when 35.8 g of CH4(g) reacts completely with O2(g) to form CH3OH(g) at constant pressure?

7) A 1.800 g sample of phenol (C6H5OH) was burned in a bomb calorimeter whose total heat capacity is 11.66 kJ/°C. The temperature of the calorimeter plus contents increased from 22.45 to 28.27°C.

Write a balanced chemical equation for the bomb calorimeter reaction.

b) What is the heat of combustion per gram of phenol?

c) What is the heat of combustion per mole of phenol?

8. Given the data.

N2(g) + O2(g) 2 NO(g) + O2(g)

2 NO(g) 2 NO2(g)

H = +180.7 kJ

H = 113.1 kJ H = 163.2 kJ

2 N2O(g) 2 N2(g) + O2(g)
Use Hess’s Law to calculate H for the reaction

N2O(g) + NO2(g) 3 NO(g) , Show your work.

9. Calcium carbide (CaC2) reacts with water to form acetylene (C2H2) and Ca(OH)2. From the following enthalpy of reaction data and data in Appendix C in textbook, calculate Hf° for CaC2(s):

CaC2(s) + 2 H2O(l) Ca(OH)2(s) + C2H2 (g) H = 127.2kJ

10. Using average bond enthalpies, predict which of the following reactions will be most exothermic:

a) C(g) + 2 F2(g)CF4(g)

b) CO(g) +3 F2(g) CF4(g) + OF2(g)

c) CO2(g) + 4 F2(g) CF4(g) + 2 OF2(g)

11. Ammonia is produced directly from nitrogen and hydrogen. The chemical reaction is N2(g) + 3 H2(g) 2 NH3(g)

NN

a) Use Table 8.4 p 326 in textbook (or you can find the table in the chapter 5 slides on the UR course), to estimate the enthalpy change using the bond enthalpy for the reaction. Is it exothermic or endothermic?

b) Calculate the standard enthalpy change for this reaction, using standard enthalpies of formation Hf°.

Explanation / Answer

5. q = mCpdT

81 = g x 0.450 x (32.5-17.9)

mass of Fe = 12.33 g

6. a) The reaction is exothermic delta H +ve

b) moles CH3OH = 24/32.04186 = 0.75 mols

totla heat transferred = 252 x 0.75 = 188.75 kJ/mol

c) Given 252 kJ heat for 2 moles of CH4 production,

so for 82.1 it would be = 0.65 mols of CH4 produced

grams of CH4 produced = 0.65 x 16 = 10.4 g

d) moles of CH4 = 35.8/16 = 2.24 mols

heat released = - 252 x 2.24/2 = -282.24 kJ heat released

7. q = deltaH = 1.8 x 1166 x 1000 x (28.27 - 22.45) = 122.15 kJ

b) per gram heat = 122.15/1.8 = 67.86 kJ/g

c) per mole heat = 67.86 x 94.11 = 6386.41 kJ/mol

8. the way data is put here is very confusing

9. From the given data and standard data,

-127.2 = (-561 + 226.8) - (CaC2 + 2 x -285.8)

dHf = CaC2 = -364.6 kJ

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