7. Write out a balanced combustion reaction for methane (CH 4 ). Given that H rx

Question

7. Write out a balanced combustion reaction for methane (CH4). Given that Hrxn for this reaction is -890 kJ/mol, what is Hrxn for the reaction of 0.5 mol CO2 with sufficient water, producing methane and oxygen?

8. Given that the specific heat of water is 4.184 J/gK, how much heat (in Joules) does it take to raise the temperature of 500.0 mL of water by 50.0C (density of water is 1g/mL)?

9. 25.0 mL of 2.00M HCl and 25.0 mL of 1.00M NaOH are mixed in a coffee cup calorimeter. The temperature increases from 25.6C to 28.2C. What is the enthalpy of the reaction?

10. If a sample of food is burned in a bomb calorimeter with a heat capacity of 15.2kcal/C and the temperature increases from 28.3C to 35.5C, what is the caloric content of the food (report answer in FOOD CALORIES).

11. Calculate the Hrxn for 2 S(s) + 3 O2(g) 2 SO3(g), given the following:

S(s) + O2(g) SO2(g), Hrxn = -296.8 kJ/mol

2 SO3(g) 2 SO2(g) + O2(g), Hrxn = +198.4 kJ/mol

12. Write out a balanced reaction for formation of ethanol (C2H5OH(l)) from elements, assuming the most stable elemental form of carbon is C(graphite).

Explanation / Answer

7. CH4 + 2O2 --> CO2 + 2H2O ......dHrxn = -890 kJ/mol

So for CO2 it would become = 890 x 0.5 = 445 kJ

This is for the reverse reaction so sign of dH has changed

8. With density being 1 g/ml

mass of water = 500 g

q = mCpdT

   = 500 x 4.184 x 50 = 104.6 kJ

9. total volume = 25 + 25 = 50 ml

let the mass = 50 g [since density of water is 1 g/ml]

dH = q = mCpdT = 50 x 4.184 x (28.2 - 25.6)

= 543.92 J

10. The caloric content of food in this case would be = dH/dT

= 15.2/(35.5 - 28.3) = 2.11 kJ is the caloric content of food.

11. Multiply the first equation by a factor of 2

2S(s) + 2O2(g) ---> 2SO2(g) ........dHrxn = -593.6 kJ/mol

reverse the second reaction,

2SO2(g) + O2(g) ---> 2SO3 .........dHrxn = -198.4 kJ/mol

Add both,

2S(s) + 3O2(g) ----> 2SO3(g) ......dHrxn = -792 kJ/mol

12. The balanced required euqation would be,

3C(graphite) + 3H2O ----> C2H5OH + CO2

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