Name: (Last, First) 3. (21 Points) Chemical Formulas and Thermochemistry A (6Poi

Question

Name: (Last, First) 3. (21 Points) Chemical Formulas and Thermochemistry A (6Points) Dimethylhydrazine is often used as a rocket fuel. It is an organic acid made only of the elements carbon, hydrogen and nitrogen. Determine the smpirical formula for dimethylhydrazine if it is 39 97% carbon and 13.41% hydrogen by mass. 12 3 33 C H? B. (8 Points )Chemists are currently studying how to use formic acid as an alternative fuel to operate small portable electronics such as cell phones. The Lewis structure for formic acid appears below. Calculate the total energy change in kJ when 7.25 grams of formic acid under rgoes combustion, given the following standard enthalpy of formation data. AHP CO (g)- -393,5 kJ/mol AH H:0(0) -285.5 kJ/mol formic ackd t- aG1_22C0wt2 ) - )-5 04 420@ Combustion Equation: 2H COc Har ( 2Y-285.5)( 393.s -57/ -1353 465 (7 Points) From the bond energy data given below, write a balanced equation for the FORMATION of formic acid from its elements and calculate its reaction energy in k. C-H 415 kJ/mol C. O-H 460 kJ/mol 0-0 490 kJ/mol H-H 436 kJ/mol C-O C-C C-O 360 kJ/mol 800 kJ/mol 799 kJ/mol C-C 345 kJ/mol Czthata,A- ????? Formation Equation of Formic Acid:

Explanation / Answer

Ans 3

Part a

Moles of C = mass/molecular weight

= 39.97/12.011 = 3.327

Moles of H = mass/molecular weight

= 13.41/1.008 = 13.304

Moles of N = mass/molecular weight

= (100-39.97-13.41)/14.01 = 3.327

3.327 is the lowest so divide all moles by 3.327

Moles of C = 3.327/3.327 = 1

Moles of H = 13.304/3.327 = 4

Moles of N = 3.327/3.327 = 1

Empirical formula = CH4N

Part b

2HCOOH + O2 = 2CO2 + 2H2O

Heat of combustion = sum of heat of formation of products - sum of heat of formation of reactants

= 2*Hf(H2O) + 2*Hf(CO2) - Hf(O2) - 2*Hf(HCOOH)

= 2*(285.5) + 2*(-393.5) - 0 - 2*(-425.8)

= 635.6 kJ/mol

Moles of formic acid = mass/molecular weight

= 7.25g / 46g/mol

= 0.1576 mol

Total energy change = 635.6 kJ/mol x 0.1576 mol

= 100.17 kJ

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