2. The coabustion of 1.00 mol of liquid ethanol (CH,CH,OH) at constant pressure

Question

2. The coabustion of 1.00 mol of liquid ethanol (CH,CH,OH) at constant pressure in excess oxygen rekases adkJ od beat per mole of.thanol. What is the ett halpy dang. (in ?) kr burning of 10.0g of ethaad? pay attention to the sign! (a)Write down the reaction of combustion of ethano (b) What bs the conaection between beat at coastant pressure and eathalpy? c) Is enthalpy positive or negative? d) How many moles are in 10.0g of ethanol e) What is the enthalpy for burning 10.0g of ethamol? 3 Caleulate the AH of the Sollowing reaction 2274 241.8,O CO 393.5 285.6 (a) Write down the formation reaction of CH and its enthalpy b) Write down the Sormation reaction of CO, and its enthalp (c) Write down the formation reaction of H,Oa and its enthalpy d) Write down the formation reaction of O, and its enthalpy (c) Use Hess's law to calculate the esthalpy of the reaction

Explanation / Answer

The combustion of ethanol (C2H5OH) is

C2H5OH+ 3O2 ----à2CO2+3H2O

At constant pressure,

Enthalpy , deltaH= CpdT, Cp is specific heat at constant pressure.

Here heat is released. Hence enthalpy change is –ve.

Moles = mass/molar mass

Molar mass of C2H5OH =24+6+16= 46 g/mole

Moles of ethanol= 10/46=0.22 moles

Heat released per moles of ethanol= 500Kj

0.22 moles releases 500*0.22=110Kj

10 gm releases 500Kj, which is also the enthalpy deltaH=-500Kj ( since heat is released)

2. The standard enthalpy change of formation of a compound is the enthalpy change which occurs when one mole of the compound is formed from its elements under standard conditions, and with everything in its standard state. Since oxygen is element existing as diatomic molecules, its equation for standard heat of formation cannot be written.

2C+ H2 -----àC2H2, deltaH= 227.4 Kj/mole (1)

Reversing this gives C2H2 -----à2C+H2,deltaH= -227.4 (1A)

Heat of formation reaction for CO2 is

C+ O2------àCO2, deltaH=-393.5 Kj/mole (2)

H2+0.5O2------àH2O, deltaH=-285.6 Kj/mole (3) is the heat of formation reaction for water.

For the reaction 2C2H2+5O2-------à4CO2+ 2H2O , (3)

Heat of reaction = sum of enthalpy change of products- sum of enthalpy change of products

Eq.1A*2+ 4*Eq.2+Eq.3*2 gives the required enthalpy change

2C2H2+5O2 ---------à 4CO2+2H2O

=2*(-227.4)*2+4*(-393.5)+ 2*(-285.6)=-2600 Kj

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