Question

Write and balance the combustion reaction for ethanol, C2H5OH(l), under standard thermochemical conditions (25'C). Use Hess' law and standard heats of formation to find deltaHrxn. Remember, this combustion is a reaction with oxygen to produce carbon dioxide and liquid water.

Given 2.72g of ethanol and 5.06g of oxygen gas are allowed to react, determine 1) The mass in grams of each reactant/product after complete reaction. Use the rxn table method to find the limiting reactant and mass of products. 2)The heat released during this combustion 3)If the heat above is absorbed by a styrofoam calorimeter containing 250.0g of water a 25.0'C, what will be the final temperature of the water? Assume the styrofoam calorimeter has a heat capacity of 20.0 J/C

Answer 1

Ans 1) First part

The balanced chemical equation for combustion of ethanol is

C₂H₅OH + 3O₂ -> 2CO₂ + 3H₂O

Using Hess law ,the above reaction is formed in three steps

a) C+O₂ ->CO₂                = -393.5

b) H₂ + 1/2O₂ -> H₂O        = -286

c) 2C+3H₂ + 1/2O₂ -> C₂H₅OH   = -278

Now we know

              = [2(-393.5)+3(-286)]-[(-278)+0]

                = -1367kj/mol

Ans1) The reaction will be

C₂H₅OH + 3O₂ -> 2CO₂ + 3H₂O

Now we will calculate the mass of CO₂ formed from the mass of 2.72g ethanol and 5.06g of oxygen

Now from ethanol we get

2.72g ethanol x 1mol ethanol/46.06g ethanol x 2mol CO₂ /1mol ethanol x44g CO₂ /1molCO₂

= 5.19g CO₂

Similarly from oxygen we get

5.06g O₂ x3molO₂/32gO₂ x 2molCO₂ /1molO₂ x 44gCO₂ /1molCO₂

=41.748g CO₂

Now we know the reactant that produces a smaller amount is the limiting reagent

Therefore C₂H₅OH is the limiting reagent

Ans 2) The heat released during the combustion process will

               = [2(-393.5)+3(-282)]-[(-278)+0]

               = -1367kj/mol

Ans 3) Here given mass = 250g

Here T₁ =25⁰C

Therefore

c=20J/c

q=-1367Kj/mol = 1367000

Now

T₁ -25        = 1367000/250x20

T₁-25 = 273.4

T₁ =298.4⁰C