Question

Ethanol is added to gasoline because the oxygen it contains improves gasoline's burning efficiency. Its combustion reaction is given below. CH3CH2OH(l) 3O2(g) → 2CO2(g) 3H2O(g) ∆H
A) Is this an exothermic or endothermic reaction? B) If 15.3 grams of pure ethanol is completely converted to products, calculate how much heat is absorbed or released, in (1) kJ and in (2) kcal. C) If 42.7 g of water vapor is produced, how much heat, in kl, is absorbed or released?

Answer 1

Answer:

A) Exothermic

B) (1) -410.5 kJ  (2) -98.1 kcal

C) -976.44 kJ

Explanation:

A) By the value of ΔH (-1236 kJ), we can see that it's negative, it means that the heat is being lost by the reaction, and, because of that, the reaction is exothermic.

B) The molar mass of ethanol is 46.07 g/mol, so, at 15.3 grams, the number of moles is:

n = mass/molar mass

n = 15.3/46.07

n = 0.3321 mol

By the equation

1 mol of ethanol           ----------------------- -1236 kJ

0.3321 mol of ethanol ----------------------- x

By a simple direct three rule:

x = -410.5 kJ

1 kJ --------------- 0.2390 kcal

-410 kJ ---------- y

y = -98.10 kcal

C) The molar mass of water is 18 g/mol, so the number of moles at 42.7 g is:

n = 42.7/18

n = 2.37 moles

By the equation

3 moles of water     ----------------------- -1236 kJ

2.37 moles of water --------------------- x

By a simple direct three rule:

3x = -2929.32

x = -976.44 kJ