Question

consider the reaction of ethanol $c_2h_5oh$ and $o_2$ shown in the accompanying diagram
part a
write a balanced equation for the combustion reaction that occurs between ethanol and oxygen. express your answer as a chemical equation.
a chemical reaction does not occur for this question.
part b
which reactant is the limiting reactant?
$c_2h_5oh$
$o_2$

Explanation:

Step1: Write unbalanced combustion equation

$\text{C}_2\text{H}_5\text{OH} + \text{O}_2
ightarrow \text{CO}_2 + \text{H}_2\text{O}$

Step2: Balance C atoms first

There are 2 C on left, so add 2 to $\text{CO}_2$:
$\text{C}_2\text{H}_5\text{OH} + \text{O}_2
ightarrow 2\text{CO}_2 + \text{H}_2\text{O}$

Step3: Balance H atoms

There are 6 H on left, so add 3 to $\text{H}_2\text{O}$:
$\text{C}_2\text{H}_5\text{OH} + \text{O}_2
ightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$

Step4: Balance O atoms

Right side has $2\times2 + 3\times1 = 7$ O. Left side has 1 O in $\text{C}_2\text{H}_5\text{OH}$, so $\text{O}_2$ needs to provide 6 O: add 3 to $\text{O}_2$:
$\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2
ightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$

Step5: Identify limiting reactant

From the diagram: 2 $\text{C}_2\text{H}_5\text{OH}$ molecules, 8 $\text{O}_2$ molecules.
From balanced equation, 1 $\text{C}_2\text{H}_5\text{OH}$ reacts with 3 $\text{O}_2$.
For 2 $\text{C}_2\text{H}_5\text{OH}$, required $\text{O}_2 = 2\times3 = 6$. We have 8, so $\text{C}_2\text{H}_5\text{OH}$ is used up first.

Answer:

Part A:

$\text{C}_2\text{H}_5\text{OH}(l) + 3\text{O}_2(g)
ightarrow 2\text{CO}_2(g) + 3\text{H}_2\text{O}(l)$

Part B:

$\text{C}_2\text{H}_5\text{OH}$