Question

the thermodynamics of ethanol
combustion reaction
a researcher is working to identify some of the thermodynamic properties of
ethanol. their intention is to use combustion data from calorimetry for their data
collection.

the formula for ethanol is \\(\ce{c2h5oh}\\).

what is the correct balanced chemical reaction for the combustion of ethanol?

• \\(\ce{2c(s,gr) + 1/2o2(g) + 3h2(g) -> c2h5oh(l)}
• \\(\ce{c2h5oh(l) + 3o2(g) -> 2co2(g) + 3h2o(g)}
• \\(\ce{co2(g) + h2o(g) -> c2h5oh(l) + o2(g)}
• \\(\ce{c2h5oh(l) + 7o2(g) -> 4co2(g) + 6h2o(g)}

Explanation:

To balance the combustion reaction of ethanol ($\ce{C_{2}H_{5}OH}$), we know that combustion reactions involve reacting with oxygen ($\ce{O_{2}}$) to produce carbon dioxide ($\ce{CO_{2}}$) and water ($\ce{H_{2}O}$).

• For the carbon atoms: Ethanol has 2 C atoms, so we need 2 $\ce{CO_{2}}$ molecules (since each $\ce{CO_{2}}$ has 1 C).
• For the hydrogen atoms: Ethanol has 6 H atoms (in $\ce{C_{2}H_{5}OH}$, $5 + 1 = 6$ H), so we need 3 $\ce{H_{2}O}$ molecules (since each $\ce{H_{2}O}$ has 2 H, $3\times2 = 6$ H).
• For the oxygen atoms: Let's calculate the number of O atoms on the product side. From 2 $\ce{CO_{2}}$: $2\times2 = 4$ O, from 3 $\ce{H_{2}O}$: $3\times1 = 3$ O, total O on product side: $4 + 3 = 7$ O. On the reactant side, ethanol has 1 O, so the number of $\ce{O_{2}}$ molecules needed: Let $x$ be the number of $\ce{O_{2}}$ molecules. The O from $\ce{O_{2}}$ is $2x$, and from ethanol is 1. So $2x + 1 = 7 \implies 2x = 6 \implies x = 3$. So the balanced reaction is $\ce{C_{2}H_{5}OH(l) + 3O_{2}(g)

ightarrow 2CO_{2}(g) + 3H_{2}O(g)}$.

Checking the other options:

• The first option is a formation reaction, not a combustion reaction.
• The third option is the reverse (decomposition or reverse of combustion), not a combustion reaction.
• The fourth option has incorrect balancing (ethanol has 2 C, but it produces 4 $\ce{CO_{2}}$, which is wrong).

Answer:

B. $\ce{C_{2}H_{5}OH(l) + 3O_{2}(g)
ightarrow 2CO_{2}(g) + 3H_{2}O(g)}$