Question

1. choose the best answer.

which of the following reactions shows that heat has been added?
$\ce{h_{2}o_{2} ->pd h_{2} + o_{2}}$
$\ce{c + o_{2} <- co_{2}}$
$\ce{fe + 2s ->delta fes_{2}}$
$\ce{2na + cl_{2} -> 2nacl + energy}$
decomposition reactions
the opposite of a synthesis reaction is a decomposition reaction, a chemical reaction in which a single compound undergoes a chemical reaction that produces two or more simpler substances. the general equation for a decomposition reaction is as follows:
$\ce{ab -> a + b}$

Explanation:

Step1: Analyze each reaction

• For $\ce{H_{2}O_{2}->[Pd]H_{2} + O_{2}}$: Pd is a catalyst, no heat symbol, so no heat added.
• For $\ce{C + O_{2} <- CO_{2}}$: The reverse of combustion (exothermic), but the arrow direction and no heat symbol for addition.
• For $\ce{Fe + 2S ->[\Delta] FeS_{2}}$: The $\Delta$ symbol means heat is added (supplied) for the reaction.
• For $\ce{2Na + Cl_{2} -> 2NaCl + energy}$: Energy is released (exothermic), so heat is produced, not added.

Step2: Identify the correct reaction

The reaction with $\Delta$ (heat added) is $\ce{Fe + 2S ->[\Delta] FeS_{2}}$.

Answer:

$\boldsymbol{\ce{Fe + 2S ->[\Delta] FeS_{2}}}$ (the third option: $\boldsymbol{\ce{Fe + 2S \xrightarrow{\Delta} FeS_{2}}}$)