Question

which reaction is endothermic?
\\(\ce{hcl + naoh \
ightarrow nacl + h_{2}o + 58\\,kj}\\)
\\(\ce{2na + cl_{2} \
ightarrow 2nacl + energy}\\)
\\(\ce{6co_{2} + 12h_{2}o + energy \
ightarrow c_{6}h_{12}o_{6} + 6o_{2} + 6h_{2}o}\\)
\\(\ce{2c_{2}h_{6} + 7o_{2} \
ightarrow 4co_{2} + 6h_{2}o + 2,502\\,kj}\\)

Explanation:

An endothermic reaction requires energy input (energy is a reactant), while exothermic reactions release energy (energy is a product).

1. The first reaction releases 58 kJ (energy as product, exothermic).
2. The second reaction releases energy (energy as product, exothermic).
3. The third reaction has energy as a reactant (energy is absorbed, endothermic; this is photosynthesis).
4. The fourth reaction releases 2,502 kJ (energy as product, exothermic).

Answer:

$\boldsymbol{6CO_2 + 12H_2O + energy
ightarrow C_6H_{12}O_6 + 6O_2 + 6H_2O}$