5.5: Enthalpy Changes of Chemcial Reactions

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Endothermic or Exothermic Reactions

Exercise \(\PageIndex{1}\)

Decide whether each of these reactions is exothermic or endothermic:

When two chemicals mix their temperature rises
A solid burns brightly and releases heat, light and sound
When two chemicals are mixed their temperatures drops
Two chemicals will only react if you heat them continually
Plants take in light energy for photosynthesis

Answer

exothermic
exothermic
endothermic
endothermic
endothermic

Enthalpy Change per Mole

Exercise \(\PageIndex{2}\)

What quantity, in moles, of hydrogen is consumed when 185.6 kJ of energy is evolved from the combustion of a mixture of H₂(g) and O₂(g)?

H₂(g) + O₂(g) → H₂O(l); Δ_rH° = –285.8 kJ/mol-rxn

Answer

0.6494 moles

\(\frac{185.6 kJ}{285.8 kJ/mol} = 0.649405 moles\)

Exercise \(\PageIndex{3}\)

The following reaction of iron oxide with aluminum is an exothermic reaction.

Fe₂O₃(s) + 2 Al(s) → 2 Fe(s) + Al₂O₃(s)

The reaction of 7.50 g of Fe₂O₃ with excess Al(s) evolves 31.5 kJ of energy in the form of heat. Calculate the enthalpy change per mole of Fe₂O₃.

Answer: -6.71*10² kJ*mol^-1 (heat evolved, negative sign)

Exercise \(\PageIndex{4}\)

How much heat is liberated at constant pressure if 0.675 g of calcium carbonate reacts with 45.1 mL of 0.498 M hydrochloric acid?

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g); Δ_rH° = –15.2 kJ/mol-rxn

Answer: ΔH = -0.103 kJ