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1.1.4—Standard enthalpy change (ΔH⦵)

Syllabus
First assessment 2025
Objective
1.1.4
Level
HL

Standard Enthalpy from Calorimetry

Q=mcΔTandΔH=−Q/nQ = mcΔT and ΔH = −Q/n

At constant pressure, calculate heat transferred from mass, specific heat capacity, and temperature change, then divide by reacting moles and apply the sign convention. Check standard conditions and units.

Worked calculation: if 100.0 g of solution warms by 6.0 K and c=4.18 J g−1 K−1c=4.18\,\mathrm{J\,g^{-1}\,K^{-1}}, q(solution)=mcΔT=(100.0)(4.18)(6.0)=2.51×103 J=+2.51 kJq(\mathrm{solution})=mc\Delta T=(100.0)(4.18)(6.0)=2.51\times10^3\,\mathrm{J}=+2.51\,\mathrm{kJ}. Therefore q(reaction)=−2.51 kJq(\mathrm{reaction})=-2.51\,\mathrm{kJ}. If 0.0500 mol0.0500\,\mathrm{mol} of limiting reactant reacted, ΔH=−2.51/0.0500=−50.2 kJ mol−1\Delta H=-2.51/0.0500=-50.2\,\mathrm{kJ\,mol^{-1}}: the negative sign means the reaction released energy. Heat loss or ignored calorimeter heat capacity usually makes the measured magnitude too small.

A standard molar enthalpy change belongs to the balanced reaction as written, with substances in their stated standard states under standard conditions. A classroom calorimetry value is an experimental estimate: report its conditions and uncertainty separately from a data-book or theoretical standard value. Heat loss to the surroundings or ignored calorimeter heat capacity commonly makes the measured magnitude too small.

Calculating Enthalpy Change

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What is the standard enthalpy change, ΔHcombustion ⊖\Delta H_{\text {combustion }}^{\ominus}, according to the data?

Amount of fuel burned =0.110 mol=0.110 \mathrm{~mol}
Mass of water =200 g=200 \mathrm{~g}
Initial temperature of water =21.0∘C=21.0^{\circ} \mathrm{C}
Final temperature of water =25.0∘C=25.0^{\circ} \mathrm{C}
Specific heat capacity of water, cw=4.18 J g−1 K−1c_{\mathrm{w}}=4.18 \mathrm{~J} \mathrm{~g}^{-1} \mathrm{~K}^{-1}
Q=mcΔTQ=m c \Delta T

Measuring Enthalpy Summary

Retrieve the route: define system/surroundings transfer, classify endothermic or exothermic, read stability from energy profiles, then calculate Q and ΔH with the correct sign.

Check the energy direction, surroundings temperature, reactant/product levels, moles, units, and ΔH sign.