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1.1 Measuring enthalpy changes

Syllabus
First assessment 2025
Topic
1.1
Level
HL

Learning objectives

1.1.1Energy transfer• Chemical reactions transfer energy between system and surroundings• Total energy conserved• Heat vs. temperature1.1.2Endothermic vs. exothermic• Endothermic: energy absorbed (T decreases)• Exothermic: energy released (T increases)1.1.3Stability and energy• Relative stability determines endo/exothermic nature• Energy profiles• Sketch and interpret labelled energy profiles for endothermic and exothermic reactions1.1.4Standard enthalpy change (ΔH⦵)• Heat transferred at constant pressure, standard conditions• Q = mcΔT• ΔH = −Q/n• Calculate enthalpy change from calorimetry data

Energy Transfer in Reactions

A chemical reaction transfers energy between the system and surroundings. Total energy is conserved. Heat is energy transferred because of a temperature difference; temperature describes the thermal state.

State which part is the system, which part is the surroundings, and the direction of energy transfer before interpreting a temperature change.

For an exothermic hand-warmer reaction, define the reacting chemicals as the system: energy leaves that system and enters your hand and the air as surroundings. A temperature rise is evidence about the surroundings, while heat names the energy crossing the boundary; neither quantity is 'stored temperature'.

Describing Energy Transfer

1 mark

Consider a reaction mixture that is in thermal equilibrium with the surroundings. When a reaction takes place, the temperature of the mixture decreases.

Which row correctly shows the changes in energy when the new thermal equilibrium is established?

Energy of system

Energy of surroundings

decreases

decreases

increases

increases

decreases

increases

increases

decreases

Endothermic and Exothermic

Type Energy direction Surroundings temperature
Endothermic Absorbed by the system Decreases
Exothermic Released by the system Increases
Products lie below reactants for an exothermic reaction and above them for an endothermic reaction, setting the sign of delta H.

Classify from the direction of energy transfer, then check that the observed temperature change of the surroundings agrees.

Connect three representations: exothermic means energy flows out of the system, the surroundings warm, and ΔH for the system is negative; endothermic gives the opposite pattern and positive ΔH. State the observed part before inferring the reaction type.

Classifying Energy Changes

1 mark

Which statement about an exothermic reaction is correct?

Stability and Energy Profiles

Lower relative energy corresponds to greater relative stability. In an exothermic reaction the products are lower in energy than the reactants; in an endothermic reaction the products are higher.

products are lower in enthalpy than reactants; Ea runs from the reactant level to the transition-state peak; ΔH points downward and is labelled negative; heat release is assigned to the surroundings.
products are higher in enthalpy than reactants; Ea runs from the reactant level to the transition-state peak; ΔH points upward and is labelled positive; heat absorption is assigned from the surroundings.

Label the horizontal axis reaction coordinate and the vertical axis potential energy. Then read reactant and product levels, identify the sign and direction of ΔH, and distinguish ΔH from activation energy.

On a profile, ΔH is the vertical difference between product and reactant levels, while activation energy rises from reactants to the peak. A catalyst lowers the peak by changing the pathway but leaves the two energy levels, ΔH and the relative stability of reactants and products unchanged.

An energy profile identifies an activation barrier but does not by itself determine an observed rate: temperature, particle concentrations/collision frequency and the available pathway also matter. Use the diagram to compare energetic barriers only when the profiles and conditions make that comparison valid.

Interpreting Energy Profiles

3 marks

The forward reaction is endothermic, uses iron(III) oxide as a catalyst, and takes place at 900 K .

Sketch the energy profile for the reaction, both with and without the catalyst, labelling ΔH\Delta H and the activation energies.

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

1 mark

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.