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(a) Energetics

Syllabus
2024
Topic
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Level
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Learning objectives

3.1Exothermic and endothermic changesKnow that chemical reactions in which heat energy is given out are described as exothermic, and those in which heat energy is taken in are described as endothermic3.2Temperature change practicalDescribe simple calorimetry experiments for reactions such as combustion, displacement, dissolving and neutralisation3.3Energy level diagramsCalculate the heat energy change from a measured temperature change using the expression Q = mcΔT3.4Bond energy calculationsCalculate the molar enthalpy change (ΔH) from the heat energy change, Q35C And explain energy level diagrams to representDraw and explain energy level diagrams to represent exothermic and endothermic reactions36C Bond-breaking is an endothermic process andKnow that bond-breaking is an endothermic process and that bond-making is an exothermic process37C Bond energies to calculate the enthalpy changeUse bond energies to calculate the enthalpy change during a chemical reaction3.8Calorimetry practicalPractical: investigate temperature changes accompanying some of the following types of change:• salts dissolving in water• neutralisation reactions• displacement reactions• combustion reactions.

Distinguish exothermic from endothermic change

Exothermic reactions transfer heat energy from the reacting chemicals to the surroundings; endothermic reactions take in heat energy from the surroundings. The direction of transfer defines the term.

Reaction type Heat-energy transfer Typical measured effect on the surroundings Sign of ΔH\Delta H
exothermic reaction →\rightarrow surroundings temperature rises negative
endothermic surroundings →\rightarrow reaction temperature falls positive

Combustion and many neutralisation reactions are exothermic. Dissolving ammonium nitrate in water is endothermic: the solution cools because energy is taken from its surroundings.

A cold mixture has not 'released cold'. It has absorbed heat from the surroundings. Temperature change is evidence of the transfer, while exothermic and endothermic describe the reaction's energy direction.

Measure temperature change by simple calorimetry

Simple calorimetry estimates the heat transferred by measuring the temperature change of water or a solution. Insulation, stirring and prompt temperature readings make the measured change closer to the true change.

Change Core method Measurement
combustion place a known mass of water in a metal can; record its initial temperature; burn a measured mass of fuel beneath it; stir and record the highest temperature water mass, temperature rise and fuel mass burned
displacement, dissolving or neutralisation put measured reactant in an insulated polystyrene cup with a lid; record initial temperature; add the other reactant, replace lid, stir and record the highest or lowest temperature total solution mass and temperature change

Use the same reactant amounts and concentrations when comparing reactions. Keep the flame-to-can distance constant for combustion, start solutions at the same temperature, repeat trials and calculate a mean after investigating any anomaly.

No school calorimeter is perfectly insulated. Heat loss, heating the apparatus, incomplete combustion and fuel evaporation usually make an exothermic temperature rise—and the calculated energy magnitude—too small.

Calculate heat energy with Q = mcΔT

The heat energy transferred to or from a substance depends on its mass, specific heat capacity and temperature change.

Q=mc\Delta T

Symbol Meaning Common unit
QQ heat energy change J
mm mass of water or solution g
cc specific heat capacity J g−1 ∘C−1\mathrm{J\,g^{-1}\,^{\circ}C^{-1}}
ΔT\Delta T final temperature minus initial temperature; use the magnitude when finding energy transferred ∘C^{\circ}\mathrm C

For 25.0 g25.0\,\mathrm g of solution with c=4.18 J g−1 ∘C−1c=4.18\,\mathrm{J\,g^{-1}\,^{\circ}C^{-1}} warming from 19.019.0 to 31.5 ∘C31.5\,^{\circ}\mathrm C, ΔT=12.5 ∘C\Delta T=12.5\,^{\circ}\mathrm C and Q=25.0×4.18×12.5=1306 J≈1.31 kJQ=25.0\times4.18\times12.5=1306\,\mathrm J\approx1.31\,\mathrm{kJ}.

Match units before substituting: QQ is in joules when cc is in joules per gram per degree Celsius. For dilute aqueous solutions, volume in cm3\mathrm{cm^3} may be converted to mass using the supplied density; do not assume that conversion when no density is given.

Convert heat energy into molar enthalpy change

Molar enthalpy change, ΔH\Delta H, is the heat-energy change for one mole of the reaction quantity specified. Divide the measured energy by the relevant amount in moles, convert joules to kilojoules and attach the sign from the reaction direction.

\Delta H=\frac{Q}{n}

Stage Action
1 calculate QQ and convert J to kJ
2 calculate the reacting amount nn in mol; use n=m/Mrn=m/M_r when needed
3 divide the energy magnitude by nn
4 use −- for exothermic or ++ for endothermic and report kJ mol−1\mathrm{kJ\,mol^{-1}}

If an exothermic displacement reaction releases 1.30 kJ1.30\,\mathrm{kJ} while 0.0125 mol0.0125\,\mathrm{mol} reacts, ∣ΔH∣=1.30/0.0125=104 kJ mol−1|\Delta H|=1.30/0.0125=104\,\mathrm{kJ\,mol^{-1}}, so ΔH=−104 kJ mol−1\Delta H=-104\,\mathrm{kJ\,mol^{-1}}.

Do not divide joules by moles and label the result kJ mol−1\mathrm{kJ\,mol^{-1}}. The mole quantity must match the reaction basis named in the question, and the sign is not supplied by the division alone.

Draw and explain energy level diagrams

An energy level diagram compares the total energy of reactants and products. Their vertical separation is the enthalpy change, ΔH\Delta H; lower products mean energy has been transferred out, while higher products mean energy has been taken in.

Reaction Relative levels ΔH\Delta H arrow and sign Explanation
exothermic products below reactants downward; negative products store less energy, so the difference is released
endothermic products above reactants upward; positive products store more energy, so the difference is absorbed

Draw a vertical energy axis, then horizontal labelled lines for the balanced reactants and products at the correct relative heights. Draw a vertical arrow from the reactant level to the product level and label it ΔH\Delta H with its value and sign when supplied.

This objective needs energy levels, not a reaction-profile hump. Activation energy and profile curves belong to the later rates Topic. The ΔH\Delta H arrow follows reactants to products; reversing it reverses the sign.

Track energy through bond breaking and making

Breaking a covalent bond requires energy and is endothermic. Making a covalent bond releases energy and is exothermic because the bonded particles move to a lower-energy arrangement.

Overall reaction Comparison of the two energy totals
exothermic energy released making product bonds is greater than energy taken in breaking reactant bonds
endothermic energy taken in breaking reactant bonds is greater than energy released making product bonds

Every reaction normally includes both processes: reactant bonds are broken and product bonds are made. The overall energy change depends on the difference between the totals, not on only one bond.

Bonds do not release energy when they break. Statements such as 'the reaction is exothermic because bonds are broken' reverse the energy direction; an exothermic result requires bond making to release the larger amount.

Calculate ΔH from bond energies

Bond energy is the energy needed to break one mole of a specified gaseous covalent bond. Count every bond in the balanced reaction, total the bonds broken and made, then subtract in the correct order.

\Delta H=\sum E(\text{bonds broken})-\sum E(\text{bonds made})

For CX2HX4+HX2→CX2HX6\ce{C2H4 + H2 -> C2H6}, unchanged C–H bonds cancel. Using E(C=C)=612E(\ce{C=C})=612, E(H−H)=436E(\ce{H-H})=436, E(C−C)=348E(\ce{C-C})=348 and E(C−H)=412 kJ mol−1E(\ce{C-H})=412\,\mathrm{kJ\,mol^{-1}}: broken =612+436=1048=612+436=1048; made =348+2(412)=1172=348+2(412)=1172; therefore ΔH=1048−1172=−124 kJ mol−1\Delta H=1048-1172=-124\,\mathrm{kJ\,mol^{-1}}.

Count bonds from displayed or structural formulae and include coefficients. A negative result is exothermic because making the product bonds releases more energy; do not reverse the subtraction or count bonds that remain unchanged on only one side.

Investigate temperature changes fairly

A temperature-change investigation compares the initial temperature with the highest or lowest temperature after a controlled change. The method must capture the extreme temperature while limiting heat exchange with the surroundings.

Change investigated Suitable reactants and method
salt dissolving measure water in an insulated cup, record its temperature, add a measured salt mass, replace lid, stir and record the minimum or maximum
neutralisation mix measured acid and alkali volumes in an insulated cup and record the maximum
displacement add an excess measured metal to a measured metal-salt solution in an insulated cup and record the maximum
combustion heat a known water mass with a measured fuel mass and record the maximum and mass burned

Change one independent variable at a time. Keep reactant amounts, concentrations, starting temperature, apparatus, stirring and—when burning fuel—the flame distance constant. Repeat each condition, identify anomalies before calculating a mean, and use eye protection and small quantities.

Record the highest or lowest temperature, not a reading at an arbitrary fixed time. A polystyrene cup reduces heat transfer for solution reactions, while a metal can conducts heat from a flame to water; these apparatus choices serve different purposes.