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8873 / 2027
Chemical Energetics: Thermochemistry

Lesson 1 of 3

Meaning of energy

Define a process and read its profile.

A-Level 8873, revised syllabus (2026-2027)

  • I can explain exothermic and endothermic changes.Syllabus

    Syllabus 8873, 6(a). Explain exothermic and endothermic changes.

  • I can construct and interpret energy profiles.Syllabus

    Syllabus 8873, 6(b). Construct and interpret energy profiles.

  • I can define the specified energy terms.Syllabus

    Syllabus 8873, 6(c). Define the specified energy terms.

  • I can explain lattice-energy magnitude.Syllabus

    Syllabus 8873, 6(e). Explain lattice-energy magnitude.

Breaking needs energy; forming releases it

A reaction can release energy overall and still need an initial barrier to be crossed.

Chemical reactions rearrange bonds. Breaking bonds absorbs energy; forming bonds releases energy. The enthalpy change is the net difference. At constant pressure, an exothermic reaction transfers heat to the surroundings and has negative ΔH; an endothermic reaction absorbs heat from them and has positive ΔH.

Read the barrier and the overall drop

Reactants are at zero, the barrier is 120 kJ mol-1 above them, and products are 80 kJ mol-1 below them. Forward activation energy is 120, reaction enthalpy is -80, and reverse activation energy is 200.

Illustrative values relative to the reactants. Reaction progress is not time, and the vertical axis is not temperature.

Read ΔH = H(products) - H(reactants). Read the forward activation energy from the reactant level to the barrier, not from the page baseline. In the figure, the reverse reaction starts at -80, so its barrier is 120 - (-80) = 200 kJ mol-1. A catalyst changes the pathway and barrier, but not the reactant/product levels or ΔH.

Worked exampleConstruct an endothermic profile

Sketch a reaction with Delta H = +30 kJ mol-1 and forward activation energy 80 kJ mol-1.

Try it first. Then show the worked stepsHide the worked steps
  1. Label the vertical axis enthalpy and the horizontal axis reaction progress. Choose the reactants as a zero reference.
  2. Place products 30 kJ mol-1 above the reactants and the barrier 80 kJ mol-1 above them.
  3. Join the levels with a smooth rise to the barrier and fall to the products. Label Delta H from reactants to products and Ea from reactants to the barrier.
Answer

The product level is higher than the reactant level. The reverse activation energy is 80 - 30 = 50 kJ mol-1.

Watch out for this

A student says: "The thermometer reading rose, so the reaction gained energy and its delta H is positive." What is wrong with this?

Show the answer

The temperature of the surroundings rises during an exothermic reaction. Their heat gain is positive, but the reacting system loses that energy, so its delta H is negative.

Check your understanding

A reaction has delta H = -40 kJ mol-1 and a forward activation energy of 60 kJ mol-1. What is the activation energy of the reverse reaction?

  1. 60 kJ mol-1, the same as the forward barrier
  2. 20 kJ mol-1, the forward barrier minus delta H
  3. 100 kJ mol-1, from the products to the peak
Show the answer

100 kJ mol-1, from the products to the peak

The peak is 60 kJ mol-1 above the reactants, and the products are 40 kJ mol-1 below the reactants. So the peak is 60 + 40 = 100 kJ mol-1 above the products.

Name the exact process and amount

An enthalpy value belongs to a stated equation, physical states and conditions.

A standard enthalpy change refers to species in their standard states at a stated temperature, commonly 298 K, with standard pressure 100 kPa. Aqueous standard data use a specified standard concentration, conventionally 1 mol dm-3 at this level. Standard does not by itself mean a reaction occurs at 0 degrees C. Always retain the states and temperature supplied with the data.

Required enthalpy terms
TermOne-mole referenceIllustration
Enthalpy change of reactionThe reaction as its equation is writtenDoubling the equation doubles its enthalpy change.
Standard enthalpy of formationOne mole of compound from its elements in their standard statesC(graphite,s) + O2(g) → CO2(g)
Standard enthalpy of combustionOne mole of substance completely burned in oxygen under standard conditionsCH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Enthalpy of neutralisationOne mole of water formed when an acid reacts with a baseH+(aq) + OH-(aq) → H2O(l)
Bond energyOne mole of specified gaseous covalent bonds brokenH2(g) → 2H(g); positive
Lattice energyOne mole of ionic solid formed from separated gaseous ionsNa+(g) + Cl-(g) → NaCl(s); negative

The formation enthalpy of an element in its standard state is zero by convention. This is not a claim that the element contains no energy. For standard formation of water, H2(g) + ½O2(g) → H2O(l) forms exactly one mole; fractional coefficients are therefore useful.

Lattice energy becomes greater in magnitude when ionic charges increase or ion radii decrease: the opposite charges attract more strongly at shorter separation. MgO, with Mg2+/O2-, has a much more negative lattice energy than NaCl, with Na+/Cl-. For equal charges, a smaller ion generally makes lattice formation more exothermic.

Check your understanding

A data table gives a positive value for separating an ionic lattice into gaseous ions. How should you use it for H1 lattice energy?

  1. Halve it, since the value covers both cation and anion.
  2. Use it as given, since lattice energy is quoted as a positive value.
  3. Change its sign, since H1 lattice energy is for forming the lattice.
Show the answer

Change its sign, since H1 lattice energy is for forming the lattice.

Separating the lattice is the reverse of forming it, so the enthalpy has the same size but the opposite sign. Read the equation, not just the name.