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D2.2 · Write thermochemical equations with ΔH or heat terms

Learn to write thermochemical equations with δh or heat terms through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Energy Changes and Rates of Reaction

SCH4U D2.2: Show heat using ΔH or a heat term

A hand warmer may become warm as a chemical reaction occurs. In another reaction, a container may feel cooler because the reaction takes in heat from its surroundings. These observations tell us that reactions can transfer heat. A thermochemical equation records that energy change alongside the chemical change. The equation must still conserve atoms, and it must make clear whether heat is released or absorbed. This lesson focuses on writing that information using either ΔH or a heat term.

What you will learn

  • Explain what a thermochemical equation communicates.
  • Write a balanced chemical equation with a ΔH value and the correct sign.
  • Represent heat as a reactant or product when writing a thermochemical equation.
  • Relate the sign and amount of ΔH to the reaction as written.

1. From a balanced equation to an energy change

A chemical equation shows which substances react and which substances form. Before adding energy information, check that the equation is balanced. A balanced equation has the same number of each type of atom on both sides. The coefficients show the relative amounts of substances in the reaction.
A thermochemical equation is a chemical equation that also states the heat change for the reaction as written. The symbol ΔH means the change in enthalpy, a course-level measure used to report heat transferred during a reaction at constant pressure. For this lesson, treat ΔH as the reaction’s heat change. Its unit is usually kilojoules, written kJ.
The sign of ΔH tells the direction of heat transfer. If the reaction releases heat to its surroundings, it is exothermic and ΔH is negative. If the reaction absorbs heat from its surroundings, it is endothermic and ΔH is positive. The sign belongs to the reaction as written; it is not a label for a particular substance.
The physical states can matter when the equation describes a particular reaction. State symbols identify solid, liquid, gas, and aqueous substances: (s)(s), (l)(l), (g)(g), and (aq)(aq). Include states when they are supplied or relevant to the stated reaction. The same substances in different states can have different heat changes, so do not silently change a state while keeping the same ΔH.
ΔH=Hproducts−Hreactants\Delta H = H_{\mathrm{products}} - H_{\mathrm{reactants}}
  • Balance atoms before adding the thermochemical information.
  • Negative ΔH means heat is released; positive ΔH means heat is absorbed.
  • ΔH applies to the complete equation as written.

2. Two ways to show heat

There are two common ways to write the energy information. In the first, write the balanced chemical equation and report ΔH beside it or after it. For example, a negative value identifies a heat-releasing reaction, while a positive value identifies a heat-absorbing reaction. Keep the sign and unit with the value.
In the second way, include heat as a term in the chemical equation. For an exothermic reaction, heat is written on the product side because the reaction releases it. For an endothermic reaction, heat is written on the reactant side because the reaction takes it in. This placement is a bookkeeping convention that makes the direction of energy transfer visible.
The heat-term form and the ΔH form express the same energy change. If heat is written as a term, do not also attach a ΔH value unless the task asks for both. When both are requested, make sure their signs and placements agree. A product-side heat term corresponds to a negative ΔH; a reactant-side heat term corresponds to a positive ΔH.
Coefficients matter. A thermochemical equation describes the amounts indicated by its coefficients. If every coefficient is multiplied by the same factor, the amount of reaction represented changes by that factor, so the heat change must also be multiplied by it. If the equation is reversed, heat transfer reverses too: the sign of ΔH changes, and heat moves to the opposite side in the heat-term version.
exothermic: ΔH<0endothermic: ΔH>0\text{exothermic: }\Delta H<0\qquad\text{endothermic: }\Delta H>0
  • Exothermic: heat on the product side and ΔH is negative.
  • Endothermic: heat on the reactant side and ΔH is positive.
  • Changing the equation’s coefficients or direction changes the stated heat amount or sign.

3. A reliable writing routine

Start with the chemical change given in the question. Write the correct formulas and balance the equation without using heat to balance atoms. Heat is an energy term, not an element, so it cannot fix an imbalance in the chemical formula side of the equation.
Next, decide whether the reaction releases or absorbs heat. Use the information in the question, such as an explicitly stated heat change or the description of heat transfer. Do not infer a numerical ΔH value from a description such as “warm” unless the value is provided or can be calculated from information the task supplies.
Then choose the requested form. If the question asks for ΔH, write the balanced equation and place the signed value with its unit. If it asks for a heat term, put heat on the product side for release or on the reactant side for absorption. Check that the direction shown by the heat term agrees with the sign, if a sign is also given.
Finally, check the scope of the stated amount. If the equation is doubled, the heat value must double. If it is reversed, the sign changes. Keep units attached to numerical heat values. Do not add extra digits: report the precision supported by the given data. In a question that supplies a value to three significant figures, retain three significant figures when the value is scaled.
  • Balance the chemical substances first.
  • Use the prompt’s information to determine the direction and amount of heat transfer.
  • Check coefficients, equation direction, sign, and units before finishing.

4. Connect the equation to the particles

At the particle level, a reaction rearranges atoms as reactant particles become product particles. In an exothermic reaction, the chemical change transfers heat to the surroundings. In an endothermic reaction, it takes in heat from the surroundings. The thermochemical equation summarizes both the rearrangement of substances and this heat transfer.
The equation does not mean that heat is a chemical particle. Writing heat on one side is a way to show whether the reaction gives energy to the surroundings or takes energy in. This distinction helps prevent a common error: treating heat as though it were an atom or changing a chemical formula to account for energy.
The value of ΔH is tied to the reaction exactly as written, including its coefficients and states. It is not automatically the value for one mole of every substance in the equation. Read the equation and its heat value together. If you change the amount represented by the equation, adjust the heat value consistently.
This representation is useful because it lets a reader see the chemical change and energy direction in one statement. It also makes equations comparable, provided their formulas, states, coefficients, and reaction directions are considered.
  • The particles rearrange; heat describes energy transferred during the reaction.
  • The heat value belongs to the complete equation, not independently to each formula.
  • Read state symbols and coefficients as part of the thermochemical statement.

Worked example

Write the heat term and ΔH

A reaction is represented by the balanced equation N2(g)+3H2(g)→2NH3(g)\mathrm{N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)}. The reaction releases 92.2 kJ92.2\ \mathrm{kJ} as written. Write it once with a ΔH value and once with a heat term.
  1. Keep the chemical equation balanced
    The starting equation has two nitrogen atoms and six hydrogen atoms on each side. Its coefficients already balance the atoms, so they do not need to change.
    N2(g)+3H2(g)→2NH3(g)\mathrm{N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)}
  2. Assign the sign
    The prompt says the reaction releases heat, so it is exothermic. A heat release has a negative ΔH. The stated amount is for the equation exactly as written.
    ΔH=−92.2 kJ\Delta H = -92.2\ \mathrm{kJ}
  3. Write both forms
    In the ΔH form, keep the balanced chemical equation and report the negative value. In the heat-term form, put the released heat on the product side. The heat-term amount is positive as an amount of heat, while its product-side placement shows release.
    N2(g)+3H2(g)→2NH3(g)+92.2 kJ\mathrm{N_2(g) + 3H_2(g) \rightarrow 2NH_3(g) + 92.2\ kJ}
Answer: ΔH form: N2(g)+3H2(g)→2NH3(g)\mathrm{N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)}, ΔH=−92.2 kJ\Delta H = -92.2\ \mathrm{kJ}. Heat-term form: N2(g)+3H2(g)→2NH3(g)+92.2 kJ\mathrm{N_2(g) + 3H_2(g) \rightarrow 2NH_3(g) + 92.2\ kJ}.
Check: The sign is negative because heat is released. The heat term appears with the products. Both forms describe the balanced equation as written.

Common mistakes and how to avoid them

Writing a positive ΔH for a reaction that releases heat.
Correction: Use a negative ΔH for heat release. A positive ΔH indicates heat absorption.
Putting heat on the reactant side for an exothermic reaction.
Correction: Place released heat on the product side. Place absorbed heat on the reactant side.
Changing a coefficient but leaving the original ΔH unchanged.
Correction: Scale the heat change by the same factor as all equation coefficients.
Reversing the chemical equation without changing the heat information.
Correction: Reversing the equation reverses heat transfer, so change the sign of ΔH or move the heat term to the opposite side.

Lesson summary

  • A thermochemical equation combines a balanced chemical equation with information about heat.
  • For heat release, write ΔH as negative or place heat among the products.
  • For heat absorption, write ΔH as positive or place heat among the reactants.
  • The heat amount belongs to the equation as written; changing its scale or direction requires changing the heat information.

Check your understanding

Question 1

A balanced reaction absorbs heat. Which statement is correct?
  1. ΔH is negative, and heat is written among the products.
  2. ΔH is positive, and heat is written among the reactants.
  3. ΔH is positive, and heat is written among the products.
  4. correctIndex 1: 1Please ensure valid JSON. Set the index to the zero-based position of the correct answer and remove this entire line before submission. Use the natural-language explanation field to explain the correct answer.
Show answer and explanation
ΔH is positive, and heat is written among the reactants.
Absorbing heat is endothermic, so ΔH is positive. In the heat-term form, absorbed heat is written on the reactant side.

Question 2

A reaction has ΔH = −48.0 kJ-48.0\ \mathrm{kJ} as written. If every coefficient is doubled, what is the new ΔH?
  1. −24.0 kJ-24.0\ \mathrm{kJ}
  2. −48.0 kJ-48.0\ \mathrm{kJ}
  3. −96.0 kJ-96.0\ \mathrm{kJ}
  4. correctIndex 2: 2Please ensure valid JSON. Set the index to the zero-based position of the correct answer and remove this entire line before submission. Use the natural-language explanation field to explain the correct answer.
Show answer and explanation
−96.0 kJ-96.0\ \mathrm{kJ}
Doubling every coefficient represents twice the reaction amount, so the heat change also doubles: 2×(−48.0 kJ)=−96.0 kJ2\times(-48.0\ \mathrm{kJ})=-96.0\ \mathrm{kJ}.

Question 3

A thermochemical equation is reversed. What happens to its ΔH?
  1. Its sign changes.
  2. Its value doubles.
  3. Its value and sign stay the same.
  4. correctIndex 0: 0Please ensure valid JSON. Set the index to the zero-based position of the correct answer and remove this entire line before submission. Use the natural-language explanation field to explain the correct answer.
Show answer and explanation
Its sign changes.
Reversing the reaction reverses the direction of heat transfer. Therefore, the ΔH sign changes while its magnitude stays the same.

Key terms

Thermochemical equation
A balanced chemical equation that also shows the reaction’s heat change.
Enthalpy change, ΔH
The heat change reported for a reaction at constant pressure; its sign indicates heat release or absorption.
Exothermic
Describes a reaction that releases heat to its surroundings.
Endothermic
Describes a reaction that absorbs heat from its surroundings.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation D2.2. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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