What is enthalpy and why is it important in the study of energy resources?

Enthalpy is a measure of the total energy of a thermodynamic system, including internal energy and the energy required to make room for it by displacing its environment. It is important in the study of energy resources because it helps us understand how energy is absorbed or released during chemical reactions, which is crucial for evaluating the efficiency of energy production and conversion processes.

How is enthalpy change calculated in a chemical reaction?

The enthalpy change (ΔH) in a chemical reaction is calculated by subtracting the total enthalpy of the reactants from the total enthalpy of the products. It can be expressed as ΔH = H_products - H_reactants. This calculation helps determine whether a reaction is exothermic (releases energy) or endothermic (absorbs energy), which is vital for understanding energy transformations in science.

What is the difference between enthalpy and internal energy?

Enthalpy and internal energy are related but distinct concepts. Internal energy refers to the total energy contained within a system, including kinetic and potential energy at the molecular level. Enthalpy, on the other hand, includes internal energy plus the energy required to displace the system's environment, represented as pressure-volume work. This distinction is important in thermodynamics and energy resource studies, as it affects how energy changes are measured and interpreted.

Why is enthalpy considered a state function in thermodynamics?

Enthalpy is considered a state function because its value depends only on the current state of the system, not on the path taken to reach that state. This means that the enthalpy change between two states is the same regardless of how the system transitions between them. This property is crucial for simplifying calculations in thermodynamics and energy resource management, as it allows scientists to focus on initial and final states rather than the specific processes involved.

How does enthalpy relate to the concept of energy conservation in the IB MYP Science curriculum?

In the IB MYP Science curriculum, enthalpy is related to the concept of energy conservation through the First Law of Thermodynamics, which states that energy cannot be created or destroyed, only transformed. Enthalpy changes in chemical reactions illustrate how energy is conserved by showing the balance between energy absorbed and released. Understanding this relationship helps students grasp fundamental principles of energy conservation and efficiency in energy resource management.

Why is "Saying exothermic ΔH is positive." a common mistake in Enthalpy?

Why it happens: Reactions release energy — feels positive. How to avoid it: Exothermic releases energy — total energy of system DECREASES → ΔH NEGATIVE. Endo: surroundings cool → ΔH positive.

Why is "Believing a catalyst changes ΔH." a common mistake in Enthalpy?

Why it happens: Catalysts feel powerful. How to avoid it: Catalyst only lowers Ea (the hump). ΔH stays the same because reactants and products are unchanged.

Why is "Ignoring heat loss to surroundings in calorimetry." a common mistake in Enthalpy?

Why it happens: The formula doesn't include it. How to avoid it: Real measurements UNDERESTIMATE the true energy because some heat escapes. Insulating the cup reduces but doesn't eliminate this.

Energy resourcesIB MYP Sciences (Years 1-5)

Enthalpy

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Enthalpy — IB MYP Sciences: exothermic and endothermic reactions and reaction profiles

Why does burning fuel warm the air but dissolving certain salts cool it? This MYP Sciences note covers the enthalpy change of a reaction (ΔH), the difference between exothermic and endothermic, and how to read a reaction profile.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Define exothermic and endothermic reactions.
MYP Sciences A — Draw and label a reaction profile for both types.
MYP Sciences A — Identify exothermic and endothermic from a temperature change.
MYP Sciences C — Calculate ΔH from a calorimetry experiment.
MYP Sciences D — Evaluate energy use in common everyday reactions.

Exothermic and endothermic reactions

Exo releases energy (surroundings warm up). Endo absorbs (surroundings cool).

Every chemical reaction involves bond breaking (which costs energy) and bond making (which releases energy). The DIFFERENCE between these is the enthalpy change ΔH.

Exothermic reactions release more energy from new bonds than they used breaking old ones. ΔH is NEGATIVE. The surroundings WARM UP.

Examples:

Combustion of fuels (a candle, a gas hob).
Neutralisation (HCl + NaOH).
Respiration in living things.
Most precipitation and salt-formation reactions.
Hand warmers (slow oxidation of iron).

Endothermic reactions absorb more energy than they release. ΔH is POSITIVE. The surroundings COOL DOWN.

Examples:

Thermal decomposition (CaCO₃ → CaO + CO₂).
Photosynthesis (CO₂ + H₂O → glucose + O₂, driven by sunlight).
Dissolving ammonium nitrate (used in sports cold packs).
Citric acid + sodium hydrogen carbonate (a chemistry trick to make a cold solution).

A simple test: drop a thermometer in the reaction mixture and watch the reading. UP = exothermic; DOWN = endothermic.

Exothermic: ΔH < 0, surroundings get hotter.
Endothermic: ΔH > 0, surroundings get colder.
Direction of temperature change is the clue.

Reaction profiles

A graph of energy vs reaction progress shows everything at a glance.

A reaction profile plots energy on the y-axis and the progress of the reaction on the x-axis. The reactants start at one energy level, climb over an activation-energy hump, and settle at the products' energy level.

Exothermic: products lower than reactants (energy released). Endothermic: products higher (energy absorbed). The hump in the middle is the activation energy.

Things to identify on any profile:

Reactants energy line on the left.
Activation energy (Ea) = height of the hump above the reactant line.
Products energy line on the right.
ΔH = product energy − reactant energy. Negative for exothermic, positive for endothermic.

A catalyst LOWERS the hump (lower Ea) but DOES NOT change ΔH — reactants and products remain at the same energies.

Profile: energy vs progress.
Hump = Ea. Difference = ΔH.
Exothermic: products lower than reactants.
Endothermic: products higher.

Measuring energy change

Calorimetry — measure a temperature change to calculate the energy released.

In a simple calorimetry experiment, you measure how much a known mass of water warms up (or cools down) as a reaction happens. Then:

$Q = m \, c \, \Delta T$

where Q is the energy transferred (J), m is the mass of water (g), c is the specific heat capacity of water (4.18 J/g/°C), and ΔT is the temperature change.

Worked example. 50 cm³ of water (= 50 g) absorbs heat from a burning ethanol burner. Temperature rises from 20 °C to 60 °C. How much energy?

Q = m c ΔT = 50 × 4.18 × 40 = 8 360 J ≈ 8.4 kJ.

This is an estimate of the energy released by the ethanol — though some heat will be lost to the surroundings rather than warming the water. So real ΔH values are USUALLY larger (in magnitude) than the calorimetry measurement.

Q = m × c × ΔT (water).
c (water) ≈ 4.18 J/g/°C.
Measurement underestimates due to heat loss.

How it’s examined

Criterion A asks for definitions, classification of reactions, and reading profile diagrams. Criterion C: calorimetry calculations with Q = mcΔT.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — Enthalpy

Step-by-step solutions to past-paper-style questions on enthalpy, written exactly the way a tutor would explain them at the board.

1Exo or endo?
Getting started• classification
Question
Classify each as exothermic or endothermic: (a) burning natural gas, (b) photosynthesis, (c) cold pack of NH₄NO₃ + water, (d) human respiration.
Step-by-step solution
1. Step 1
  (a) Heat is RELEASED — exothermic.
2. Step 2
  (b) Energy is ABSORBED from sunlight — endothermic.
3. Step 3
  (c) Surroundings get COLDER — endothermic.
4. Step 4
  (d) Body produces heat — exothermic.
Answer
(a) exo, (b) endo, (c) endo, (d) exo.
2Calorimetry calculation
Building confidence• Q = mcΔT
Question
100 cm³ of water is heated by a burning candle. Temperature rises from 22 °C to 32 °C. Calculate the energy transferred. (c_water = 4.18 J/g/°C.)
Step-by-step solution
1. Step 1
  Mass = 100 cm³ × 1 g/cm³ = 100 g.
2. Step 2
  ΔT = 32 − 22 = 10 °C.
3. Step 3
  Q = m c ΔT = 100 × 4.18 × 10 = 4 180 J = 4.18 kJ.
Answer
Q = 4 180 J = 4.18 kJ.
3Reading a reaction profile
Building confidence• reaction profile
Question
A reaction profile shows reactants at 200 kJ, products at 50 kJ, and a peak at 350 kJ. Find (a) Ea, (b) ΔH, (c) classify the reaction.
Step-by-step solution
1. Step 1
  (a) Ea = peak − reactants = 350 − 200 = 150 kJ.
2. Step 2
  (b) ΔH = products − reactants = 50 − 200 = −150 kJ.
3. Step 3
  (c) ΔH negative → exothermic.
Answer
Ea = 150 kJ. ΔH = −150 kJ. Exothermic.
4Catalyst and ΔH
Stretch• catalyst
Question
A reaction has ΔH = −200 kJ/mol. A catalyst is added. What happens to (a) Ea, (b) ΔH?
Step-by-step solution
1. Step 1
  (a) Catalyst lowers Ea by providing an alternative pathway.
2. Step 2
  (b) ΔH is unchanged. Reactants and products still have the same energies; only the route between them is easier.
Answer
(a) Ea decreases. (b) ΔH unchanged at −200 kJ/mol.

Key Formulae — Enthalpy

The formulae you need to memorise for enthalpy on the IB MYP Sciences paper, with every variable defined in plain English and a note on when to use it.

Calorimetry
$Q = m × c × ΔT$
$Q$
energy transferred (J)
$m$
mass of water (g)
$c$
specific heat capacity of water (4.18 J/g/°C)
$ΔT$
temperature change (°C)
When to use
Measuring the energy released or absorbed by a reaction with a water bath.

Key Definitions and Keywords — Enthalpy

Definitions to memorise and the exact keywords mark schemes credit for enthalpy answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Enthalpy (H)
Examiner keyword
Total energy content of a substance. Changes (ΔH) are what we measure.
Exothermic
Examiner keyword
A reaction that releases energy to the surroundings. ΔH < 0.
Endothermic
Examiner keyword
A reaction that absorbs energy from the surroundings. ΔH > 0.
Activation energy (Ea)
Examiner keyword
The minimum energy that reactant particles need for a successful collision.
Reaction profile
A graph of energy against the progress of the reaction, showing reactants, products, ΔH and Ea.

Common Mistakes and Misconceptions — Enthalpy

The traps other students keep falling into on enthalpy questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Saying exothermic ΔH is positive.
Why it happens
Reactions release energy — feels positive.
How to avoid it
Exothermic releases energy — total energy of system DECREASES → ΔH NEGATIVE. Endo: surroundings cool → ΔH positive.
✕Believing a catalyst changes ΔH.
Why it happens
Catalysts feel powerful.
How to avoid it
Catalyst only lowers Ea (the hump). ΔH stays the same because reactants and products are unchanged.
✕Ignoring heat loss to surroundings in calorimetry.
Why it happens
The formula doesn't include it.
How to avoid it
Real measurements UNDERESTIMATE the true energy because some heat escapes. Insulating the cup reduces but doesn't eliminate this.

Practice questions

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Enthalpy — frequently asked questions

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Enthalpy

Short Study Notes in the form of Slides

Short Study Notes — Enthalpy

Detailed Notes

What you’ll learn

How it’s examined

1Exo or endo?

2Calorimetry calculation

3Reading a reaction profile

4Catalyst and ΔH

Calorimetry

Enthalpy (H)

Exothermic

Endothermic

Activation energy (Ea)

Reaction profile

✕Saying exothermic ΔH is positive.

✕Believing a catalyst changes ΔH.

✕Ignoring heat loss to surroundings in calorimetry.

Practice questions

Past paper style quiz

Assess your understanding

Enthalpy — frequently asked questions