8.1-8.4 - Enthalpy Changes

Heat Change At Constant Pressure

When magnesium reacts with dilute hydrochloric acid, the test tube becomes warm. When ammonium nitrate dissolves in water in a cold pack, the surroundings become colder. In both cases, the important question is not just "did the temperature change?" but "which way did energy move between the chemical system and the surroundings?"

The chemical reaction or process being studied is the system. The rest of the mixture, apparatus, air, thermometer and your hand are part of the surroundings used to observe the heat transfer.

Use this sign convention:

If the system releases heat to the surroundings, the products have lower enthalpy than the reactants and $\Delta H$ is negative. This is an exothermic change. If the system absorbs heat from the surroundings, the products have higher enthalpy than the reactants and $\Delta H$ is positive. This is an endothermic change.

The sign belongs to the system, not to the thermometer. A temperature rise in the surroundings usually means the system has lost heat, so the reaction enthalpy change is negative.

Worked example: assigning the sign

Question: A reaction in an insulated cup causes the solution temperature to rise. Is the reaction exothermic or endothermic, and what is the sign of $\Delta H$?

Model answer route:

1. The reacting chemicals are the system.
2. The bulk solution and cup are the immediate surroundings used to observe the heat change.
3. The surroundings warm up, so heat has been transferred from the system to the surroundings.
4. The system has lost enthalpy, so the products are at lower enthalpy than the reactants.
5. The reaction is exothermic and $\Delta H$ is negative.

Chemical interpretation: the temperature observation is evidence for the direction of heat transfer, but the sign is defined for the chemical system.

Constructing Enthalpy Level Diagrams

An enthalpy level diagram shows the relative enthalpy of reactants and products. It is not a picture of individual molecules moving, and the horizontal spacing is not usually a measured time or distance. The key feature is the vertical difference between the reactant level and the product level.

[DIAGRAM: asset_name: Enthalpy Level Signs; asset_slug: edexcel_a_level_chemistry_l042_enthalpy_level_signs; recommended_method: deterministic_chart; description: Two enthalpy level diagrams comparing exothermic and endothermic reactions, with reactants and products at different enthalpy levels and Delta H arrows showing negative and positive signs.]

To construct a level diagram:

1. Draw a vertical axis labelled $\text{Enthalpy}, H$.
2. Put reactants and products on horizontal levels.
3. For an exothermic reaction, draw products lower than reactants.
4. For an endothermic reaction, draw products higher than reactants.
5. Draw a vertical arrow showing the enthalpy change.
6. Attach the correct sign: negative for exothermic, positive for endothermic.

For example, if a reaction has $\Delta H = -92 \mathrm{kJ\,mol^{-1}}$, the product level must be below the reactant level. If a reaction has $\Delta H = +180 \mathrm{kJ\,mol^{-1}}$, the product level must be above the reactant level.

Common-error contrast

Do not decide the sign from whether the arrow points "down the page" or "up the page" alone. The sign comes from the enthalpy difference:

• products lower than reactants → $\Delta H$ negative
• products higher than reactants → $\Delta H$ positive

A badly drawn arrow can be corrected; a wrong relative level shows a wrong model.

Guided practice

Level Diagrams And Reaction Profiles

One distinction matters here: activation energy is not shown in enthalpy level diagrams, but it is shown in reaction profile diagrams. A level diagram compares the enthalpy of the start and the finish. A reaction profile adds a pathway with an energy barrier between them.

[DIAGRAM: asset_name: Level Vs Profile; asset_slug: edexcel_a_level_chemistry_l042_level_vs_profile; recommended_method: deterministic_chart; description: Comparison of an enthalpy level diagram, which shows only Delta H between reactants and products, with a reaction profile diagram, which also shows activation energy.]

The reaction profile is useful when the question is about rate, catalysts or activation energy. The enthalpy level diagram is useful when the question is about the overall enthalpy change and its sign.