science

Heat of dilution

Heat of dilution is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Heat of dilution rather than just read about it. In short: In thermochemistry, the heat of dilution, or enthalpy of dilution, refers to the enthalpy change associated with the dilution process of a component in a solution at a constant pressure. If the initial state of the component is a pure liquid (presuming the solution is liquid), the dilution process is equal to its dissolution process and the heat of dilution is the same as the heat of solution.

Key takeaways

Heat of dilution belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Heat of dilution to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Heat of dilution from memory before moving on to harder problems.

Reference excerpt

In thermochemistry, the heat of dilution, or enthalpy of dilution, refers to the enthalpy change associated with the dilution process of a component in a solution at a constant pressure. If the initial state of the component is a pure liquid (presuming the solution is liquid), the dilution process is equal to its dissolution process and the heat of dilution is the same as the heat of solution. Generally, the heat of dilution is normalized by the amount of the solution and its dimensional units are energy per unit mass or amount of substance, commonly expressed in the unit of kJ/mol (or J/mol).

Definition The heat of dilution can be defined from two perspectives: the differential heat and the integral heat. The differential heat of dilution is viewed on a micro scale, which is associated with the process in which a small amount of solvent is added to a large quantity of solution. The molar differential heat of dilution is thus defined as the enthalpy change caused by adding a mole of solvent at a constant temperature and pressure to a very large amount of solution. Because of the small amount of addition, the concentration of dilute solution remains practically unchanged. Mathematically, the molar differential heat of dilution is denoted as:

Δ dil d H = ( ∂ Δ dil H ∂ Δ n i ) T , p , n B {\displaystyle \Delta _{\text{dil}}^{d}H=\left({\frac {\partial \Delta _{\text{dil}}H}{\partial \Delta n_{i}}}\right)_{T,p,n_{B}}}

where ∂∆ni is the infinitesimal change or differential of mole number of the dilution. The integral heat of dilution, however, is viewed on a macro scale. With respect to the integral heat, consider a process in which a certain amount of solution diluted from an initial concentration to a final concentration. The enthalpy change in this process, normalized by the mole number of solute, is evaluated as the molar integral heat of dilution. Mathematically, the molar integral heat of dilution is denoted as:

Δ dil i H = Δ dil H n B {\displaystyle \Delta _{\text{dil}}^{i}H={\frac {\Delta _{\text{dil}}H}{n_{B}}}}

If the infinite amount of solvent is added to a solution with a known concentration of solute, the corresponding change of enthalpy is called as integral heat of dilution to infinite dilution. The dilution between two concentrations of the solute is associated to an intermediary heat of dilution by mole of solute.

Dilution and dissolution The process of dissolution and the process of dilution are closely related to each other. In both processes, similar final statuses of solutions are reached. However, the initial statuses can be different. In a dissolution process, a solute is changed from a pure phase—solid, liquid, or gas—to a solution phase. If the pure phase of the solute is a solid or gas (presuming the solvent itself is liquid), the process can be seen in two stages: the phase change into a liquid, and the mixing of liquids. The dissolution process is generally expressed as:

solute(s,l,g) + solvent(l) → solute(l) + solvent(l) → solute(sln) + solvent(sln) {\displaystyle {\textrm {solute(s,l,g)}}+{\textrm {solvent(l)}}\rightarrow {\textrm {solute(l)}}+{\textrm {solvent(l)}}\rightarrow {\textrm {solute(sln)}}+{\textrm {solvent(sln)}}}

The notation "sln" stands for "solution", which represents a status of the solvent or solute being part of the solution. In a dilution process, on the other hand, the solution is changed from one concentration to another, illustrated as:

solute(sln 1 ) + solvent(sln 1 ) → solute(sln 2 ) + solvent(sln 2 ) {\displaystyle {\textrm {solute(sln}}_{1}{\textrm {)}}+{\textrm {solvent(sln}}_{1}{\textrm {)}}\rightarrow {\textrm {solute(sln}}_{2}{\textrm {)}}+{\textrm {solvent(sln}}_{2}{\textrm {)}}}

Consider an extreme condition for the dilution process. Let the initial status be the pure liquid. The dilution process is then described as:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Heat of dilution

Start with the simplest possible case. Write down what Heat of dilution claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Heat of dilution before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Heat of dilution ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Heat of dilution

In research: Heat of dilution appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Heat of dilution in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Heat of dilution is common in secondary-school and first-year university syllabi. It links to neighbouring topics Enthalpy, so understanding it makes those chapters shorter.
In everyday life: Look for Heat of dilution outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Heat of dilution in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Heat of dilution means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Heat of dilution out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Heat of dilution in simple terms?

Why does Heat of dilution matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Heat of dilution?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Heat of dilution.