Lydersen method

Key takeaways

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

Reference excerpt

The Lydersen method is a group contribution method for the estimation of critical properties temperature (Tc), pressure (Pc) and volume (Vc). The method is named after Aksel Lydersen who published it in 1955. The Lydersen method is the prototype for and ancestor of many new models like Joback, Klincewicz, Ambrose, Gani-Constantinou and others. The Lydersen method is based in case of the critical temperature on the Guldberg rule which establishes a relation between the normal boiling point and the critical temperature.

Equations

Critical temperature

T c = T b 0.567 + ∑ G i − ( ∑ G i ) 2 {\displaystyle T_{c}={\frac {T_{b}}{0.567+\sum G_{i}-\left(\sum G_{i}\right)^{2}}}}

Guldberg has found that a rough estimate of the normal boiling point Tb, when expressed in kelvins (i.e., as an absolute temperature), is approximately two-thirds of the critical temperature Tc. Lydersen uses this basic idea but calculates more accurate values.

Critical pressure

P c = M ( 0.34 + ∑ G i ) 2 {\displaystyle P_{c}={\frac {M}{\left(0.34+\sum G_{i}\right)^{2}}}}

Critical volume

V c = 40 + ∑ G i {\displaystyle V_{c}\,=\,40+\sum G_{i}}

M is the molar mass and Gi are the group contributions (different for all three properties) for functional groups of a molecule.

Group contributions

Example calculation

Acetone is fragmented in two different groups, one carbonyl group and two methyl groups. For the critical volume the following calculation results: Vc = 40 + 60.0 + 2 * 55.0 = 210 cm3 In the literature (such as in the Dortmund Data Bank) the values 215.90 cm3, 230.5 cm3 and 209.0 cm3 are published.

References

Worked examples

Example 1 — a first encounter with Lydersen method

Start with the simplest possible case. Write down what Lydersen method claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, 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 Lydersen method 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 Lydersen method 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 Lydersen method

In research: Lydersen method appears in physics 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 Lydersen method 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: Lydersen method is common in secondary-school and first-year university syllabi. It links to neighbouring topics Physical chemistry, Thermodynamic models, so understanding it makes those chapters shorter.
In everyday life: Look for Lydersen method 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.

How to study Lydersen method in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Lydersen method 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 Lydersen method out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Lydersen method in simple terms?

The Lydersen method is a group contribution method for the estimation of critical properties temperature (Tc), pressure (Pc) and volume (Vc). The method is named after Aksel Lydersen who published it in 1955.

Why does Lydersen method matter?

Because it connects several physics 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 Lydersen method?

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 Lydersen method.