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chemistry

Thermochemistry

Thermochemistry is a chemistry 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 Thermochemistry rather than just read about it. In short: Thermochemistry is the study of the heat energy which is associated with chemical reactions and/or phase changes such as melting and boiling. A reaction may release or absorb energy, and a phase change may do the same.

Thermochemistry — main illustration
Thermochemistry — illustration

Key takeaways

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

Reference excerpt

Thermochemistry is the study of the heat energy which is associated with chemical reactions and/or phase changes such as melting and boiling. A reaction may release or absorb energy, and a phase change may do the same. Thermochemistry focuses on the energy exchange between a system and its surroundings in the form of heat. Thermochemistry is useful in predicting reactant and product quantities throughout the course of a given reaction. In combination with entropy determinations, it is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable. Endothermic reactions absorb heat, while exothermic reactions release heat. Thermochemistry coalesces the concepts of thermodynamics with the concept of energy in the form of chemical bonds. The subject commonly includes calculations of such quantities as heat capacity, heat of combustion, heat of formation, enthalpy, entropy, and free energy.

Thermochemistry is one part of the broader field of chemical thermodynamics, which deals with the exchange of all forms of energy between system and surroundings, including not only heat but also various forms of work, as well the exchange of matter. When all forms of energy are considered, the concepts of exothermic and endothermic reactions are generalized to exergonic reactions and endergonic reactions.

History Thermochemistry rests on two generalizations. Stated in modern terms, they are as follows:

Lavoisier and Laplace's law (1780): The energy change accompanying any transformation is equal and opposite to energy change accompanying the reverse process. Hess' law of constant heat summation (1840): The energy change accompanying any transformation is the same whether the process occurs in one step or many. These statements preceded the first law of thermodynamics (1845) and helped in its formulation. Thermochemistry also involves the measurement of the latent heat of phase transitions. Joseph Black had already introduced the concept of latent heat in 1761, based on the observation that heating ice at its melting point did not raise the temperature but instead caused some ice to melt. Gustav Kirchhoff showed in 1858 that the variation of the heat of reaction is given by the difference in heat capacity between products and reactants: dΔH / dT = ΔCp. Integration of this equation permits the evaluation of the heat of reaction at one temperature from measurements at another temperature.

Calorimetry The measurement of heat changes is performed using calorimetry, usually an enclosed chamber within which the change to be examined occurs. The temperature of the chamber is monitored either using a thermometer or thermocouple, and the temperature plotted against time to give a graph from which fundamental quantities can be calculated. Modern calorimeters are frequently supplied with automatic devices to provide a quick read-out of information, one example being the differential scanning calorimeter.

Systems Several thermodynamic definitions are very useful in thermochemistry. A system is the specific portion of the universe that is being studied. Everything outside the system is considered the surroundings or environment. A system may be:

a (completely) isolated system which can exchange neither energy nor matter with the surroundings, such as an insulated bomb calorimeter a thermally isolated system which can exchange mechanical work but not heat or matter, such as an insulated closed piston or balloon a mechanically isolated system which can exchange heat but not mechanical work or matter, such as an uninsulated bomb calorimeter a closed system which can exchange energy but not matter, such as an uninsulated closed piston or balloon an open system which can exchange both matter and energy with the surroundings, such as a pot of boiling water

Processes A system undergoes a process when one or more of its properties changes. A process relates to the change of state. An isothermal (same-temperature) process occurs when temperature of the system remains constant. An isobaric (same-pressure) process occurs when the pressure of the system remains constant. A process is adiabatic when no heat exchange occurs.

See also

Calorimetry Chemical kinetics Cryochemistry Differential scanning calorimetry Isodesmic reaction Important publications in thermochemistry Photoelectron photoion coincidence spectroscopy Principle of maximum work Reaction Calorimeter Thermodynamic databases for pure substances Thermodynamics Thomsen-Berthelot principle Julius Thomsen

References

External links Walker, James (1911). "Thermochemistry" . Encyclopædia Britannica. Vol. 26 (11th ed.). pp. 804–808.

Illustrations

Thermochemistry: The world's first ice-calorimeter, used in the winter of 1782–83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat evolved in various chemical changes; calculations which were based on Joseph Black's prior discovery of latent heat. These experiments mark the foundation of thermochemistry.
The world's first ice-calorimeter, used in the winter of 1782–83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat evolved in various chemical changes; calculations which were based on Joseph Black's prior discovery of latent heat. These experiments mark the foundation of thermochemistry.

Worked examples

Example 1 — a first encounter with Thermochemistry

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

In research
Thermochemistry appears in chemistry 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 Thermochemistry 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
Thermochemistry is common in secondary-school and first-year university syllabi. It links to neighbouring topics Branches of thermodynamics, Physical chemistry, Thermochemistry, so understanding it makes those chapters shorter.
In everyday life
Look for Thermochemistry 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 Thermochemistry in 20 minutes

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

Frequently asked questions

What is Thermochemistry in simple terms?

Thermochemistry is the study of the heat energy which is associated with chemical reactions and/or phase changes such as melting and boiling. A reaction may release or absorb energy, and a phase change may do the same.

Why does Thermochemistry matter?

Because it connects several chemistry 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 Thermochemistry?

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 Thermochemistry.

Tags

  • Branches of thermodynamics
  • Physical chemistry
  • Thermochemistry

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