7.2 Thermochemistry Essentials

Learning Objectives

  • Define how temperature and heat are understood on the molecular level by Collision Theory.
  • Learn about the absolute scale of the Kelvin and its relationship to degrees Celsius.
  • Define the three types of heat transfer: conduction, convection and radiation.
  • Define the three types of thermodynamic systems: open, closed and isolated, and the allowed transfer of matter and energy in each.

Collision Theory

To understand thermochemistry, we must first talk about the basics of collision theory. Collision theory is a chemical model which applies physics principles to understand the mechanisms of reactions. Let’s consider the formation of water: [latex]\ce{2H2 + O2 -> 2H2O}[/latex]. Under this model, we will represent all molecules as balls, as demonstrated in Figure 7.2.1.

Oxygen and hydrogen molecules floating in space as dark red and white balls, respectively.
Figure 7.2.1: Oxygen and hydrogen molecules floating in space.

From your own experience (in the lab or cooking on a stove), you may know that the temperature of a system correlates to the rate of a reaction. According to collision theory, temperature is directly related to the kinetic energy of molecules. As temperature rises, the speed at which molecules move increases (see Figure 7.2.2).

Three squares of red and white balls, representing floating oxygen and hydrogen molecules. In the first square, the molecules are still. But as heat increases, the speed of the balls also increase. Here, the molecules are drawn to appear as though they are vibrating and moving faster in each consecutive square, becoming more blurry and spread out.
Figure 7.2.2: As substances heat up, molecules move faster and faster.

With molecules moving faster, they are more likely to collide with each other with enough energy, causing a reaction to take place. Higher temperatures allow more successful collisions to occur more often, increasing the reaction rate.

The Kelvin

With this understanding, we must revisit how temperature is traditionally measured. The majority of the world utilises degrees Celsius (°C) – a system based on the freezing (0°C) and boiling (100°C) point of water. However, a reading of 0°C does not mean that no energy exists within a system; a lower temperature, such as -5°C, can still be achieved. For our energy calculations, an absolute value is needed: the Kelvin (K). The conversion from degrees Celsius to Kelvin is as follows:

[latex]K = °C + 273.15[/latex]

Under the Kelvin system, a value of 0 K represents that a molecule has no kinetic energy and is completely still. This is known as absolute zero[1]. Energy calculations within science are generally performed in Kelvin for this reason.


With our understanding of temperature being a measure of kinetic energy, we can appreciate how heat occurs. Heat is the transfer of energy from one body to another. This can only occur when a temperature difference is present. A hotter body will impart its energy onto a colder body until thermodynamic equilibrium is established. As heat is transferred, the hotter body will cool down as it loses its kinetic energy, while the colder body will begin to rise in temperature (see Figure 7.2.3).

Two rectangles in contact, one red and one blue. An arrow in between them shows the transfer of thermal energy—resulting in both boxes with having equal energy, andturning the same colour in thermodynamic equilibrium.
Figure 7.2.3: All bodies will change temperature until they reach thermodynamic equilibrium – where both contain the same amount of thermal energy.
A hand holding an ice cube.
Figure 7.2.4: Holding an ice cube feels cold, as the thermal energy within your hand is being transferred to the ice cube. That thermal energy is enough to melt the ice. Image attribution: child holding ice cubes – winter © BarbaraKrupa – stock.adobe.com.

Therefore, what we consider to be hot or cold isn’t due to the temperature itself – but the temperature difference. Something appears to feel cold because our hands are hotter than it is (see Figure 7.2.4), and vice versa. Our morning coffee on a particularly chilly day might appear to be warmer than usual because our bodies are colder than normal. This is also why some people find it hard to check their own temperature by touch when feverish – if their whole body is getting hotter, nothing will feel out of the ordinary even though body temperatures have risen.

What we have just described is conduction: the movement of thermal energy between two bodies in physical contact. It is the simplest of three main ways to transfer energy. From cooking a steak on a cast-iron pan to the cold feeling of placing an icepack on your body – conduction is the most direct form of heat transfer.


Thermal transfer is, however, not limited to solid objects. When a pot of water is heated up, how does that thermal energy spread itself around the container?

Diagram of a pot of water over a flame with the heading convection. The warmer molecules move from bottom to the top while the cooler molecules from top to lower, creating a circular current.
Figure 7.2.5: Molecules warmed through convection become less dense and rise to the top of the system, where colder molecules take their place. This flow is known as a convection current. Image attribution: Convection currents vector illustration labeled diagram © VectorMine – stock.adobe.com

All fluids exhibit convection. As liquids or gasses are heated, hotter areas become less dense and begin to rise, allowing colder molecules to replace them and be heated themselves. This movement creates a convection current (see Figure 7.2.5).

An air-fryer or convection oven uses this concept to its advantage. By heating up food through a convection current of air, a more uniform temperature can be achieved. While a frying pan can only heat food from one side at a time, a convection oven can heat from everywhere at once. To assist in this movement, fans are used to improve efficiency – allowing cooking temperatures to be reduced in fan-forced systems.


So far, the models we have looked at rely on matter to be present to transfer thermal energy. However, there is no matter in space, so how does the sun transfer heat to our planet?

The sun produces a wide range of electromagnetic radiation. Our eyes are able to detect only a small range of wavelengths (known as the visible spectrum; see Figure 7.2.5) – but a variety of other wavelengths are emitted.

Diagram of electromagnetic spectrum, with the visible light portion pulled out.
Figure 7.2.5: The electromagnetic spectrum. Transcript. Image attribution: Spectrum wavelength. Visible spectrum color range. Educational physics light line. Light wave frequency. Wavelengths of the visible part of the spectrum for human eyes © designer_things – stock.adobe.com.
Photo of lady through IR camera. Warmer areas are present in a red colour against the darker blue background.
Figure 7.2.6: IR cameras are able to measure the infrared radiation emitted from hot objects. This allows for temperature to be measured from a distance and is the main technology by night-vision cameras. Image attribution: Vector graphic of Thermographic image of a woman face showing different temperatures in a range of colors from blue showing cold to red showing hot. Medical thermal imaging of human female face. © Cipta – stock.adobe.com.

Infrared is the most common form of radiation generating heat. All objects with heat emit radiation, although hotter bodies manifest higher energy waves (see Figure 7.2.6). These wavelengths collide with molecules – causing some kinetic energy to be imparted. With more kinetic energy, a higher temperature is achieved and the objected is heated.

As such, thermal radiation requires no physical matter between the source and a target of interest to transfer energy. In fact, all forms of electromagnetic radiation do not require a medium to travel through, explaining why the sun can impart light and energy onto the Earth even through the vacuum of space.

Three methods of heat transfer. On the left is radiation, showing a woman with sunlight shining on her and an arrow pointing towards her, indicating radiant energy absorption. In the center, covection is shown with a pot on a stove; the water inside the pot has arrows moving in a circular pattern, which are the convection currents as heat transfers through the pot's liquid. To the right is conduction, showing a hand holding one end of a metal rod and the opposite end in contact with a candle flame. There is a colour gradient on the rod, illustrating the direct heat conduction along the rod from the flame to the hand.
Figure 7.2.7: The 3 types of heat transfer: radiation, convection, and conduction. Image attribution: Heat transfer types with radiation, convection and conduction types outline diagram. Labeled educational scheme with thermal energy exchange methods vector illustration. Hot temperature sources list. © VectorMine – stock.adobe.com.

Systems of Heat

Diagram of thermodynamic concepts with a green area labeled system representing the subject of study. System is enclosed by a dotted red line designated as the system boundary, distinguishing it from the outer area labelled surroudings. Above the entire diagram, the word universe indicates the larger environment encompassing both the system and its surroundings.
Figure 7.2.8: All thermodynamic studies have spatial positions that can be labelled. Image attribution: Thermodynamic system, boundary, system and surroundings © Reuel Sa – stock.adobe.com

With all 3 forms of heat transfer discussed (see Figure 7.2.7), we can understand how different thermal systems operate. In this context, a system refers to the part of the universe under study. Between the universe and the system is its direct surroundings – which can exchange heat and matter depending on the system (see Figure 7.2.8). The type of exchange depends on the system utilised. We observe three main types: open, closed, and isolated (see Figure 7.2.9).

An open system is one where both matter and energy can be exchanged with the environment. A boiling pot of water can be considered an open system: heat can radiate and conduct out to the surrounding environment, and water (in the form of steam) can escape. This is an important system for distillation and extraction methods. A closed system limits the transfer of matter but allows the transmittance of heat. By placing a lid on our pot, we are preventing matter from escaping the system as heat. We can, however, still heat up or cool down our system. An isolated system prevents both mass and heat from escaping into the surroundings. Heat transfer via convection can be limited through the implementation of a vacuum surrounding the system, while a reflective surface limits energy loss through radiation.

<a href="https://rmit.pressbooks.pub/rmitchemistrybridgingcourse/chapter/7-2-thermochemistry-essentials-2/">Transcript.</a>
Figure 7.2.9: Types of systems within thermochemistry. Transcript. Image attribution: Thermochemistry heat exchange as thermodynamics study brunch outline diagram. Labeled educational open, closed and isolated systems with mass and heat physical forces type scheme vector illustration. © VectorMine – stock.adobe.com.

Energy studies performed within thermochemistry can involve all of these different types of systems. Understanding what flows from a system to its surroundings and the larger universe, alongside accounting for it, allows us to perform many different thermochemical calculations and experiments.

Key Takeaways

  • Temperature is a measure of the kinetic energy of molecules. As temperature raises, the speed which molecules move does as well.
  • In science, the Kelvin is the preferred measure of temperature when performing calculations. 0 K is known as absolute zero.
  • All bodies in contact will attempt to reach thermodynamic equilibrium.
  • There are three types of heat transfer: conduction (physical contact); convection (fluid currents); and radiation (electromagnetic waves).
  • There are three types of systems: open (matter and energy exchange); closed (only energy exchanged), and isolated (no transfer)


Practice Questions



  1. . Absolute zero is a theoretical concept and cannot physically be achieved.


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