Video 7.2.1: Collision Theory and Rates of Reaction. An introduction into Collision Theory. Video attribution: “Collision Theory and Rates of Reaction” by RMIT Library. © 2024 RMIT Library, licensed under CC BY-NC-SA 4.0.

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.

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).

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.

Conduction

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).

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.

Convection

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?

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.

Radiation

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.

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.

Systems of Heat

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.

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.