We all take these super-cool appliances for granted, but without the power of Chemistry, refrigerators would not exist!
If you’re preparing for your IB Chemistry exams, it’s very important that you have a firm understanding of how refrigerators work and what their environmental impact is.
Here at Save My Exams, we know that learning all these fridge facts is easier said than done. But help is on hand, in the form of our IB Chemistry tutor team.
They’ve scoured your exam syllabus in order to put together this simple guide that includes all of the essential exam knowledge. So grab a cold drink, chill out, and let’s study together.
It all boils down to state changes
The key to understanding the refrigeration process is to remind yourself about the changes of state that you learnt about in Topic 1 of the Chemistry IB syllabus (something that frequently confuses students). This diagram shows the flow of energy when substances turn from liquids to gases, and vice-versa.
States of matter diagram.
Melting and boiling involves separating particles, so energy flows in from the surroundings.
Freezing and condensing involves new forces being created between particles, which releases heat energy to the surroundings.
The role of refrigerants
Inside your fridge, you’ll find a substance called a refrigerant. Modern fridges use a material called HFC-134a, which chemists call 1,1,1,2-tetrafluoroethane.
The characteristics of a good refrigerant include:
- Low boiling point
- High heat of vaporization
- Low toxicity
HFC-134a boils at -26.3°C and ticks most of the boxes.
In liquid HFC-134a, the molecules are attracted to each other by intermolecular forces. These forces must be overcome in order to separate the molecules, and this is achieved using heat energy. The energy is used to break the attractive forces as the substance transitions from liquid to gas.
So how does my refrigerator keep food cool?
Now we need to combine our knowledge of state changes and refrigerant properties to understand exactly how a fridge functions.
Let’s consider how the system works.
The first diagram below shows the cycle of refrigeration, and the second shows the flow of heat from the outside to the inside of the fridge. (The numbers 1-4 correspond between the diagrams)
Diagram of a fridge (rear view)
The expansion-compression cycle of a fridge
Stage 1: The refrigerant is compressed in vapour form (which requires an energy input)
Stage 2: The compressed refrigerant is forced into a liquid state and travels through tubes at the back of the fridge. As it changes state, heat energy is released as intermolecular forces are reformed, so the refrigerant gets hot. This is why you need a cooling panel at the back of the fridge to dissipate the heat.
Stage 3: The refrigerant enters the inside of the fridge, where it evaporates (becomes a vapour). This process requires an energy input from the environment, which means that the inside of the fridge is cooled (because the refrigerant has used the thermal energy to break intermolecular forces and evaporate). This is a bit like what happens when you put a few drops of alcohol on your skin- it feels cold as the alcohol absorbs heat from your skin as it evaporates.
Stage 4: The refrigerant vapour is pumped back to the compressor, completing the cycle. The cycle is driven by the electric pump at the back of the fridge. If you leave the door open, the thermostat will kick in and trigger the pump into switching on. This will produce more cold air inside the fridge, but unfortunately, also more hot air at the back of the fridge at the same time. So you can’t use your fridge to keep your kitchen cool!
What about air conditioner units?
Air conditioners work in exactly the same way, but with a key difference in that the heat and cooling parts of the cycle are physically separated. You may have seen the heating units placed outside or on top of a building.
Are fridges bad for the environment?
That’s a yes and no. As an IB student , you are expected to know that the negative environmental impacts of refrigeration and air conditioning systems are significant. Aside from the fact that cooling units consume energy, the use of CFCs (chlorofluorocarbons) as refrigerants has been a major contributor to ozone depletion.
In 1987, the Montreal Protocol was a treaty designed to phase out the use of CFCs – which scientists found had been causing great damage to the ozone layer, a protective barrier in the stratosphere that screens out a lot of the sun’s harmful ultraviolet radiation. CFCs from old fridges and air conditioners break down in the upper atmosphere and release chlorine radicals:
Ozone naturally breaks down in UV light, but it is re-generated when oxygen radicals combine with oxygen molecules:
This maintains a constant level of ozone in the atmosphere in a process called the Chapman cycle
These chlorine radicals are very reactive and scavenge around for electrons which they can find from ozone molecules, breaking them down in a chain reaction:
One chlorine radical can irreversibly damage thousands of ozone molecules, which is really bad news for humans down on Earth. Increased levels of UV can cause skin cancer, cataracts and great harm to wildlife.
The good news is that since the Montreal Protocol the rate of ozone depletion has been falling. This has been achieved by using more ozone friendly refrigerants like HFC-134a. The bad news is that HFC-134a has a Global Warming Potential (GWP) of 1420, meaning it is 1420 times more potent than carbon dioxide in warming the atmosphere. Luckily, the amounts of HFC-134a released into the atmosphere are tiny, so it’s not just the GWP of a gas that is the problem, but the amount present as well.
Chemists are working on phasing out HFC-134a and looking for replacements that are both ozone and global warming friendly. That is part of the challenge that will be faced by the next generation of IB Chemists – could you be the person who discovers the solution?
Ready to test your knowledge? Explore our brand new collection of Chemistry IB Topic Questions for the SL course, and check your technique and understanding against our teacher-written Model Answers.
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