Skip to 0 minutes and 3 seconds So here we are in the Central Teaching Hub of the University of Liverpool. There’s a lab class going on around me at the moment, but I’d like us just to focus for a moment on this bomb calorimeter here. There are trillions of cells in our body undergoing thousands of processes every day– breaking down energy, converting molecules. The energy comes from our food, and we can measure the energy content of our food using a bomb calorimeter like this one here. We can place a piece of food in this little grip here and put it inside the bomb, seal it up, and then we place the bomb inside the calorimeter surrounded by a bath of water.
Skip to 0 minutes and 46 seconds And we use the first law of thermodynamics to assess the energy released during combustion by measuring the temperature rise of the water that surrounds the bomb. If we combust the food in a fixed volume like this, then we’ll get one result. If we do it with a fixed pressure and allow the cylinder to expand a little bit, then we’ll get a slightly different answer. And that’s because if we use the same fuel and the same oxygen, we’ll get the same release of energy, but the CO2 and the water from the combustion occupy more space than the oxygen and the fuel at the beginning, and so the cylinder would expand if allowed to.
Skip to 1 minute and 22 seconds And some of the energy is used as work to form that expansion, and so less is available as internal energy that we measure as a temperature rise in the water. Enthalpy is what we use to assess those changes. So enthalpy is the total energy of the system– it’s the internal energy plus the pressure times the volume inside the calorimeter. It’s really an accounting device - it doesn’t have a physical meaning, except for a system that’s free to expand. So the heat released by combustion is normally known as Δ H, the change in enthalpy of a fuel.
Skip to 1 minute and 57 seconds It’s often more convenient for us to use open furnaces, and so we’re interested, then, in the enthalpy change that occurs in those open furnaces. For example, if we combust one litre of petrol, the enthalpy released is about 33 megajoules. And the difference between the enthalpy change and the internal energy change is only about 130 kilojoules. So not much is lost by making the combustion process occur in an open furnace– it’s about 0.4% loss. It’s useful to know the definition of enthalpy and internal energy in terms of temperature change and the specific heat capacities.
Skip to 2 minutes and 35 seconds So the specific heat capacity at constant volume is the gradient of the plot of internal energy versus temperature, whereas the heat capacity at constant pressure is the gradient of the plot of enthalpy versus temperature.
Bombs and enthalpy
Eann visits the Central Teaching Laboratory in Liverpool to talk about bomb calorimeters which are used to measure the energy content of our food. He also talks about enthalpy and its connection to specific heat capacity.
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