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Skip to 0 minutes and 8 seconds The hydrogen-powered car that we saw earlier gets its energy from reacting together hydrogen and oxygen in a fuel cell to produce water. The energy of this reaction can also be released as heat and light by setting light to hydrogen in air. Where does this energy come from? Chemical energy is a kind of potential energy. As you’ll recall, this is energy something has because of its position or state, rather than its movement. When we talk about a high energy compound in biology or chemistry, we aren’t talking about a hot compound which is rapidly jiggling around like the molecules in a hot gas. Rather, we’re talking about a compound whose structures stores potential energy.

Skip to 1 minute and 0 seconds To understand how, we need to look deeper into the structure of matter beyond what the 19th century thermodynamicists knew into the structures of atoms and molecules. As you probably know, each atom has at its centre a positively charged nucleus, which contains protons. Surrounding the nucleus are the same number of tiny negatively charged electrons. These are kept from flying off into space by their attraction to the positively charged nucleus. This model of the atom reminds us of planets held in orbit around the sun by the force of gravity. Although in reality, quantum mechanics complicates the picture at the atomic scale.

Skip to 1 minute and 48 seconds And remember, it isn’t gravity that holds atoms together– gravity is much too weak– but the attractive electrostatic force between opposite charges. Still, the gravitational analogy is valuable. Electrons can occupy orbitals at different distances from the nucleus, just as planets can be nearer or further from the sun. The electrons that are furthest from the nucleus have the greatest potential energy, just like an object further from the earth has a greater potential energy than one nearer the surface. When atoms bind together to make molecules, the electronic orbitals around the atoms merge together to make molecular orbitals. This means that the electrons are shared by two or more atoms. They act as a negatively charged glue, electrostatically sticking together the positively charged nuclei.

Skip to 2 minutes and 53 seconds The electrons in the molecular orbitals have a lower potential energy than they did when they were orbiting a single atom because they now have more than one positively charged partner. The electrons can lose their potential energy as heat. The more they lose, the more stable the molecule is and the more heat is released. The bonding electrons aren’t always shared equally, however. Atoms of different elements have different sizes and different numbers of protons in their nuclei. The closer an electron can get to a strongly positive nucleus, the stronger the electrostatic force it feels and the stronger the bond is. Oxygen is a small atom with eight protons in its nucleus, and it is highly attractive to electrons in a bond.

Skip to 3 minutes and 49 seconds We say it has a high electronegativity. Given the chance to spend time around oxygen, electrons will jump at it because they’re able to lose a lot of their potential energy and form strong bonds. In a molecule of water, electrons spend most of their time around the oxygen atom, making the molecule polar, a partial negative charge at the oxygen end and a partial positive charge at the hydrogen end. That’s why you saw water molecules sticking to one another in the simulation When molecules react with one another, electrons are rearranged, breaking some bonds and forming others. The difference between the strengths of the bonds in the reactants and the products is released or absorbed as heat.

Skip to 4 minutes and 42 seconds If the products have stronger bonds than the reactants, then heat is emitted because the electrons in the products have lower potential energies than they started with. That’s the case with hydrogen and oxygen reacting together to make water. The polar OH bonds in a mole of water are 286 kilojoules stronger than the bonds that are broken in the reacting gases. And it’s that energy that’s released as heat when hydrogen burns in air.

The energy in chemicals

Chemical energy is potential energy. It is related to the energies of the bonding electrons in covalent compounds.

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Understanding Biological Energy

Royal Holloway, University of London

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