Basically the title: but what about a substance makes it makes it’s specific heat capacity higher or lower than another? From what I understand, SHC is the amount of energy absorbed by a substance in order for it to be raised by 1C. But on a molecular level, what determines this and why is it different between different substances or different states of a substance? eg liquid water has SHC of 4.184J/JC, ice has SPC of 2.11J/JC and vapour has SPC of 2J/JC. Very convoluted question but an explanation would be greatly appreciated 🙂
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it’s actually super complicated, but the short answer is “what does the substance do with energy besides vibrate more?” substances with more and bigger ways to store energy have higher specific heat.
for solids and liquids, this is often a matter of how they bond to the atoms or molecules they’re made of. water is famous for its high heat capacity because of its hydrogen bonding. basically, the hydrogen in water is very attracted to the oxygen in other water molecules, even though it’s already bonded to one. it’s a lot like a spring. pulling those hydrogen bonds apart takes a lot of energy, and that energy is stuck there, not being temperature, until the bond breaks (the water boils) or the energy has somewhere else to go.
Even though different molecules are not bonded together like the atoms in each molecule, there are intermolecular forces between molecules. This is what keeps solids solid. In liquids, these forces keep the molecules together but only loosely. In gases, the intermolecular forces are too weak to keep the molecules close to each other.
In the specific case of a water molecule, there’s two hydrogen atoms and an oxygen atom that are bonded together in the shape of the letter V. Even though the molecule as a whole is electrically neutral, the oxygen atom in a water molecule has some negative charge while the hydrogen atoms have positive charges. Because of this, the oxygen atoms of each water molecule are attracted to the hydrogen atoms of other water molecules. This is called electrostatic force.
Now, temperature describes the average amount of kinetic energy particles have within a substance. Basically, the more the particles vibrate and bounce around, the larger temperature we measure. Forces like electrostatic force make it more difficult for molecules to move around, so we need to use more energy to make them move. Specific heat capacity tells us how much energy we need to make those particles move a specific amount more than they already were moving.
Also, since temperature description the *average* amount of kinetic energy particles have, denser substances generally have higher specific heat capacity simply because there’s more particles in them and all of them require energy to move faster.
One of the biggest factors is the mass of the atoms involved. The lower the atomic mass, the higher the specific heat in inverse proportion. This relationship holds accurately between molecules made of just single atoms, like helium, neon, argon, etc. The relationship also works fairly well amongst metals. For other molecules the structure itself plays a role but, still, two of the compounds with very high specific heats, ammonia (NH3) and water (H2O), are considerably helped by all those very-light hydrogen atoms. On the other hand, uranium, with a very high atomic mass, has an extremely low specific heat.
Fundamentally it comes down to the fact that temperature is a measure of the average energy of the particles (the atoms and molecules). The kinetic energy of a moving particle is proportional to its mass multiplied by the square of its velocity. A particle that is 4 times lighter needs to move 2 times faster to have the same energy.
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