Enthalpy facts for kids
Enthalpy is a scientific idea used to figure out how much heat and work is added to or taken away from something. It's especially useful when things change at a steady pressure. The word "enthalpy" comes from the Greek word "enthalpos," which means "to put heat into." A Dutch scientist named Heike Kamerlingh Onnes first came up with this idea and the word in 1909.
Enthalpy is a bit like energy, but it's not exactly the same. When a substance changes, like expanding or shrinking, energy is used or released. Enthalpy helps scientists keep track of this energy, especially the part related to changes in volume. That's why scientists often look at the change in enthalpy instead of just the change in energy.
Imagine you have a fixed amount of gas in a container. If you add heat to this gas, two main things can happen:
- The gas's internal energy can go up, making it hotter.
- The gas can expand, doing work on its surroundings.
Enthalpy helps us measure both of these things together. It's like a total package that includes the gas's internal energy plus any work it does by changing its volume. The formula for enthalpy (H) is: H = E + (P x V) Here, H is enthalpy, E is the internal energy, P is the pressure, and V is the volume.
Contents
Enthalpy and Chemical Reactions
When a chemical reaction takes place, substances can either get warmer or colder. If they get warmer, they release heat to their surroundings. If they get colder, they absorb heat from their surroundings. If the pressure stays the same during this process, the amount of heat released or absorbed tells us how much the enthalpy changed.
Exothermic Reactions: Releasing Heat
Let's say you burn some gasoline in the open air. This reaction releases a lot of heat, making the surroundings warmer. If 100 kilojoules (kJ) of heat were released, it means the enthalpy of the gasoline decreased by 100 kJ. So, the change in enthalpy (written as ∆H) for this reaction would be –100 kJ.
When a chemical reaction gives off heat, it's called an exothermic reaction. In these reactions, the enthalpy of the substances involved goes down, so the value of ∆H is negative. Think of it like losing heat.
Endothermic Reactions: Absorbing Heat
On the other hand, some chemical reactions absorb heat from their surroundings, making things cooler. This is called an endothermic reaction. In these reactions, the enthalpy of the substances goes up, so the value of ∆H is positive. It's like gaining heat.
Related pages
Images for kids
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T–s diagram of nitrogen. The blue curves are isenthalps (curves of constant enthalpy). The values are indicated in blue in kJ/kg.
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A diagram showing throttling, where fluid moves from point 1 to point 2.
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A diagram showing a compressor, where fluid enters at point 1 and leaves at point 2.
See also
In Spanish: Entalpía para niños
