ΔH is the change in enthalpy, or the heat exchanged at constant pressure during a chemical reaction or physical change. In Intro to Chemistry, a negative ΔH means heat is released and a positive ΔH means heat is absorbed.
ΔH is the enthalpy change for a process in Intro to Chemistry, and it tells you whether the system gives off heat or takes in heat at constant pressure. If ΔH is negative, the process is exothermic. If ΔH is positive, the process is endothermic.
The cleanest way to think about it is as the heat flow attached to a reaction or phase change. When bonds form, energy is released into the surroundings. When bonds break or particles separate in a way that requires more energy than is released, the system absorbs heat. That heat exchange is what you track with ΔH.
At constant pressure, which is the usual setup for open beakers, test tubes, and most classroom labs, the heat measured by the surroundings equals the enthalpy change of the system. That is why ΔH shows up when you compare temperature changes in a calorimetry lab. If the cup warms up, the reaction was exothermic. If the cup cools down, the reaction was endothermic.
ΔH is a state function, so it depends on the starting and ending conditions, not on the exact path taken. That matters because you can calculate it indirectly with thermochemical equations or Hess’s law instead of measuring every possible route. In class, you may use tabulated enthalpies, bond energy ideas, or reaction equations to find the overall ΔH for a process.
This term also shows up in phase changes and dissolving. Melting, vaporizing, or dissolving a solute in water can absorb or release heat depending on how much energy it takes to separate particles and how much is released when new interactions form. For example, if a salt dissolves and the solution gets colder, the dissolution has a positive ΔH for that process.
ΔH is one of the first thermodynamic numbers you use to connect a visible change, like warming or cooling, to particle-level energy changes. In Intro to Chemistry, that connection comes up in calorimetry, reaction energy diagrams, solution chemistry, and any topic where you compare what the system does with what the surroundings feel.
It also gives you a quick way to classify processes. Exothermic and endothermic are not just labels, they tell you the direction of heat flow. That helps when you read a graph, interpret a lab result, or decide whether a reaction mixture should heat up, cool down, or stay near room temperature.
ΔH matters for dissolution too. Some solutes dissolve with a temperature drop, others with a temperature rise, and that difference comes from the balance between breaking old interactions and forming new solute-solvent interactions. If you can track the sign of ΔH, you can explain why two dissolving processes feel different in the lab.
It also sets up the bigger idea that heat alone does not decide whether a process happens on its own. A process can have a positive ΔH and still be spontaneous if entropy changes make the overall free energy favorable. That is why Intro to Chemistry keeps ΔH linked to spontaneity instead of treating it like the whole story.
Keep studying Intro to Chemistry Unit 16
Visual cheatsheet
view galleryEnthalpy
Enthalpy is the state function that ΔH measures the change in. When you see ΔH in a reaction or a phase change, you are looking at how the enthalpy of the system shifts from reactants to products or from one physical state to another. In practice, the two terms are tightly linked, but enthalpy is the property itself and ΔH is the difference you calculate or measure.
Dissolution Process
The dissolution process is one of the most common places you see ΔH outside of reaction equations. As a solute separates and mixes with a solvent, energy is spent breaking existing attractions and energy is released when new attractions form. The net result can be a positive or negative ΔH, which is why some solutions warm up and others cool down.
ΔG
ΔG connects ΔH to spontaneity, so it answers a different question than enthalpy does. ΔH tells you the heat change, but ΔG tells you whether the process is thermodynamically favorable under a given set of conditions. A process can be endothermic and still have a negative ΔG if the entropy term is large enough.
ΔH_soln
ΔH_soln is the enthalpy change specifically for dissolving a substance. It helps you separate a general heat change from the heat change tied to solution formation. In lab work, this is the value you think about when you compare salts, predict temperature changes, or explain why one solute makes water warmer while another makes it colder.
A quiz question or free-response item usually asks you to identify the sign of ΔH, interpret a temperature change, or connect a graph to exothermic or endothermic behavior. If the container gets warmer, you should link that to negative ΔH for the system, because heat left the reaction mixture and went to the surroundings. If the container gets colder, the process absorbed heat and ΔH is positive.
In a problem set, you may also calculate ΔH from thermochemical data, a Hess’s law setup, or a simple calorimetry table. The move is not just plugging numbers in. You have to keep track of what counts as the system, what counts as the surroundings, and whether the sign should be positive or negative based on the direction of heat flow.
ΔH and ΔG get mixed up because both involve energy changes, but they answer different questions. ΔH is about heat at constant pressure, while ΔG tells you whether a process is spontaneous. A reaction can have a negative ΔH and still not be spontaneous if entropy does not favor it.
ΔH is the enthalpy change for a process, and in Intro to Chemistry it usually means the heat absorbed or released at constant pressure.
A negative ΔH means exothermic behavior, so heat leaves the system and the surroundings warm up.
A positive ΔH means endothermic behavior, so the system absorbs heat and the surroundings cool down.
ΔH shows up in reactions, phase changes, and dissolving, especially when you track temperature changes in a lab.
ΔH tells you about heat flow, but not by itself whether a process is spontaneous.
ΔH is the change in enthalpy, which is the heat exchanged by a system at constant pressure. It tells you whether a reaction or physical change releases heat or absorbs heat. Negative ΔH is exothermic, and positive ΔH is endothermic.
Look at the direction of heat flow. If the reaction mixture gives off heat and the surroundings warm up, ΔH is negative. If the mixture takes in heat and the surroundings cool down, ΔH is positive.
No. ΔH only tells you the heat change, not whether the process happens on its own. Spontaneity depends on ΔG, which combines enthalpy and entropy. A process can be endothermic and still be spontaneous.
When a solute dissolves, energy is used to separate particles and energy is released when the solute and solvent interact. The net heat change is the dissolution enthalpy, and it can be positive or negative depending on which effect is larger.