Heat and enthalpy show up together in almost every thermochemistry passage, and they’re easy to blur together! Both come in joules, and both tell you whether a reaction takes in energy or gives it off.
In short, heat (q) is energy moving between things at different temperatures. Enthalpy (H) is a property of the system, and at constant pressure its change, ΔH, equals the heat the system absorbs or releases.
We’ll walk through heat vs enthalpy one piece at a time, with the formulas, the sign conventions and worked examples, plus the thermochemistry MCAT traps that cost students points. Let’s get started!
Heat vs Enthalpy: What’s the Difference?
Heat is energy in transit. It exists only while it’s flowing from a hotter object to a cooler one, so a system can absorb heat or release it, and it never has a stored amount of heat.
Enthalpy is something the system has at every moment, like its temperature or volume. You can’t measure H itself, only its change (ΔH) between a starting state and a final one.
At constant pressure, with only pressure-volume work, ΔH = q.
Thermochemistry falls under general chemistry, which the AAMC estimates at about 30% of the Chem/Phys section. Our thermochemistry overview maps out the rest of the unit if you’d like the bigger picture!
What Is Heat (q)?
Heat is the energy that moves between objects because of a temperature difference. It always flows from the hotter object to the cooler one, and it stops once they reach the same temperature (thermal equilibrium).
Set a pot of water on a stove, and heat flows from the burner into the pot and then into the water. Heat is measured in joules (J) or calories (cal).
Heat vs temperature
Temperature measures the average kinetic energy of the particles in a sample. Heat is the energy that changes hands. A bathtub of warm water holds more thermal energy than a cup of hot coffee, even though the coffee has the higher temperature!
Adding heat usually raises the temperature. During a phase change it doesn’t, and we’ll come back to that with the heating curve!
How heat is transferred
Heat travels in three ways.
- Conduction passes heat through direct contact. A metal spoon heats up in a pot of boiling soup.
- Convection carries heat with a moving fluid (a liquid or a gas). Warm air rises and cool air sinks in a room.
- Radiation carries heat as electromagnetic waves, with no medium needed. Sunlight warms the Earth across empty space!
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Calculating Heat With q = mcΔT
To find the heat that changes a sample’s temperature, use the formula
q = mcΔT,
where m is the mass, c is the specific heat and ΔT is the final temperature minus the initial one.
Specific heat is the energy it takes to raise 1 g of a substance by 1 °C, and a 1 K step is the same size.
Water’s specific heat is 4.18 J/(g·°C), or 1 cal/(g·°C). The MCAT usually gives you c, but knowing water’s value by heart saves time!
How much heat does it take to warm 200 g of water from 20 °C to 45 °C?
ΔT = 45 °C − 20 °C = 25 °C, so q = (200 g)(4.18 J/(g·°C))(25 °C) = 20,900 J, or about 20.9 kJ. The sign is positive, because the water absorbs heat. Nice work if you got there first!
If the sample cools, ΔT is negative, so q comes out negative too. That minus sign means heat left the sample!
In a calorimetry problem, the heat one object loses equals the heat the other gains, so qhot = −qcold.
Keep in mind that q = mcΔT only works while the temperature is changing! During melting or boiling, you’ll need q = mL (more on that below).
What Is Enthalpy (H)?
Enthalpy (H) equals a system’s internal energy (U) plus the product of its pressure and volume (PV).
H = U + PV
Internal energy is the kinetic and potential energy of the particles in the system. The PV term is the energy it took to make room for the system by pushing its surroundings out of the way.
Chemists can only measure the change in enthalpy, ΔH = Hproducts − Hreactants. That’s the number you’ll see in MCAT passages, usually in kJ/mol!
Endothermic and exothermic reactions
A positive ΔH means the system absorbs heat, so the process is endothermic. A negative ΔH means it releases heat, so the process is exothermic.
Burning methane is the classic exothermic example! CH4 + 2O2 → CO2 + 2H2O releases about 890 kJ per mole of methane, so ΔH ≈ −890 kJ/mol.
Forming nitric oxide goes the other way. N2 + O2 → 2NO absorbs heat, so its ΔH is positive. In the atmosphere, lightning supplies that energy!
State Functions vs Path Functions
A state function depends only on where a system starts and where it ends. A path function depends on the route it took between them.
Both routes in the graphic take the system from the same start to the same finish. Route A takes in 300 J of heat and does 100 J of work on the surroundings.
Route B takes in 250 J and does 50 J. Both end with ΔU = +200 J, so the energy change matches while the heat doesn’t!
Hess’s law works because H is a state function
Since enthalpy is a state function, you can add up the ΔH of each step to get the ΔH of the overall reaction. That’s Hess’s law!
Carbon can burn straight to CO2 (ΔH = −393.5 kJ), or it can go through CO first. C + ½O2 → CO has ΔH = −110.5 kJ, and CO + ½O2 → CO2 has ΔH = −283.0 kJ.
Add the two steps and you get −393.5 kJ, the same as the one-step route!
Each of those steps runs at constant pressure, so its heat matches its ΔH. Run a step at constant volume and q changes, while ΔH stays put.
(Optional) If you’d like to practice spotting state and path functions in full passages, our Chem/Phys & Bio/Biochem Mastery Course has 100+ strategy lessons from 130+ scorers, including 70+ AAMC passage walkthroughs.
Sign Conventions for q, w and ΔH
Every sign is written from the system’s point of view. Heat flowing into the system is positive, and heat flowing out is negative.
The first law of thermodynamics says energy is conserved. It moves between the system and its surroundings or changes form, and the total stays the same.
Chemistry texts write it as ΔU = q + w, where w is work done on the system.
Physics texts write ΔU = Q − W, where W is work done by the system. The two agree, because work done on the system is the negative of work done by it.
Why Constant Pressure Makes ΔH Equal Heat
At constant pressure, ΔH = ΔU + PΔV. In the chemistry form of the first law, expansion work is w = −PΔV, so ΔU = q − PΔV. Put that in, and the PΔV terms cancel to leave ΔH = q.
That’s often written qp, the heat at constant pressure, and it holds when pressure-volume work is the only work done.
Most reactions on the MCAT run open to the air at constant pressure, so you can usually treat the heat of reaction and ΔH as the same number!
Coffee-cup vs bomb calorimeters
ΔH and ΔU differ by the PΔV term. For reactions in solution that gap is small, and it grows when a reaction changes the number of moles of gas.
Phase Changes, Latent Heat and Enthalpy
When ice melts, it absorbs heat while its temperature stays at 0 °C. That energy goes into pulling the water molecules apart, so ΔT = 0, and q = mcΔT can’t describe it.
For a phase change, use q = mL, where L is the latent heat for that change. At constant pressure, latent heat is the enthalpy change per gram.
L for melting is the enthalpy of fusion (ΔHfus), and L for boiling is the enthalpy of vaporization (ΔHvap).
Melting and boiling absorb heat (ΔH > 0). Freezing and condensing release the same amount, so their ΔH is negative.
How much heat melts 50 g of ice at 0 °C? Use L = 334 J/g.
That gives q = (50 g)(334 J/g) = 16,700 J, or about 16.7 kJ. In molar terms, 50 g is 2.78 mol, and 2.78 mol × 6.01 kJ/mol ≈ 16.7 kJ, the same answer!
Our thermodynamics of phase changes notes cover phase diagrams and heating curves in more depth!
Heat vs Enthalpy Traps on the MCAT
In heat vs enthalpy questions, a negative ΔH tells you a reaction releases heat. It doesn’t tell you whether the reaction happens on its own. That depends on Gibbs free energy, ΔG = ΔH − TΔS.
Ice melting at room temperature is endothermic (ΔH > 0), and it still happens on its own, because the entropy gain (TΔS) outweighs ΔH. Our Gibbs free energy notes and entropy notes go further if you’re curious!
Our heat calculation notes have more multistep problems if you’d like extra practice.
(Optional) If you’d like to work through thermochemistry passages with a top scorer, our 6-Week Live MCAT Accelerator runs 12 live strategy classes across every MCAT section, taught by a 520+ scorer.
Heat vs Enthalpy Practice Questions
Try these before you open the answers :)
Question 1. Which of the following is an example of heat transfer by convection?
A. A metal spoon heating up in hot soup
B. Warm air rising and cool air sinking in a room
C. Heat from the sun warming your face
D. Ice melting in a glass of water
Show the answer
B. Convection moves heat with a flowing fluid. Warm air rising and cool air sinking in a room is convection. The spoon is conduction, and the sun is radiation!
Question 2. What is the sign of the enthalpy change (ΔH) for an exothermic reaction?
A. Positive
B. Negative
C. Zero
D. It can’t be determined
Show the answer
B. An exothermic reaction releases heat, so ΔH is negative.
Question 3. A reaction in a coffee-cup calorimeter gives off 5.0 kJ of heat. What is its enthalpy change?
A. +5.0 kJ
B. −5.0 kJ
C. 0 kJ
D. It can’t be found without the volume
Show the answer
B. A coffee-cup calorimeter runs at constant pressure, so q = ΔH. Heat left the system, so ΔH = −5.0 kJ. Nice work if you caught the constant pressure!
Question 4. In an insulated cup, 100 g of water at 80 °C is mixed with 100 g of water at 20 °C. What is the final temperature?
A. 40 °C
B. 50 °C
C. 60 °C
D. 100 °C
Show the answer
B. The heat the hot water loses equals the heat the cool water gains. With equal masses and the same c, (80 − T) = (T − 20), so T = 50 °C, right in the middle!
Question 5. Which of the following is a path function?
A. Enthalpy
B. Internal energy
C. Heat
D. Temperature
Show the answer
C. Heat depends on the route between two states. Enthalpy, internal energy and temperature depend only on the states themselves.
If these feel shaky, 1:1 MCAT tutoring with a 515+ scorer can walk you through the reasoning behind each one!
Heat vs Enthalpy Terms to Know
Our thermodynamic terminology notes define systems, processes and the rest of the vocabulary, if you’d like more!
Heat vs Enthalpy FAQs
Is enthalpy the same as heat?
No. Heat is energy moving because of a temperature difference, and enthalpy is a property of the system. At constant pressure, the change in enthalpy (ΔH) equals the heat absorbed or released, which is why the two get paired in heat vs enthalpy questions so often!
Is ΔH heat?
ΔH equals the heat transferred at constant pressure, when pressure-volume work is the only work. At constant volume, the heat equals ΔU instead.
What’s the difference between enthalpy and temperature?
Temperature measures the average kinetic energy of the particles. Enthalpy also depends on how much substance there is, and during melting or boiling it rises while the temperature stays flat!
Is enthalpy the same as thermal energy?
Thermal energy is the kinetic energy of the particles, which is part of the internal energy (U). Enthalpy adds the PV term on top of all of U, so H = U + PV.
Is latent heat the same as enthalpy?
At constant pressure, yes. Latent heat is the enthalpy change of a phase change per gram, like 334 J/g for melting ice. ΔHfus is the same quantity per mole, about 6.01 kJ/mol!
What’s the difference between heat of reaction and enthalpy of reaction?
Heat of reaction is the heat a reaction gives off or takes in. At constant pressure, it equals the enthalpy of reaction, ΔHrxn, so textbooks often use the two names for the same number.
Final Thoughts on Heat vs Enthalpy
In heat vs enthalpy, heat is energy on the move, and enthalpy is a property the system carries. Hold on to two ideas. At constant pressure, ΔH = q, and every sign is written from the system’s point of view!
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Supplemental Resources
Want to go deeper on this topic? Here's where to read, watch and practice it next.
Read
- Kaplan MCAT General Chemistry Review 2026-2027, §7.3 and §7.4
- The Princeton Review MCAT Physics and Math Review, section 7.4
- The Princeton Review MCAT General Chemistry Review, sections 7.1 through 7.4
Watch
- “Enthalpy” (Khan Academy)
- “Heat transfer” (Khan Academy)
Official AAMC practice that tests this topic
- Chemistry Question Pack, Passage 11, Q62
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