Enthalpy
Enthalpy change (ΔH) is the heat flow at constant pressure, which is how most lab reactions run. ΔH is negative for exothermic reactions (heat released) and positive for endothermic reactions (heat absorbed).
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Analogy
A hot water bottle gives out heat (exothermic). An ice pack takes heat in (endothermic).
Example
CH₄ + 2O₂ → CO₂ + 2H₂O(l)ΔH = −890 kJ/molNegative: heat is released
In practice
Burning methane has a large negative ΔH, which is why natural gas heats homes.
Practice
Flashcards
- What sign does ΔH have for an exothermic reaction?Negative: heat is released.
Interview questions
- Use standard enthalpies of formation to calculate ΔH for burning methane, and say if it is endothermic or exothermic.
- ΔH = ΣΔHf°(products) − ΣΔHf°(reactants)
- Use CO₂ −393.5, H₂O(l) −285.8, CH₄ −74.8
- Result about −890 kJ/mol
- Negative, so exothermic
- Describe how a coffee-cup calorimeter finds the enthalpy of a neutralisation reaction.
- Measure the temperature change of the solution
- Use q = mcΔT
- Heat gained by water = heat lost by reaction
- Divide by moles reacted to get ΔH per mole
Questions people ask
What is the difference between ΔU and ΔH?
ΔU is the heat flow at constant volume, and ΔH is the heat flow at constant pressure. They differ by the work of gas expansion: ΔH = ΔU + PΔV. For reactions with no change in the number of gas molecules, the two values are almost the same.
What does a thermochemical equation tell you?
It gives the balanced equation together with its ΔH for the amounts shown. If you double the equation, ΔH doubles. If you reverse it, the sign of ΔH flips. State symbols such as (g), (l) and (s) matter, because changing the physical state changes ΔH.