Understand how heat (q) and work (w) affect a system's internal energy (ΔU).
Calculate P–V work done during gas expansion or compression.
Determine changes in enthalpy (ΔH) and internal energy (ΔU).
Identify and explain state functions (values determined only by the system’s state).
Term | Definition |
---|---|
Thermodynamics | Study of energy, heat, and work in systems. |
System | The part of the universe under study (e.g., gas in a cylinder). |
Surroundings | Everything else that can exchange energy with the system. |
Heat (q) | Energy transferred due to temperature difference. |
Work (w) | Energy transferred when a force moves an object (e.g., gas expansion). |
Internal Energy (U) | Sum of potential and kinetic energies of particles in a system. |
Enthalpy (H) | Heat content of a system at constant pressure: H=U+PVH = U + PVH=U+PV. |
State Function | Depends only on the system’s state, not the path taken (e.g., ΔU, ΔH). |
Path Function | Depends on the process/path taken (e.g., q, w). |
First Law of Thermodynamics
ΔU=q+w\Delta U = q + wΔU=q+w
q>0q > 0q>0: heat into the system (endothermic)
w>0w > 0w>0: work done on the system
q<0q < 0q<0: heat out of the system (exothermic)
w<0w < 0w<0: work done by the system
P–V Work (Gas Expansion/Compression)
w=−PΔVw = -P \Delta Vw=−PΔV
PPP in Pascals (Pa)
ΔV\Delta VΔV in m³
Note: Convert L to m³ → 1 L=0.001 m31 \text{ L} = 0.001 \text{ m}^31 L=0.001 m3
Enthalpy Change at Constant Pressure
ΔH=ΔU+PΔV=qp\Delta H = \Delta U + P\Delta V = q_pΔH=ΔU+PΔV=qp
ΔH>0\Delta H > 0ΔH>0: endothermic
ΔH<0\Delta H < 0ΔH<0: exothermic
Type of Transfer | Sign of q or w | Effect on Internal Energy (ΔU) |
---|---|---|
Heat into system | q>0q > 0q>0 | ΔU>0\Delta U > 0ΔU>0 |
Heat out of system | q<0q < 0q<0 | ΔU<0\Delta U < 0ΔU<0 |
Work on system | w>0w > 0w>0 | ΔU>0\Delta U > 0ΔU>0 |
Work by system | w<0w < 0w<0 | ΔU<0\Delta U < 0ΔU<0 |
State Function (depends only on initial & final states) | Path Function (depends on route taken) |
---|---|
Internal energy (U) | Heat (q) |
Enthalpy (H) | Work (w) |
Temperature (T), Pressure (P), Volume (V) | — |
📝 Example: Heating a substance from 25°C to 75°C:
ΔT = 50°C no matter how you got there → state function.
The heat or work involved may differ → path function.
CO₂(s) → CO₂(g)
Heat is absorbed → q>0q > 0q>0
Work is done by the gas → w<0w < 0w<0
Use:
ΔU=q+w\Delta U = q + wΔU=q+w
ΔV=0⇒w=0\Delta V = 0 \Rightarrow w = 0ΔV=0⇒w=0
Then ΔU=qv\Delta U = q_vΔU=qv
If only P–V work:
ΔH=qp\Delta H = q_pΔH=qp
✅ Define internal energy and describe how it changes.
✅ Use ΔU=q+w\Delta U = q + wΔU=q+w to solve word problems.
✅ Calculate work done by or on a gas using w=−PΔVw = -P\Delta Vw=−PΔV.
✅ Identify if a quantity is a state function or not.
✅ Determine if a process is exothermic or endothermic based on sign of qqq or ΔH\Delta HΔH.
✅ Explain why ΔH=qp\Delta H = q_pΔH=qp under constant pressure.
Memorize sign conventions.
Practice problems calculating qqq, www, ΔU\Delta UΔU, and ΔH\Delta HΔH.
Understand examples involving gas expansion, sublimation, or constant volume.
Compare state and path functions with real-world analogies (like altitude vs. distance).
Let me know if you want a quiz or practice problems for Chapter 5.4!
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