Difference between 'q' and 'ΔH' in thermochemistry?

Click For Summary
SUMMARY

The primary distinction between 'q' and 'ΔH' in thermochemistry lies in their definitions: 'q' represents the amount of heat transferred, while ΔH denotes the change in enthalpy, encompassing total energy changes, including heat, pressure/volume work, and entropy. ΔH equals q only under conditions of constant pressure and volume, with no entropy changes. Additionally, Gibbs' free energy (ΔG) is defined as the difference between enthalpy and entropy, reflecting the useful energy available for work in the system.

PREREQUISITES
  • Understanding of thermodynamic concepts such as enthalpy and entropy
  • Familiarity with the first law of thermodynamics
  • Knowledge of Gibbs' free energy and its significance
  • Basic principles of heat transfer in chemical reactions
NEXT STEPS
  • Study the relationship between enthalpy and entropy in thermodynamic processes
  • Learn about the conditions under which ΔH equals q in chemical reactions
  • Explore the implications of Gibbs' free energy in reaction spontaneity
  • Investigate the first law of thermodynamics and its applications in heat transfer
USEFUL FOR

Chemistry students, thermodynamics researchers, and professionals in chemical engineering seeking to clarify the concepts of heat transfer and energy changes in thermochemical processes.

zorro
Messages
1,378
Reaction score
0
What is the basic difference between 'q' and 'ΔH' in thermochemistry? I get confused between them!
Is there any criteria for ΔH to become equal to q?
 
Chemistry news on Phys.org


Abdul Quadeer said:
What is the basic difference between 'q' and 'ΔH' in thermochemistry? I get confused between them! Is there any criteria for ΔH to become equal to q?

Q is an amount of heat being transferred and only heat. The change in enthalpy (ΔH), is the change in total energy of the system. That includes heat, but also pressure/volume work and entropy.

Gibbs' free energy, ΔG, is a measure of the change of the useful (i.e. work-producing) energy of the system, given no change in temperature or pressure. So it's simply the enthalpy minus the entropy.

So the heat transferred to a system in a reaction, Q, equals ΔH only if there is no change in pressure/volume or entropy. (You also neglect how the change in temperature caused by the heat from ΔH changes ΔH itself)
 

Similar threads

When Should I Apply ΔH After Calculating Q in Thermodynamics?
  • · Replies 2 ·
Replies
2
Views
1K
Why Does Thermochemistry Equate Q of Solution to Negative Q of Reaction?
  • · Replies 1 ·
Replies
1
Views
2K
When do you use q=mc(Tf-Ti) versus q=c(Tf-Ti) in thermochemistry calculations?
  • · Replies 2 ·
Replies
2
Views
2K
Understanding the Relationship Between ΔH and q in Thermodynamics
  • · Replies 12 ·
Replies
12
Views
76K
What is enthelpy for an adiabatic process
  • · Replies 8 ·
Replies
8
Views
27K
Gibbs Free Energy Equation: why is the entropy change of the system not q/T ?
  • · Replies 2 ·
Replies
2
Views
3K
Classical Find the Best Thermodynamics Books for Clear Insight
  • · Replies 4 ·
Replies
4
Views
3K
Where does ΔH fit into this? is it interchangeable with Q?
  • · Replies 1 ·
Replies
1
Views
3K
Thermochemistry relating delta H and q of surroundings
  • · Replies 1 ·
Replies
1
Views
2K
Understanding Time Measurement in Physics: Uncertainty and Calculations
  • · Replies 6 ·
Replies
6
Views
2K