Heat of Formation: Is Q Always Positive?

  • Thread starter Thread starter angelababyy
  • Start date Start date
  • Tags Tags
    Formation Heat
Click For Summary
SUMMARY

The discussion centers on the heat of formation in the context of an exothermic reaction involving magnesium and hydrochloric acid. The participant calculated a theoretical heat of formation of -461.96 kJ/mol but obtained a value of 386.1 kJ/mol in their trials. The participant questions whether the heat (Q) can ever be negative and seeks clarification on the definitions of exothermic and endothermic reactions. It is established that an exothermic reaction releases heat, leading to a temperature increase in the solution, contrary to the participant's assumption about temperature changes.

PREREQUISITES
  • Understanding of thermodynamics concepts, specifically heat transfer.
  • Familiarity with the equation Q=ms∆T and its components.
  • Knowledge of exothermic and endothermic reactions.
  • Basic laboratory skills for measuring temperature changes in chemical reactions.
NEXT STEPS
  • Study the principles of thermodynamics, focusing on heat transfer in chemical reactions.
  • Learn about the calculations involved in determining heat of formation for various substances.
  • Explore the differences between exothermic and endothermic reactions in detail.
  • Investigate laboratory techniques for accurately measuring temperature changes during chemical reactions.
USEFUL FOR

Chemistry students, educators, and laboratory technicians involved in thermodynamics and chemical reaction studies.

angelababyy
Messages
1
Reaction score
0

Homework Statement


When using the equation, Q=ms∆T, is the heat always positive?

Homework Equations


heat of formation = q/mol (but of course the moles used in this lab is a positive number)

The Attempt at a Solution


I'm working on the energy of magnesium lab where magnesium ribbon contacts and reacts with hydrochloric acid. The theoretical heat of formation is -461.96 kJ/mol, but I got 386.1 kJ/mol of Mg as the largest amount (I'm turning in three trials).
m & s are given as 100 g and 4.184 J/g Celsius respectively. Therefore, the only variable that would make the energy negative would be the temperature change. However, if the temperature change was negative that means that the temperature cooled down. I don't think an exothermic reaction would have the solution cool down.
 
Last edited:
Physics news on Phys.org
What's the definition of exothermic?

(Hint: How does exo- differ from endo-thermic?)Also, since you're doing this in a lab - aren't you measuring the temperature of the solution?
 

Similar threads

Chemistry Understanding the calculation of enthalpy of the formation of benzene
  • · Replies 3 ·
Replies
3
Views
3K
Chemistry Does Combusting Different Amounts of Propane Change Final Temperature?
  • · Replies 5 ·
Replies
5
Views
2K
Calculating Heat Released During Water Freezing
  • · Replies 1 ·
Replies
1
Views
2K
Calculating S-F Bond Energy in SF6 using Standard Heat of Formation Values
  • · Replies 3 ·
Replies
3
Views
3K
Chemistry Thermodynamics: The molar enthelpy of a solution
  • · Replies 2 ·
Replies
2
Views
2K
Constant pressure heat of reaction -> constant volume q
  • · Replies 3 ·
Replies
3
Views
6K
How to calculate heat from enthelpy of reaction
  • · Replies 5 ·
Replies
5
Views
3K
Chemistry Calculation of bond energy of oxygen gas at 0K
  • · Replies 2 ·
Replies
2
Views
2K
How Do You Calculate Combustion Energy in Bomb Calorimetry?
  • · Replies 3 ·
Replies
3
Views
2K
Chemistry How Does Doubling Zinc Mass Affect Temperature Change in ZnO Formation?
  • · Replies 3 ·
Replies
3
Views
3K