Heat of reaction in constant pressure calorimeter (Chemistry)

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SUMMARY

The discussion centers on calculating the final temperature of a reaction mixture in a constant pressure calorimeter involving a 30.0 mL sample of 0.937M A and 16.7 mL of 0.904M B, with a reaction enthalpy (ΔH) of 41.2 kJ. The initial temperature of both solutions is 24.9 degrees Celsius. The user calculated the final temperature as 28.1 degrees Celsius, but the expected result is 21.7 degrees Celsius, indicating an error in the calculations. The limiting reagent was identified as B, and the user seeks clarification on whether the reaction is exothermic or endothermic.

PREREQUISITES
  • Understanding of thermochemical equations and reaction enthalpy.
  • Knowledge of calorimetry principles, specifically constant pressure calorimetry.
  • Familiarity with the equation q = mcΔT for heat transfer calculations.
  • Ability to identify limiting reagents in chemical reactions.
NEXT STEPS
  • Review the concept of enthalpy changes in chemical reactions.
  • Learn about the principles of constant pressure calorimetry.
  • Study the calculation of heat transfer using specific heat capacity.
  • Examine examples of exothermic and endothermic reactions to understand temperature changes.
USEFUL FOR

Chemistry students, educators, and anyone involved in thermodynamics or calorimetry who seeks to understand reaction heat calculations and the implications of enthalpy changes.

mwamba
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Homework Statement


A 30.0mL sample of 0.937M A is mixed with 16.7mL of 0.904M B in a constant pressure calorimeter. The thermochemical equation for this reaction is:
A(aq) + B(aq) --> C(aq) + D(aq); ΔH = 41.2kJ

Both solutions are at 24.9 degrees Celsius prior to mixing and reacting. What is the final temperature of the reaction mixture? The density of all solutions is 1.00 g/mL and the specific heat is 4.184 J/gC.

Homework Equations


q = mcΔT where m is mass, c is specific heat and ΔT is the change in temperature.


The Attempt at a Solution



q = (30mL + 16.7mL) * 1.00g/mL * 4.184 J/gC * (xC - 24.9C) where C stands for Celsius and x is the final temperature (what we want to find).

Finding limiting reagent:
?mL A = 16.7mL B * (1L/1000mL) * ((0.904mol B)/1L) * ((1mol A)/0.937)*1000mL
= 16.11mL A < 16.7mL B so B is the limiting reagent

Finding q:
41200J = 1mol B * qJ/0.0151
q = 622.12J

Substituting q into the original equation to find x:


622.12J = (30mL + 16.7mL) * 1.00g/mL * 4.184 J/gC * (xC - 24.9C)
x = 28.1 degrees Celsius

This is apparently wrong. The answer is supposed to be 21.7 degrees Celsius. Does anyone see where I went wrong?
 
Physics news on Phys.org
Sign. Is the reaction exothermic (temperature goes up) or endothermic (temperature goes down)?
 

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