Register to reply

Determination of the enthalpy of combustion of Magnesium using Hess's law

Share this thread:
Sekminara
#1
Nov23-08, 02:45 AM
P: 2
1. The problem statement, all variables and given/known data

ok so i'm trying to find the change in enthalpy for this reaction:
Mg(s) + (1/2)O2--> MgO(s)

I've done the lab to find the temperatures of certain reactions, these were:
9.70 Celsius for the reaction Mg(s) + 2HCl(aq)--> MgCl2(aq) + H2(l)

7.60 Celsius for the reaction MgO(s) + 2HCl --> MgCl2(aq) + H2O(l) reaction

(and we know the enthalpy of change for the reaction
H2(g) + (1/2)O2(g) --> H2O(l) is 285.8 kJ)


2. Relevant equations

change in enthalpy = mass (100ml/1000ml) * specific heat (4.18) * change in temp (above)


3. The attempt at a solution

using the (above) equation i found that the enthalpy of change for the first reaction was -405 kJ per mol and the second was -318 kJ per mol.

so i built a Hess's law chart and found that [Mg(s) + 2HCl(aq)--> MgCl2(aq) + H2(l)] + [H2(g) + (1/2)O2(g) --> H2] = [MgO(s) + 2HCl --> MgCl2(aq) + H2O(l)] + [Mg(s) + (1/2)O2--> MgO(s)]

so basically the equation look like this: -405 + (-285.8) = -318 + x

therefore x should be -372.8 kJ per mol

but all the answers i've found on the net so far say that the enthalpy of combustion is approximately 602 kJ per mol

help?
Phys.Org News Partner Science news on Phys.org
Experts defend operational earthquake forecasting, counter critiques
EU urged to convert TV frequencies to mobile broadband
Sierra Nevada freshwater runoff could drop 26 percent by 2100
Borek
#2
Nov23-08, 03:49 AM
Admin
Borek's Avatar
P: 23,600
-372.8 kJ/mol agrees with your experimental data, so if it doesn't agree with tables - you have to look for experimental error.
Sekminara
#3
Nov23-08, 03:56 AM
P: 2
dang...ouch 38% error

well...i guess thats what you get when you use Styrofoam insulators...


Register to reply

Related Discussions
Enthalpy of Combustion for Magnesium Biology, Chemistry & Other Homework 8
What is the enthalpy of combustion of magnesium? Biology, Chemistry & Other Homework 3
Determination of enthalpy of solution (solubility/temp stuff) Biology, Chemistry & Other Homework 1
Enthalpy of Combustion - Alkanes Biology, Chemistry & Other Homework 2
Hess's law Chemistry 4