# Chemical Forums

## Chemistry Forums for Students => High School Chemistry Forum => Topic started by: daisychain on April 26, 2013, 06:28:13 AM

Title: Hess' Law Enthalpy Changes Literature Values
Post by: daisychain on April 26, 2013, 06:28:13 AM
Hi there, I am suppose to calculate the enthalpy changes of Na2CO3 (anhydrous sodium carbonate) and NaHCO3 (sodium hydrogencarbonate), I have calculated them ( I think they are right, if they are not please help)
The two reactions carried out are
A. 2HCl (aq) + Na2CO3(aq) → 2NaCl (aq) + CO2(g) + H2O(l)
B. HCl (aq) + NaHCO3(aq) → NaCl (aq) + CO2(g) + H2O(l)

I then am suppose to use hess's law, combining the results to obtain the enthalpy of reaction
2NaHCO3(aq) ) → Na2CO3 (aq) + CO2(g) + H2O(l)

Now I will show you my working for just the enthalpy change of Na2CO3 (because its really similar to NaHCO3)

A 100 cm3 of 2 mol dm-3 hydrochloric acid was poured into a polystyrene cup. 8 grams of anhydrous sodium carbonate was added with.

Initial temperature was 20.0 C and final temperature was 25.0 C.

(i) number of moles of HCl
1000 cm3 HCl solution contain 1 mol of HCl
Number of moles of HCl= MV/1000
M= molarity of the solution (mol dm-3)
V= volume of the solution (dm-3)
(2 x 100)/1000= 0.2 mol

(ii) number of moles of Na2CO3
n= m/M
m= given mass (g)
M= molar mass(g mol-1)

M= (2.00 x 22.99) +(1.00 x 12.01) + (3.00 x 16.00)= 106.0 g mol-1

n= 8.00/106.0= 0.0755 moles (3 s.f.)

The ratio is 2 :1 so which means Na2CO3  is the limiting reactant.

(iii) Delta temperature
TFINAL-TINITIAL
25.0 ˚C - 20.0 ˚C= 5.0˚C (±1 ˚C)

(iv) Heat released during reaction
Q= mc ΔT

m= 100 cm3= 100 ml
Density of water = 1 g mol
So in 100 ml, its 1 g mol x100 ml = 100 g

(100 g)(4.18 J/K.C)(5.0˚C)= 2090 Joules
Q= 2.09 kJ

ΔH=(-Q)/n
ΔH= -2.09/0.0755 = -27.7 kJ/ mol Na2CO3 (3 s.f.)

ΔH= -27.7 kJ/ mol-1 (± 0.02) Na2CO3 (3 s.f.)

The enthalpy change I calculated for NaHCO3 (with the same method) I got ΔH= -22.5 kJ/ mol-1  (±0.03) NaHCO3

However I have to find a literature value and I've found one in the textbook Chemistry International Baccalaureate by John Green and Sadru Damji

The literature value in the book for enthalpy change is NaHCO3 is -140 kJ/ mol-1 and for Na2CO3 -370 kJ/mol-1

My values are very off from the literature value, so I don't know if I did something wrong  ??? but I thought I calculated it right... Please tell me if I did something wrong in my calculations and how to help fix them, because I first need to have the right calculations to combine the two to find the2NaHCO3(aq) ) → Na2CO3 (aq) + CO2(g) + H2O(l)

I know this is long so any help would be appreciated, thank you so much!  ;D
Title: Re: Hess' Law Enthalpy Changes Literature Values
Post by: Borek on April 26, 2013, 07:17:25 AM
Disclaimer: I have not analyzed what you are doing, just skimmed. But one thing caught my attention immediately:

Initial temperature was 21.0 C and final temperature was 12.0 C.[/i]

Quote
(iii) Delta temperature
TFINAL-TINITIAL
25.0 ˚C - 20.0 ˚C= 5.0˚C (±1 ˚C)
Title: Re: Hess' Law Enthalpy Changes Literature Values
Post by: daisychain on April 26, 2013, 08:07:33 AM
Disclaimer: I have not analyzed what you are doing, just skimmed. But one thing caught my attention immediately:

Initial temperature was 21.0 C and final temperature was 12.0 C.[/i]

Quote
(iii) Delta temperature
TFINAL-TINITIAL
25.0 ˚C - 20.0 ˚C= 5.0˚C (±1 ˚C)

Oops that was from NaHCO3 calculations  :o my bad, ill change that