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Offline tanya06

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Chemistry Question?
« on: March 22, 2007, 10:51:27 PM »
I am having difficulty with a few chemistry problems I don't even know how to start, so if you can tell me step by step how to get the answer or start me in the right direction that would be very helpful.

1) Sucrose, C12H22O11, is found to have a standard enthalpy of combustion of -5640.9 kj mole-1. The standard enthalpies of formation or CO2(g) and H20(l) are, -393.51 and -285.83 kj mole-1. Compute ?H?f for sucrose.

2) The enthalpy of solution of NaOH by dissolving 4.00g of NaOH(s) in 180ml of pure water (destiny 0.999 g ml-1) at an inital temperature of 19.5?C. Temperature-time data collected after mixing was extrapolated back to the time of mixing to obtain a temperature change, ?T=+5.5?. The destiny of the final solution was 1.013ml-1 at 25?C, and the solution heat capacity was 4.08Jg-1deg-1. In a seperate experiment, the calorimeter constant was found to be 21 J deg-1. What was the molar concentration of the final NaOH solution at 25?C? What was the molar enthalpy of dissolution of NaOH in water?

Offline Yggdrasil

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Re: Chemistry Question?
« Reply #1 on: March 23, 2007, 02:01:20 AM »
1) Sucrose, C12H22O11, is found to have a standard enthalpy of combustion of -5640.9 kj mole-1. The standard enthalpies of formation or CO2(g) and H20(l) are, -393.51 and -285.83 kj mole-1. Compute ?H?f for sucrose.

What are the chemical equations for the formation of CO2 and H2O?  What is the chemical equation for the combustion of sucrose?  Can you add these equations to get the equation for the formation of sucrose?

Offline Sam (NG)

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Re: Chemistry Question?
« Reply #2 on: March 23, 2007, 08:16:09 AM »
2) The enthalpy of solution of NaOH by dissolving 4.00g of NaOH(s) in 180ml of pure water (destiny 0.999 g ml-1) at an inital temperature of 19.5?C. Temperature-time data collected after mixing was extrapolated back to the time of mixing to obtain a temperature change, ?T=+5.5?. The destiny of the final solution was 1.013ml-1 at 25?C, and the solution heat capacity was 4.08Jg-1deg-1. In a seperate experiment, the calorimeter constant was found to be 21 J deg-1. What was the molar concentration of the final NaOH solution at 25?C? What was the molar enthalpy of dissolution of NaOH in water?
Okay, for question two I’ll have a go at helping, but I’ve not answered this type of question in 3 years so I may be a little rusty.  I would consider the following equations:

For Calorimeter:
q(cal)=c?T
For Solution
q=mc?T

?H=q+q(cal)



?=m/V
?=density
m=mass
V=volume

The molar concentration of NaOH can be calculated from the change in mass of the solution.  The initial mass is 0.999x180 g.  The final mass is 1.013x180 g.  The difference between these two values gives you the amount of NaOH added in grams.  This can be converted into the number of moles of NaOH that have been dissolved.  From this value you can get the molarity.

From the other equations you can calculate ?H for the process, which can be divided by the number of moles to give the molar enthalpy change.  For the calorimeter, c is the calorimeter constant.  For the solution, c is the specific heat capacity of the solution and m is the mass of the solution at 25°C.

Offline AWK

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