Molar Heat Of Solution 26i3p

Molar Heat of Solution

Molar Heat of Solution Background Information: When most solids are dissolved in a solute such as water, there will usually be a change of the heat content of the solution. When this happens, heat is either absorbed or released based on the bonding energies of the solute and the solvent. If the solute gives out energy into the solution, the temperature of the solution will increase. On the other hand, if energy is absorbed by the solute, the temperature of the solution will then decrease. The amount of energy released or absorbed per mol of substance is called the "Heat of Solution" for that solute. If heat is given off, the temperature of the solution will rise and will have a negative heat of solution. If heat is absorbed the heat of solution will a positive number. (EduWebLabs)

Aim: The aim of this investigation is to find the molar heat of solution of a substance (In this investigation, NaOH and water was used.)

Hypothesis: The molar heat of solution will be a negative and the temperature of the solution will rise.

Apparatus & Materials: Quantity

Item

2

20 ml of distilled water

2

20 ml graduated cylinder

1

Alcohol thermometer

1

Copper calorimeter

1

Electronic balance

2

Filter paper

1

Spatula

As many as needed (2)

Paper

1

Beaker

Reuel Chua

Molar Heat of Solution

Safety: Item

Safety Measure/s

Glassware

Place in the centre of the table and be careful, wear safety goggles, lab coat and gloves

Mysterious/Unknown/Know Substances

Do not ingest, do not make direct with it, wear safety goggles, lab coat and gloves, use spatula or beaker when handling or transporting these substances, avoid spills

Method: Variables There are no variables in this investigation because this investigation was to find out a value and not a trend. Method 1) Obtain the 20 ml graduated cylinder and pour 20 ml of distilled water into it. 2) Pour that 20 ml of distilled water into a copper calorimeter. 3) Leave the 20 ml of distilled water and the calorimeter, make sure the lid is off. 4) Place a piece of filter paper onto the electronic balance. 5) Tare the electronic balance. 6) Using the spatula, spoon out approximately 3g of the substance. 7) Place the 3g of substance into a beaker. 8) Go back to the calorimeter. Test the temperature of the water using the alcohol thermometer. Note this temperature. 9) Place 3g of substance into the calorimeter. 10) Quickly cover the calorimeter and use the stirrer provided with the lid of the calorimeter to stir the solution. 11) Note the highest temperature. 12) Pour the solution out and wash all the materials. 13) Dry the inside of the calorimeter 14) Repeat steps 1 to 12. 15) Remove the inside of the calorimeter and dry it. 16) Turn on the electronic balance and weigh the inside of the calorimeter. Note this value.

Reuel Chua

Molar Heat of Solution

Diagram

Figure 1 - Step 9

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Molar Heat of Solution

Results: 1. Table of results Mass of Solute (g) Mass of Solvent (Water) (g) Mass of Calorimeter (g) Initial Temperature ( C) Final Temperature ( C)

Test 1 3.02 20

Test 2 2.99 20

46

46

27.9

27

51.6

50.9

Change in Temperature C Mass of Substance (g) Molar Mass of Substance (NaOH) (g) Number of Moles Q of solvent Q of Calorimeter Q total Experimental Molar heat of solution (J) Experimental Molar heat of Solution (kJ) Theoretical Molar heat of NaOH (kJ) % Difference

2. Calculations Test 2 -23.9 2.99 39.9971

Test 1 -23.7 3.02 39.9971

Average

0.075505474 0.07475542 -1983.216 -1999.952 -419.727 -423.269 -2402.943 -2423.221 -31824.75214 -32415.32196 -31.82475214 -44.51 28.49977052

-32.41532196 -44.51 27.1729455

-32.12003705 -44.51 27.83635801

Note: Theoretical molar heat of NaOH taken from the CRC Handbook of Chemistry and Physics that is listed on Wikipedia (http://en.wikipedia.org/wiki/Enthalpy_change_of_solution)

Equations used to calculate values (Where values would be in °C, g, kJ or J) ( )

( )

( )

Note: Specific heats may be found on the Internet 3. Graph A graph may not be constructed due to the lack of controlled variables such as initial temperature Reuel Chua

Molar Heat of Solution

and the mass of solute.

Discussion: In this investigation, the initial temperature had a direct influence on the molar heat of solution. Since the amount of solute used will determine the change in temperature, one may think that it would be a variable, however, the equations takes care of this problem since the energy produced by this reaction is divided by the amount of moles of the solute used. Based on the average percentage difference (28.7%), the experimental molar heat greatly differed from the theoretical molar heat of NaOH. This could have been because the equation used failed to consider the heat gained by the thermometer (that was in the water). Some heat may have also escaped due to the minor openings at the top of the calorimeter and the imperfect design of the calorimeter. A strength of the design of this investigation is that it tries to ensure that no heat escapes in the beginning reaction of the solvent and solute in step 9 and 10, because in step 10, it mentions to “quickly cover the calorimeter…” An improvement to this design is to use a perfectly design calorimeter with perhaps a built in thermometer. The calorimeter would be perfectly insulated and airtight and would mention the weight and specific heat of that specific calorimeter. A perfect calorimeter would never exist, however. Another improvement to this design would be to completely wash and clean out the insides of the calorimeter before adding the solvent and solute in. Left over particles of different chemicals may have left over from previous investigations by others. This may also be a reason why the percentage difference between the experimental and theoretical molar heat is fairly high. Having done this investigation, it would be interesting to do more of these kinds of investigation using water to dissolve a certain solute. Changing the amount of water and solute each time to come up with every possible combination of mass of solute to mass of water. After doing these investigations, perhaps an equation for finding the molar heat of that solution (certain solute and water) could be developed.

Conclusion: In conclusion, the molar heat of solution was a negative number and the temperature of the solution indeed rose. The molar heat of water and NaOH was found, however, it did differ quite a bit from the theoretical molar heat. These inconsistencies were then justified in the discussion.

Bibliography EduWebLabs. (n.d.). Heat Of Solution. Retrieved 8 3, 2012, from EduWebLabs: http://eduweblabs.com/Database/Lab_Folders/HeatofS/HeatofS.html

Reuel Chua