determining the heat of reaction for two exothermic reactions.

Lab 9 Heat of Reaction

OBJECTIVE:

To experimentally determine the heat of reaction for two exothermic reactions.

DISCUSSION:

The heat given off or gained in a reaction is called the heat of reaction (∆????????????????????????????????????).

A reaction that releases heat is known as an exothermic reaction; an endothermic reaction

is one that absorbs heat.

The heat of the reaction corresponds to one mole of the reactant and is expressed in

kilocalories per mole of reactant or in kilojoules per mole of reactant: ∆???????????????????????????????????? = ???????????????????????????????????? (????????????????)

???? (????????????) , where qreaction is the heat produced or absorbed by a certain amount of moles of

reactant, n.

The reactions for this experiment are outlined as (1) and (2) and are as follows:

(1) ???????????????? (????) + ???????????? (????) → ???????? + (????????) + ????????− + ???????????? (????) (????????) ∆???????? =?

(2) ???????????????? (????) + ???????????? (????????) → ????????+ (????????) + ????????− (????????) + ???????????? (????) ∆???????? =?

To determine Hreaction the heat released or absorbed by the reaction must be measured.

When an exothermic reaction occurs in a container, the heat which is released warms up the

container and its contents (the temperature increases). The contents of the container include

the reaction products and a solvent (water in our case). The measurement of the heat transfer

under controlled conditions is called “calorimetry” (from the energy unit, calorie), and a

container used to measure the heat transfer is called a calorimeter. The calorimeter prevents

heat transfer to or from the surroundings, thereby isolating the system that us being

measured.

When a reaction occurs in a relatively large amount of solvent (water), we can consider water

as the sole medium absorbing the heat of the reaction. This allows us to apply the law of

conservation of energy (heat released by the reaction is equal to the heat absorbed by the

water) and to calculate the heat of reaction by measuring the temperature change of the

water (even though it is actually a diluted aqueous solution of the products), according to

the following relationships:

(3) ???????????????????????????????????? = −???????????????????????? (4) ???????????????????????? = ???????????????????????? ∗ ???????????????????????? ∗ ∆????????????????????????

Where: swater is the specific heat of water, 1.00 ????????????

????∙℃ or 4.184

????

????∙℃ ; mwater is the mass of water

used (recall that an acid solution (HCl (aq)) is mostly water); ∆???????????????????????? is the difference between the initial and final temperatures of the water solution in the calorimeter.

PROCEDURES:

REACTION 1:

1. Open the virtual lab workspace for the “heat of reaction” activity. (Posted on the course website)

2. From the glassware menu, obtain a 50 mL graduated cylinder and a foam cup (“glassware”). Take out the distilled water and solid NaOH (“stockroom”) and the

balance (“tools”).

3. Transfer 50.0 mL of water to the graduated cylinder. Record the initial temperature. 4. Weigh between 1.00 and 1.50 grams of solid NaOH in the foam cup (tare the foam

cup first, on the balance). Record the mass of NaOH.

5. Pour the water into the foam cup and record the highest temperature; this is the final temperature.

REACTION 2:

1. Take the 0.50 M HCL from the stockroom, measure 50.0 mL with the graduated cylinder, and record the initial temperature.

2. Empty the foam cup (right click). The procedure is the same as in reaction 1, except that HCl is being used in place of water.

3. Record the mass of NaOH used, and the initial temperature of the acid, and the final temperature after the NaOH has been added.

Name:

Lab 9 Heat of Reaction

DATA:

Solid NaOH (g) Liquid (mL) ???????????????????????????????? (℃) ???????????????????????????? (℃) ∆???? (℃)

Reaction #1

H2O:

Reaction #2

HCl:

1. Using dimensional analysis, calculate the number of moles of NaOH (nNaOH) used in

reaction #1 (show your work): ???????????????????? (????????????) = ???????????????????? (????) × 1 ????????????

???????????????????????? (????) Where nNaOH is

the number of moles NaOH , mNaOH is the mass of sodium hydroxide used, and MMNaOH

is the molar mass of sodium hydroxide.

2. Using the density of water (???? = 1.000 ????

???????? ), calculate the mass of the 50.0 g water used

in reaction #1 using: ???????????????????????? = ???????????????????????? × ????????????????????????.

3. Using the specific heat of water (???????????????????????? = 1.000 ????????????

????∙℃ ), calculate the heat absorbed by the

water and the heat produced by the solution in reaction #1 using: ???????????????????????? = ???????????????????????? ∗ ???????????????????????? ∗ ∆???????????????????????? and ???????????????????????????????????? #1 = −????????????????????????.

a. ????????????????????????:

b. ???????????????????????????????????? #1:

4. Convert the heat produced by reaction #1 to kcal using: ???????????????????????????????????? #1(????????????????) =

???????????????????????????????????? #1 (????????????) ∗ 1 ????????????????

1000 ????????????

5. Calculate ∆???????????????????????????????????? #1 using: ∆???????????????????????????????????? #1 = ???????????????????????????????????? #1 (????????????????)

???????????????????? (????????????)

6. Using dimensional analysis, calculate the number of moles of NaOH (nNaOH) used in

reaction #2 (show your work): ???????????????????? (????????????) = ???????????????????? (????) × 1 ????????????

???????????????????????? (????) Where nNaOH is

the number of moles NaOH , mNaOH is the mass of sodium hydroxide used, and MMNaOH

is the molar mass of sodium hydroxide.

7. Using the density of water (???? = 1.000 ????

???????? ), calculate the mass of the 50.0 mL HCl used

in reaction #2 using: ???????????????? = ???????????????? × ????????????????????????.

8. Using the specific heat of water (???????????????????????? = 1.000 ????????????

????∙℃ ), calculate the heat absorbed by the

HCl and the heat produced by the solution in reaction #2 using: ???????????????????????? = ???????????????????????? ∗ ???????????????????????? ∗ ∆???????????????????????? and ???????????????????????????????????? #1 = −????????????????????????.

a. ????????????????????????:

b. ???????????????????????????????????? #2:

9. Convert the heat produced by reaction #2 to kcal using: ???????????????????????????????????? #1(????????????????) =

???????????????????????????????????? #2 (????????????) ∗ 1 ????????????????

1000 ????????????

10. Calculate ∆???????????????????????????????????? #2 using: ∆???????????????????????????????????? #2 = ???????????????????????????????????? #1 (????????????????)

???????????????????? (????????????)

11. Convert the ∆???????????????????????????????????? #1 from kcal/mol to kJ/mol using the conversion: 1.000 kcal = 4.184 kJ

12. Convert the ∆???????????????????????????????????? #2 from kcal/mol to kJ/mol using the conversion: 1.000 kcal = 4.184 kJ