chemistry lab report

INTRODUCTION

Chemical reactions are generally accompanied by some heat change. The chemical reactions which involve energy changes are referred to as thermochemical reactions. During many chemical reactions, energy is converted to heat energy and result in increase in the temperature of the surrounding. These reactions are called exothermic reactions. Some chemical changes absorb energy from the surrounding and are known as endothermic reactions.

Enthalpy change always result from changes in a chemical system, hence, the heat changes are classified according to the type of chemical reactions. For instance, enthalpy change from the dissolution of a substance is called the heat of solution. The enthalpy change involved in a chemical change stays constant regardless of the route taken during the chemical change provided the initial and final conditions remain the same. This is the Hess' law of constant heat summation.

PROCEDURE

1. The heat of neutralization of HCl (aq) with NaOH (aq).

Using a graduated cylinder, about 50.0mL of 1.0M HCl was carefully measured and transferred into the calorimeter that was setup as instructed. Then 50.0mL ofNaOH into a small beaker. The stirring plate was then switched on and the stirring bar was adjusted until it spun rapidly and smoothly. The time duration was changed to 180 seconds in the LabQuest. The probe was checked to ensure it was placed properly in the calorimeter and the green arrow in the LabQuest was pressed to continue with the experiment. NaOH solution was added into the calorimeter after 40s. After data was taken, a graph was projected onto the display and the data was then analysed as instructed.

2. The heat of solution of NaOH (s)

About 4.00g of solid NaOH was carefully weighedusing a weighing bottle. Then 100mL of distilled water was carefully added into the calorimeter. The stirring plate was switched on and the stirring bar adjusted until it spun quickly and smoothly. The duration on the LabQuest was reset to 480s r. The probe was checked to ensure that it was beneath the liquid level. Solid NaOH was poured into the calorimeter after 40s. After data was obtained, it was then analysed using the procedure given.

3. The heat of reaction of HCl (aq) with NaOH (s)

About 55.0mL of 1.0M HCl was carefully measured using and poured into the calorimeter. Using the cylinder, 45.0mL of distilled water was carefully measured and poured into the calorimeter. About 2.00g of solid NaOH was carefully weighed in a capped vial. The stirring plate was switched on and the stirring bar adjusted until it spun quickly and smoothly. The duration on the LabQuest was reset to 480s . The temperature probe was properly placed in the calorimeter. Solid NaOH was added to the calorimeter after 40.5s. After the data was obtained, a drop of phenolphthalein indicator was added into the calorimeter and mixed well. The data was then analyzed following the instructions that were given.

DATA

1. The heat of neutralization of HCl (aq) with NaOH (aq)

Volume of 1.0M HCl used	50.0mL
Volume of 1.0M NaOH used	50.0mL
Extrapolated initial temperature	20.97°C
Extrapolated final temperature	27.07°C
Change in temperature	6.19°C
Heat gained by calorimeter	24 Cal
Heat gained by solution	5.97 × 10² Cal
Total heat released by react	6.2 × 10² Cal
Heat of reaction/mole OH–	- 1.2 × 10⁴ Cal
Average heat of reaction/mole OH-	- 1.2 × 10⁴ Cal

2. The heat of solution of NaOH (s)

Volume of water used	100.0mL
Mass of NaOH and vial	4.05g
Mass of empty vial	0.00g
Mass of NaOH	4.05g
Extrapolated initial temperature	21.48°C
Extrapolated final temperature	30.39°C
Change in temperature	8.91°C
Heat gained by calorimeter	36 Cal
Heat gained by solution	8.8 × 10² Cal
Total Heat released by reaction	9.2 × 10² Cal
Heat of solution/mole OH-	- 9.0 × 10³ Cal
Average heat of solution/mole OH-	- 9.0 × 10³ Cal

3. The heat of reaction of HCl (aq) with NaOH (s)

Volume of 1.0M HCl used	55.0mL
Volume of water used	45.0mL
Mass of NaOH and vial	2.08g
Mass of empty vial	0.00g
Mass of NaOH	2.08g
Extrapolated initial temperature	21.49°C
Extrapolated final temperature	32.03°C
Change in temperature	10.54°C
Heat gained by calorimeter	42 Cal
Heat gained by solution	1.0 × 10³ Cal
Total Heat released by reaction	1.0 × 10³ Cal
Heat of reaction/mole OH -	- 1.9 × 10⁴ Cal

CALCULATIONS

The heat of Neutralization of HCl (aq) with NaOH (aq)

∆T =( 27.07–20.97) = 6.1°C

1. Heat gained by calorimeter = C∆T = (4 × 6.1) = 24.4 Cal

2. Heat gained by solution = mc∆T = (102 × 0.96 × 6.1) = 597.312 Cal

3. Heat released by reaction = Heat gained by calorimeter and solution = ( 597.312 + 24.4) = 621.712 Cal

4. Heat of reaction/mole = (Heat released by reaction/moles of OH- used)

moles of OH- = molarity × volume = (1.0M × 0.05L) = 0.05moles

Heat of reaction/mole OH- = (621.712/0.05) = 12434.24 Cal/mole

The heat of Solution of NaOH (s)

∆T = (30.39 – 21.48) = 8.91°C

1. Heat gained by calorimeter = C∆T = (4 × 8.91) =35.64 Cal

2. Heat gained by solution = mc∆T = (104.05 × 0.94 × 8.91) = 959.48667 Cal

3. Total Heat released by reaction = Heat gained by calorimeter and solution = (959.48667+ 35.64) = 995.12667 Cal

4. Heat of solution/mole OH- = (Heat released by reaction/moles OH- reacted)

moles OH- reacted = (mass used/molar mass) = (4.05/40) = 0.10125moles

Heat of solution/mole OH- = (995.12667/0.10125) = 9828.41155 Cal

The heat of reaction of HCl (aq) with NaOH (s)

∆T = (32.03 – 21.49) = 10.54°C

1. Heat gained by calorimeter = C∆T = (4 × 10.54) = 42.16 Cal

2. Heat gained by solution = mc∆T = (102 × 0.96 × 10.54) = 1032.0768 Cal

3. Total Heat released by reaction = Heat gained by calorimeter and solution = ( 1032.0.768 + 42.16) = 1074.2368 Cal

4. Heat of reaction/mole OH- = (Heat released by reaction/moles OH- reacted)

moles OH- reacted = (mass used/molar mass) = (2.08/40) = 0.052moles

Heat of reaction/mole OH- = (1074.2368/0.052) = 20658.4 Cal

% Difference for Hess' Law

Value of A = (12434.24 + 9828.41155) = 22262.65155 Cal

Value of B = 20658.4 Cal

|A–B| = (22262.65155 – 20658.4) = 1604.25155 Cal

Average of A and B = (22262.65155 + 20658.4)/2 =21921.525775 Cal

% diff = (1604.25155/21921.525775)× 100 = 7.48%

DISCUSSION

During the experiment, the heat of neutralisation of HCl solution with NaOH solution, the heat of solution of solid NaOH and the heat of reaction of solid NaOH with HCl solution were determined. Using the three values of enthalpy changes measured, it was shown that;

Heat of reaction of solid NaOH with HCl solution = Heat of solution of solid NaOH + Heat of neutralisation of HCl solution with NaOH solution.

This relationship summarises the Hess' Law of constant Heat Summation. Whether solid NaOH is reacted directly with HClsution or it is first dissolved before reacting with the acid, the amount of energy expended remains the same.

The key sources of error during the experiment included wrongful reading of values during measurements due to parallax error, rounding off of values during manipulation and loss of heat to the sorrounding.