Heat of Neutralization Questions

HEAT OF NEUTRALIZATION

COURSE CH181 NAME___________________________________

The objective of this experiment is to determine the heat (intensive) evolved from a reaction of acid and base.

SAFETY: Acids and bases are corrosive. Wash hands before leaving the lab. Wear eye protection if wearing contact lenses.

INTRODUCTION: In today’s experiment, you will measure the heat generated from a strong acid reacting with a strong base. Refer to your textbook for definitions of acids and bases. The reaction proceeds:

ACID + BASE  SALT + WATER + heat

Example: HA (aq) + MOH (aq) MA (aq) + H2O (l) + heat (molecular)

H+(aq) + OH(aq)  H2O (l) + heat (net ionic)

Hydronium

H3O+(aq) + OH(aq)  2H2O (l) + heat (net ionic)

REMINDER

The moles of reactant are given via molarity. Molarity  (moles solute)/(liter of solution). Therefore, to determine the molarity of a reactant, multiply the concentration unit, molarity, by the amount of liters used (convert mL to L).

MOLES OF REACTANT = (MOLARITY OF REACTANT)(LITERS OF SOLUTION)

Example: given 25 mL of a 4.0 M ammonia  = 0.10 moles NH3.

The acid ion reacts with the hydroxide ion to form water. This is an exothermic process. The reaction is open to the atmosphere which supplies a constant pressure process. Changes in enthalpy for exothermic reactions are always negative. The heat is picked up by the solution causing the solution temperature to rise. The transfer of energy is

The solution’s heat increase may be expressed as:

Since the solution is mostly water it is reasonable that the solution’s heat capacity nearly equals the heat capacity for water (1.00 calorie/(g∙oC)). Specifically, it takes 1.00 calorie of energy to raise the temperature of one gram of water from 14.5 oC to 15.5 oC. Note that 4.184 joules is equal in energy to 1.00 calorie. As an example, an exothermic reaction in 500-mL of mostly aqueous solution at 25.0 oC produces 92.3 kilojoules of heat. Determine the final temperature of the solution.

Therefore, H = -92.3 kJ and “q” of the solution picks up 92,300 J.

92,300 J = (~500 g)(4.184 )(Tf – 25.0 oC)

Solving for Tf, the final temperature is 69.1 oC. This same equation may be used to calculate the change in enthalpy when the initial and final solution temperatures are known.

PROCEDURES: Watch the following video to obtain data for this experiment: https://www.youtube.com/watch?v=PJRhLVnfQUo&t=6s

Make Reactant concentrations as specified by the instructor (written on the chalkboard). Use volumetric ware for solution preparations and dilutions. Enter the name and the concentrations with correct significant figures in the table below.

run

NAME OF ACID

ACID MOLARITY

NAME OF BASE

BASE MOLARITY

1

Phosphoric acid

0.60 M

Sodium Hydroxide

1.85 M

  1. Use a graduated cylinder to measure 100 mL of acid. Add a stir bean (a Teflon coated magnet) to a foam cup for mixing the solution. Record the mass of the foam cup and stir bean.

Use a 400-gram balance! Mass of the foam cup and stir bean _________________ g.

  1. Pour the acid solution into the foam cup with stir bean (to be used as a calorimeter). Use the standard thermometer from your drawer for temperature readings. NOTE: YOUR TEMPERATURE READINGS SHOULD BE REPORTED TO A TENTH OF A DEGREE (EITHER XX.X oC). After addition of the acid to the cup, wait several minutes to allow the temperature to equilibrate (the temperature remains constant).
  2. Use a clean, dry graduated cylinder to measure 1.01 x 102-mL of base (you can estimate the 1-mL over a hundred in the graduated cylinder or 50.0 mL + 51.0 mL for a fifty-milliliter graduated cylinder). The extra base will ensure the acid is the limiting reagent. Add the base solution to the acid solution and start timing. Not all the base will transfer into the acid container; is this important?
  3. Two students, working together can monitor time and temperature. One student shall monitor time telling the other student every thirty seconds to note the temperature. Collect data until the temperature has dropped at least five degrees from the maximum-recorded temperature. Essentially, the reaction was over as soon as the base was added. Due to a lag in the thermal response of the thermometer, the peak temperature was missed. Plot your data and extrapolate back to the line of mixing (start time) to obtain the peak temperature. An example is provided. Record the mass of the foam cup, stir bean and total solution.

(Use 400-gram balance!) Mass of the foam cup, stir bean, and total solution ________________ g.

CALCULATIONS:

Specific heat capacity for water

 

  1. Graphically determine the maximum temperature. ______ oC

2) Initial temperature ______ oC

 

  1. Determine T (maximum)[#1- #2] ______ oC
  2. Calculate the mass of the product solution _______ g

Since the product mass is mostly water:

  1. Determine qwater in kJ = (Swater)(masssolution)(T)(1kJ/1000J) _______ kJ
  2. Determine enthalpy change Hrxn (constant pressure) ________kJ

 

  1. Calculate the number of moles of (limiting) acid reacted ______ mol
  2. Convert reaction energy to an intensive property (#6/#7) ______kJ/mol

EXPERIMENTAL RESULTS

  1. Write the molecular reaction and write the net ionic reaction.

Molecular reaction of run 1:

Net ionic reaction of run 1:

  1. From the table of Hof in the text calculate the Hrxn for the net ionic reaction

______kJ/mol

Literature (textbook value)

  1. Calculate the percent error100% ______ %

T

{MAX}

TEMPERATURE (OC)

TIME (S)

∆T

Temperature change with time for Heat of Neutralization

~ room temperature

PROBLEMS:

  1. What is the final temperature of 13,400 liters of water at 23.200 oC if 200,900 kJ of heat was removed?

DATA

 

TEMP

(oC)

time

(s)

 

run 1

 

AVERAGE

DATA POINT

 

0

 

30

LINE OF MIXING

 

60

 

90

 

120

 

150

 

180

 

210

 

240

 

270

 

300

 

330

 

360

 

390

 

420

 

450

 

480

 

510

 

540

 

570

 

600

 

630

 

660

 

690

 

720

 

750

 

780

 

810

 

840

 

870

 

900

 

930

 

960

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