MHS Chemistry
Heat of Neutralization: Acids, Bases, and Energy

Hey gang!  Another new lab!  In this lab, we will be investigating (you guessed it) acids, bases, and energy.  You will be working with concentrated bases and acid solutions - these are as dangerous as things get in here!  You MUST use goggles or your lab privileges will be suspended for the remainder of the lab session, and an appointment will be made for you to make up the time after school.  THIS IS YOUR WARNING!

One definition of an acid is any substance that forms H+ ions when placed in aqueous solution.  A similar definition of a base is any substance that produces OH- when placed in aqueous solution.  An acid and a base mixed are said to neutralize each other, and produce water and a salt.
See the first titration lab for memory refreshment.

Concentrated hydrochloric acid will cause severe burns on the skin, and its vapors will cause damage to your nasal mucous membranes.  It must be used under the hood.

Sodium hydroxide comes in small white pellets that will also burn the skin.  DO NOT touch them - use a spatula and gloves.  Sodium hydroxide also absorbs quite a bit of water from the atmosphere, so as soon as you have obtained pellets from the container, cap it tightly (even if someone is waiting for it).  Students observed walking away from an uncapped bottle of sodium hydroxide will be penalized when the lab is graded.

Part 1  Heat of Solution

  1. We will be starting with HCl that is more concentrated than we need.  The concentration of this “stock” HCl is __________ M.  Determine the volume of this hydrochloric acid necessary to make 50.0 mL of 2.0 M solution.  Also, determine the amount of water you are going to need (hint: the second part is especially easy).
  2. To make this solution, add most of the water you will use to a 50.0 mL volumetric flask.  Use a pipet or graduated cylinder to measure the proper volume of acid from the reagent bottle in the hood.  Swirl this mixture gently, and feel the outside of the flask.  Finally, “top off” the volume with distilled water.  ALWAYS ADD ACID TO WATER, NOT THE OTHER WAY AROUND!
  3. Determine the necessary mass of sodium hydroxide to make 50.0 mL of 2.0 M solution.  Use a spatula to measure this into a weighing dish.  Make the solution in a similar way to step two above.  Feel the outside of the flask.
  4. Test each solution with blue pH paper.
  5. Test each solution with red pH paper.

Part 2  Heat of Neutralization

  1. Measure the temperature of each solution.  Quickly add the base to the acid, keeping a thermometer in the flask.  Do not cool this flask - swirl it gently.  Keep an eye on the thermometer to find the maximum temperature reached.
  2. Record the temperature of the mixture.
  3. Test the final mixed solution with blue pH paper.
  4. Test the final mixed solution with red pH paper.
  5. [If you like, carefully dip the tippiest tip of your pinkie in the mixture, and taste it.  What does it taste like?]
  6. Using the formula q = mCpDT, determine the amount of energy that the mixture gained (you can call it 100 mL or two parts of 50 mL, whichever is easier for you).  Remember that it is the water heating up, not the acid or base.

Part 3  A Penny Wasted...

  1. Place a penny in an evaporating dish.  Use a disposable pipet to add four drops of nitric acid to the penny.  What do you observe?
  2. Cover the dish with a watch glass.  What looks different?

Part 4  Clean-Up

  1. The solution from step 10 can be flushed down the drain.
  2. Wipe the thermometers dry and return them to the rack.
  3. The watch glasses and evaporating dishes can be rinsed several times to be clean.
  4. Split the penny with your group.  Use your share to buy yourself something pretty.  Don’t forget to use “common cents” in this step.  Ha ha.

Show all the work for any of these that have calculations involved.

    1.    Concentration of stock HCl __________ M
           V of concentrated HCl to make 50.0 mL of 2.0 M solution. __________ mL

           V of water needed to make 50.0 mL of 2.0 M solution. __________ mL

    2.    What does the flask feel like?

    3.    Necessary mass of NaOH to make 100 mL of 1.0 M solution. __________ g

           What did the outside of the flask feel like?

    4.    Acid + blue pH paper:

           Base + blue pH paper:

    5.    Acid + red pH paper:

           Base + red pH paper:

    6.    Starting T of acid solution    __________ °C

           Starting T of base solution   __________ °C

    7.    Maximum temperature reached (of mixture) __________ °C

    8.    Final mixed solution + blue pH paper:

    9.    Final mixed solution + red pH paper:

  10.    [What does it taste like?]

           Specific Heat of water __________ cal/gC°

  11.    Energy gained by 50.0 mL of acid solution __________ cal

           Energy gained by 50.0 mL of base solution __________ cal

           Total energy gained by both solutions __________ cal

           Balanced equation for sodium hydroxide reacting with HCl

           Moles of HCl used (see #1) __________ mol

           Moles of NaOH used (see #3) __________ mol

           Moles of water formed __________ mol

            Heat of Neutralization of HCl __________ kcal/mol

  12.    Penny + four drops of nitric acid.  What do you see?

  13.    Cover the dish with a watch glass.  What do you see?

  17.    What will you buy with your share of the penny?

[Heat of Neutralization score sheet][MHS Chem page]