280 DIY Science: Illustrated Guide to Home Chemistry Experiments
SBSTITUTIU oNS ANd modIfICATIoNS- You can prepare 50 mL of 1.0 M sodium hydroxide
solution by dissolving 2.00 g of sodium hydroxide
in about 40 mL of water in a small beaker and then
making it up to 50.0 mL in your graduated cylinder.
Allow the sodium hydroxide solution to cool to room
temperature before using it. - You can prepare 50 mL of 1.2 M hydrochloric acid
solution by adding 5.0 mL of concentrated (37%,
12 M) hydrochloric acid to about 35 mL of water in a
small beaker and then making it up to 50.0 mL in your
graduated cylinder. If you use hardware-store muriatic
acid (typically 31.45% or 10.3 M HCl), add 5.8 mL of
the acid to about 35 mL of water in a small beaker and
then make it up to 50.0 mL in your graduated cylinder.
Sodium hydroxide is the limiting reagent in this
experiment, so the exact molarity of the hydrochloric
acid is not critical, as long as HCl is in excess. Allow the
hydrochloric acid solution to cool to room temperature
before using it. - If you already have 1.0 M stock solutions of sodium
hydroxide and hydrochloric acid made up, you can
use them. Make sure the HCl is slightly in excess. For
example, use 45.0 mL of 1.0 M sodium hydroxide and
55.0 mL of 1.0 M hydrochloric acid.
Chemical reactions absorb or release energy,
usually in the form of heat (also called thermal
energy). Endothermic reactions absorb heat;
exothermic reactions release heat. If the
reaction occurs in a solution in a calorimeter,
the heat absorbed (or released) by the reaction
reduces (or increases) the temperature of
the solvent. This heat transfer to or from the
solvent can be quantified with the familiar
formula:
q = mcΔT
or, if water is the solvent:
q = (mcΔT)water
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ calorimeter£ thermometer£ graduated cylinder, 100 mL£ sodium hydroxide (NaoH) solution, 1.0 m (50 mL)£ hydrochloric acid (HCl) solution, 1.2 m (50 mL)where Q is the amount of heat transferred, m is the mass of
the water, c is the specific heat of water, and ΔT is the change
in temperature of the water. With known values for the mass
and specific heat of water and ΔT determined experimentally,
the value of Q can be calculated. (Remember that Q for an
exothermic reaction is a negative value.) Q may be expressed in
the traditional units of calories (cal) or in SI units of joules (J).
The calculation is the same in either case. Only the units for the
specific heat of water (c) are different:
cwater = 1.00 cal/(g · °C)
cwater = 4.18 g · °C)j/(
In this laboratory, we neutralize 50.0 mL of 1.0 M sodium
hydroxide solution with an equal volume of 1.2 M hydrochloric
acid solution. (We use a slight excess of HCl to ensure that all of
the sodium hydroxide is consumed by the reaction.) The balanced
equation for this reaction is:
HCl(aq) + NaoH(aq) → H 2 o + NaCl(aq)
This reaction is exothermic, so the final temperature of the
solution is higher than the starting temperature, and the value
of Q is negative. Because the starting and final solutions are
dilute, we can as a working approximation assume that their
density is the same as that of water, 1.00 g/mL, which simplifies
calculations. Filling in the known values gives us:
q = [100 g] · [1.00 cal/(g · °C)] · [ΔT]
LABORATORY 15 .4:
dETERmINE THE ENTHALpy CHANGE of A REACTIoN
Once we determine ΔT experimentally, plugging that value into
the equation gives us the value of Q—the amount of thermal
energy transferred. Determining the enthalpy change requires
one more step. Q is denominated in calories or Joules, and
the enthalpy change of reaction, ΔH°reaction, is denominated in