Chemistry 11 Lab #14
Lab #14: Exothermic and Endothermic Reactions
WARNING: This experiment requires the use of concentrated and DANGEROUSLY CORROSIVE hydrochloric acid as well as other toxic substances. Notify me immediately of spills and wear safety goggles at all times.
AND: Make sure to give the names and formulas for all reactant and products in this lab.
Theory
Enthalpy and exothermic/endothermic
reactions: In chemical reactions the internal energy states of the products
typically differs from that of the reactants.
The net result of a reaction then is to bring about a net energy change
called enthalpy change (algebraically called ΔH and measured in
kJ/mol). A reaction in which the
internal energy states is lowered/raised must
release/absorb heat energy as a result of the Law of Conservation of
Energy. Such a reaction is called
exothermic/endothermic and will have a negative/positive enthalpy change.
An example of an exothermic reaction:
N2 + 3H2
-----> 2NH3 + 92.4 kJ
N2 + 3H2 ---> 2NH3 ΔH=-92.4 kJ/mol
Metals and acids: When we study acids later this semester we will see that
one of the definitions of an acid is that it is a compound that donates a
proton (hydrogen nucleus, or positive hydrogen ion, H+) in a
reaction. We already know that metals
are defined by their tendency to give up electrons to become positive
ions. Use this knowledge to predict what
should happen when you mix a metal with an acid. In particular try to figure out the result of
the reaction between magnesium metal and hydrochloric acid (HCℓ). Hint: remember your diatomic elements.
Hydrated compounds: Ionic compounds are often made in aqueous solutions and this results in them bonding lightly with water molecules in a process called hydration. A hydrated salt (ionic compound) will have a set number of water molecules for every formula unit of ionic compound. The net result is not quite a compound and is written differently. The ionic compound formula is followed by a dot and then the number of water molecules
For example barium hydroxide in its hydrated form bonds with 8
molecules of water. We write the formula
for hydrate barium chloride as:
Ba(OH)2·8H2O
In a reaction in water the
compound dissolves as before and the water molecules simply join their fellow
molecules of solvent.
Procedure: Part 1
1. Add 10 mL of tap water to a Styrofoam cup and take a temperature reading.
2. Measure
out 1.0 g of Ba(OH)2·8H2O and 0.40 g of NH4Cℓ
(remember to use a piece of paper on the balance) and place these
into the Styrofoam cup.
3. Measure the temperature and note the most extreme temperature change.
4. What type of reaction is this, endo or exo thermic? Was there a gas given off? If so, what gas? (The smell of the gas will be a clue.) Calculate the enthalpy change. To do this you will need to balance the reaction equation and figure out which reactant was in excess.
Ba(OH)2 + NH4Cl => BaCl2 + NH3
+ H2O (Note that I have ignored the hydration of the
barium compound. You can, if you choose,
try to write the equation balanced with this added.)
Procedure: Part 2
1. Add 10 mL of hydrochloric acid to a Styrofoam cup and take a temperature reading. Rinse the thermometer off carefully after doing this.
2. Mass the strip of magnesium metal provided and place it carefully in the acid in the Styrofoam cup. Observe what happens. Is a reaction taking place? How do you know?
3. Measure the temperature once all the magnesium is consumed. Remember to rinse the thermometer well.
4. What type of reaction is this, endo or exo thermic? Was there a gas given off? If so, what gas? (We may have time to do a splint test to check out your hypothesis.)
5. Calculate the enthalpy change. To do this you will need to balance the reaction equation and use the fact that the magnesium was clearly the limiting reactant. (You were asked to figure out the reaction in the theory section.)
Other data
Density of water = 1000 kg/m3
Specific heat of water = 4186 J/kgCo
Assume the same values for the hydrochloric acid which is a solution mostly made of water.
Hydroxide =
Ammonium = NH4+