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Unformatted text preview: means that a chemical proce s s (the system) absorbed heat
from the water. Since the process absorbed energy, it was endothermic. Now consider an exothermic example.
If a chemical process (the system) releases heat to the water (the surroundings), the temperature of the water will
increase as the water absorbs heat energy and the ΔT of the water will be positive.
In Part A of the lab, you
will observe temperature changes that occur as compounds dissolve in water.
In Parts B and C, you will
observe temperature changes that occur during chemical reactions. The reaction in Part C is a neutralization.
Neutralization reactions (also known as acid- base reactions, which you will study in more detail later in the
www.webassig n.net/ebooks/wsug encheml1/lab_12/manual.html 2/7 4/15/13 Lab 12 - M easur ing Enthalpy Chang es course) usually involve transfer of H+ (a proton) from one chemical species to another. For example, the reaction
of acetic acid and sodium hydroxide is shown below.
HC2H3O2(aq) + NaOH(aq) → NaC2H3O2(aq) + H2O
Strong acids, such as HCl and HNO3, react with water as shown below.
HCl(aq) + H2O → Cl−(aq) + H3O+(aq)
HNO3(aq) + H2O → NO3−(aq) + H3O+(aq)
H3O+ is called the hydronium ion. Whenever strong acids are placed in water, this is the species that reacts.
For example, the reaction with sodium hydroxide is shown below.
H3O+(aq) + NaOH(aq) → Na+(aq) + 2 H2O
In these reactions, species are followed by "(aq)" to indicate that they are present in aqueous solution. This
notation will be used frequently in the rest of this course. In Part D of the lab, you will observe temperature
changes that occur as energy is added to water in its solid and liquid phases. Equipment
1 30 mL beaker
1 100 mL beaker
1 10 mL graduated cylinder
1 digital thermometer
4 small test tubes
1 test tube rack
1 ring stand and clamp
1 hot plate/stirrer
1 magnetic stir bar
1 deionized water squirt bottle Reagents
~2 g CaCl2(s, anydrous)
~2 g NH4NO3(s)
1 mL 1 M NaOH...
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- Spring '14