Name ____________________________
Per. ____
Date _____________
Enthalpy & Entropy: Hot & Cold Packs
1. The plastic bag that the trainer gives to the wounded quarterback gets ice cold when punched. This is an example of an
endothermic reaction.
(a) The contents of the bag are the (system/surroundings).
(b) The quarterback’s hand is the (system/surroundings).
(c) The potential energy of the products is (higher/lower) than that of the reactants.
(d) Energy goes (into/out of) the system.
(e) Energy goes (into/ out of) the surroundings.
(f) Which has less thermal energy afterwards? (system/surroundings).
2. What compounds (name and chemical formula) cited in the article feel colder when dissolved in water?
3. (a) What are salts composed of?
(b) Why are all salts solid crystals?
4. What are the steps involved in dissolving and what energy change is associated with each step?
5. Calculate the kilocalories absorbed when 159 grams of ammonium nitrate dissolves in water. Show your work.
6. (a) Lighting a match produces heat. What do we call this kind of reaction?
(b) Where does the heat energy come from?
(c) Which processes always require energy in any chemical reaction?
(d) Why is energy ultimately given off when a match is lit?
7. The “Heat Factory” is a brand of hot pack that produces the same reaction which occurs when iron metal (like a shovel)
is exposed to water and air for several days.
(a) Write the reaction
(b) Explain why it takes the hot pack just a few seconds to produce the products, not days.
8. (a) What substance “activates” the Heat Factory?
(b) What can you do to stop the reaction and restart it again later?
9. The Heat Solution works on a different principle than the Heat Factory. Explain how this heat pack works.
Name ____________________________
Per. ____
Date _____________
10. Why is the system in the Heat Solution better for our muscles than other types of hot packs?
11. What is a unique characteristic of the Heat Solution which makes it a more environmentally “green” product compared
to other types of hot packs?
12. (a) Spontaneous reactions usually result in an increase in enthalpy (heat). Why are most spontaneous reactions
exothermic?
(b) The products of an exothermic reaction have (higher/lower) potential energy than the reactants.
13. (a) Define entropy.
(b) Does entropy tend to increase or decrease in nature?
14. There are two driving forces (a natural tendency to increase) in all chemical and physical reactions. Those two forces
are enthalpy (heat spread) and entropy (matter spread). The cold pack is an example of a reaction in which enthalpy
and entropy are in opposition. Explain why dissolving the ammonium nitrate solid is a spontaneous reaction, even
though it doesn’t produce heat.
15.
G = H - TS
This equation summarizes how enthalpy (H) and entropy (S) are both factors in determining whether a reaction will
be spontaneous (G < 0) or non-spontaneous (G > 0). T is Kelvin temperature.
Think about this qualitatively, using just positive (+) signs & negative (-) signs & your knowledge of algebra.
1. If G ends up with a negative (-) sign, the reaction will be spontaneous; the reaction will GO forward!
2. If G ends up with a positive (+) sign, the reaction will be non-spontaneous; the reaction will NOT GO. It just won’t
happen!
3. If H is (-) a spontaneous reaction is favored; (G < 0).
4. If H is (+), a spontaneous reaction is not favored; (G > 0).
5. If S is (+), a spontaneous reaction is favored; (G < 0).
6. If S is (-), a spontaneous reaction is not favored; (G > 0).
7. If H and S are in opposition, T can affect the size of S (see the equation). A “high” T can increase the effect of
entropy and a “low” T can decrease the effect of entropy.
Fill in the table with the correct information and signs given each situation.
GO or NO GO
G
H T (hi, lo, any)
S
any
+
-
GO
+
low
high
+