Instant cold packs, often used to ice athletic injuries on the field, contain am
ID: 708702 • Letter: I
Question
Instant cold packs, often used to ice athletic injuries on the field, contain ammonium nitrate and water separated by a thin plastic divider. When the divider is broken, the ammonium nitrate dissolves according to the following endothermic reaction: <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
NH4NO3<?xml:namespace prefix = v ns = "urn:schemas-microsoft-com:vml" />NH4 +NO3
In order to measure the enthalpy change for this reaction, 1.25 g of NH4NO3of is dissolved in enough water to make 25.0 ml of solution. The initial temperature is 25.8 and the final temperature (after the solid dissolves) is 21.9.
Calculate the change in enthalpy for the reaction in kilojoules per mole. (Use 1.0 g/mL as the density of the solution and 4.18 J/ g.as the specific heat capacity.
Explanation / Answer
Calculate the change in enthalpy for the reaction?
dH = m C dT
dH = 25 grams water(4.18J/g-C)(-3.9Celsius) = -407.55
dH water = - 408 Joules
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if the water lost 408 Joules, (dH water = -408J)
then the ammonium nitrate gained 408J , (dH reaction = +408J)
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often times questions then ask, what was the dH per gram ammonium nitrate:
408J / 1.25 grams = 326 J /gram
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often they then ask , what was the dH per mole ammonium nitrate:
326 J /gram @ 80.04 g/mol = 26.1 kJ/mole
dH reaction =