nstant cold packs, often used to ice athletic injuries on the field, contain amm

Question

nstant 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: NH4NO3(s)NH+4(aq)+NO3(aq) In order to measure the enthalpy change for this reaction, 1.25 g of NH4NO3 is dissolved in enough water to make 25.0 mL of solution. The initial temperature is 25.8 C and the final temperature (after the solid dissolves) is 21.9 C. Part A Calculate the change in enthalpy for the reaction in kilojoules per mole. (Use 1.0g/mL as the density of the solution and 4.18J/gC as the specific heat capacity.) Express your answer to two significant figures and include the appropriate units.

Explanation / Answer

Delta H = Cp (m-water) ( delta T) / n(substance) (1000)

Here we have

Cp = 4.18J/gC

m-water = 1 * 25 { density = M/V }

= 25g

now  ( delta T)

Delta T = 25.8 - 21.9

=3.9

For n Of NH4NO3

= 1.25/80.052 (MW of NH4NO3=80.052)

=0.0156 mol

Now for Delta H

Delta H = 4.18J/gC(25 g) ( 3.9 C) / 1000 ( 0.0156 mol )

=26.125 kJ mol-1

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