RS 6 Instant cold packs, often used to ice athletic injuries on the field, conta

Question

RS 6

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:

NH4NO3(s)NH+4(aq)+NO3(aq)

In order to measure the enthalpy change for this reaction, 1.25 g of ammonium nitrate is dissolved in enough water to make 25.0 mL of solution. The initial temperature is 23.4 C and the final temperature (after the solid dissolves) is 19.7 C.

Calculate the change in enthalpy for the reaction in kilojoules per mole. (Use 1.00g/mL as the density of the solution and 4.18J/gC as the specific heat capacity.) Hrxn =______kJ/mol

Explanation / Answer

To calculate the change in enthalpy for the reaction in kilojoules per mole use the following formula:

d H= Cp* m(water) * d T / n(substance) * 1000

dT = T2 or final – T1 initial temperature

Now calculate the mass of the water by using its density:

mass=density*volume; 1.0g/mL*25.0mL=25.0 g

To calculate the number of moles of the substance, NH4NO3 by using the following formula:

moles=mass/molar mass

=1.25 g / 80.052 g/mol

= 0.0156 mol

Cp= 4.18 J/g-C
m (for water)= 25 g
Delta T= 23.4-19.7= 3.7 C
Now dH calculate as follows:

d H= Cp* m(water) * d T / n(substance) * 1000
d H= 4.18*25 g* 3.7 C / 1000 * 0.0156 mol

= 24.78 kJ mol^-1

Related Questions

Navigate

• Browse (All)
• Browse R
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.