Note: you may need to obtain D H of values from the appendices in Open Stax Chem

Question

Note: you may need to obtain D H of values from the appendices in Open Stax Chemistry, books

A chemical cold pack consists of two isolated interior pouches. One contains water and the other contains an ionic compound that dissolves in water in an endothermic reaction. Ammonium nitrate, NH4NO3, is commonly used in chemical cold packs because of its low cost and low toxicity (should the pouch happen to leak).

-Write a balanced chemical equation to show what happens when NH4NO3 dissolves in water.

- What is DHoreaction when 20.00 g of NH4NO3 dissolves in water? Useful information: Molar mass of NH4NO3 = 80.052 g/mol; DHof (NH4NO3) = -365.1 kJ/mol; DHof (NH4+) = -132.8 kJ/mol; and DHof (NO3-) = -206.57 kJ/mol.

- Suppose that a chemical cold pack contains 50.00 g of H2O at 25.00oC. If all of the heat absorbed in the reaction in (b) is released from the water in the cold pack, what is the final temperature of the cold pack?

Explanation / Answer

Lets write an equation

NH4NO3 (aq) (s) ---- > NH+ (aq) + HNO3 (aq)

Lets find delta H of above reaction

Delta Hrxn = Delta H product – delta H reactant

= ( Delta H NH4+ + delta H HNO3 ) – Delta H NH4NO3

= (-132.8 kJ/mol – 206.57 kJ/mol ) – (-365.1 kJ/mol )

= 25.73 kJ

Delta H of reaction of 20.0 g

Number of moles of NH4NO3 = 20.0 g / molar mass of NH4NO3

= 20.0 g/80.052 kJ per mol

= 0.25 mol NH4NO3

Delta H for 0.25 mol NH4NO3

= 0.25 mol NH4NO3 x 25.73 kJ/mol NH4NO3

= 6.43 kJ

Second part :

Given

Mass of water = 50.00 g

T = 25.00 deg C

We use following equation

q = m c delta T

here q is absorbed heat , ,m is mass in g , C is specific heat , delta T = Tf – Ti

Initial T= 25.00 deg C

Lets plug all the values

6.43 x 103 J = 50.00 x 4.184 J / (deg C g ) x ( Tf – 25.00 )

Tf – 25.00 deg C = 30.73 deg C

Tf = 55.73 deg C

Final T of water = 55.73 deg C

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