Question

In: Chemistry

1. Have you ever used an instant cold pack?. It actually contains two compartments: one has...

1. Have you ever used an instant cold pack?. It actually contains two compartments: one has water and the other contained ammonium nitrate solid. When you squeeze, the water bag breaks and ammonia nitrate solid dissolves in water liquid. The following equation depicts the thermochemical process of dissolving ammonia nitrate in water. NH₄NO₃ (s) + H₂O → NH₄⁺(aq) + NO₃^-(aq) DH = +27.5 kJ/mole.

Let’s assume the pack contains about 100g of water and 30.0 gram of NH₄NO₃ solid.

(a) According to the above chemical equation, is heat released or absorbed when NH₄NO₃ solid is dissolved in water? Find out exact amount of heat in Joules.

(b) Based on your answer in part (a), predict what happens to the temperature of the water inside the bag. Why it is called “instant cold pack”?

( c) After ammonia nitrate is dissolved, you have an aqueous solution of ammonia nitrate. What is the total mass of this solution do you expect it to be?

( d) The heat capacity of an aqueous ammonia nitrate solution is not going to be exactly same as pure water, but we often can approximate it as same as pure water, namely, specific heat capacity is 4.18 J/g ºC. Find out by how much the temperature drop would occur inside the bag after you applied?

Expert Solution

The sign of enthalpy change in the reaction is positive which tells us that this reaction is endothermic i.e. heat is absorbed in this reaction from surrounding. To find the exact amount of heat in joules we need to use the following equation

H =q/n where q is the heat exchanged and n is the number of moles of limiting reactant. Here the limiting reactant is NH₄NO₃. Now 30g of NH₄NO₃ corresponds to 30/80.04 = 0.375 mol NH₄NO₃ ( Molecular weight of NH₄NO₃ is 80.04g/mol)

So q= H * n = 27.5*1000*0.375 Joule = 10312.5 J

This amount of heat is absorbed which means that the temperature of water decreases because the water serves as an temperature bath. Therefore the pack gets cold and hence the name.

The total mass of the solution will simply be the mass of water + mass of NH₄NO₃ = (100 +30) g = 130g

To find out the temperature change inside the bag we need to use the following equation

q = m*c*t where m is the mass of the solution, c is the heat capacity and t is the change in temperature

So, 10312.5 = 130g * 4.18 J/g ºC*t

t = 10312.5/(130*4.18) = 18.98 ºC 19 ºC

queen_honey_blossom answered 2 years ago

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