Help student address a thirst for knowledge with this practical

The students are set the problem of designing a new drinks container which will cool 100 cm3 of a drink by 5 °C in 5 min. The students need to decide which of ammonium nitrate and ammonium chloride should be used based on the enthalpy of solution, the solubility is in water, the cost and the relevant health and safety information for each salt. They then need to trial their method and modify the quantity of salt required accordingly.

Pre-lab questions

(Remember to give full references for any information beyond A-level that you find out)

1. Give definitions for each of the following enthalpy changes. In each case include an equation to represent the process to which the enthalpy change applies;
• a) Enthalpy of solution of the ionic compound X+ Y
• b) Lattice energy of the ionic compound X+ Y
• c) Enthalpy of hydration of the gaseous ion Z+ (g)
1. The enthalpy of solution of an ionic compound can be calculated from its lattice enthalpy and the enthalpies of hydration of the individual ions. Use the enthalpy changes shown and a Born-Haber type cycle to calculate;
• a) The enthalpy of solution of ammonium chloride, NH4Cl (s)
• b) The enthalpy of solution of ammonium nitrate, NH4NO3 (s)

1. In 2001, Nescafe launched a self-heating can of coffee. To heat up the coffee a button is pressed which mixes the heating ingredients; a single step mixes calcium oxide and water to produce calcium hydroxide and generate heat;

CaO + H2O Δ Ca(OH)2 DH –82 kJmol–1

The can warms up 210 cm3 of coffee by 40◦C.

• a) Assuming that the heat capacity for coffee is the same as that of water (4.18 JK–1g –1) calculate the energy needed to warm 210 cm3 of coffee by 40◦C.
• b) Use this value to hence calculate the minimum mass of CaO needed in the can for it to function as specified.

(Question taken from 2003 International Chemistry Olympiad Booklet 2003 – Round 1)

Equipment

Apparatus

• Accurate thermometer
• Copper calorimeter (or similar), 100 cm3
• Beaker, 250 cm3
• Measuring cylinder, 100 cm3
• Top pan balance
• Stirring rod
• Spatulas
• Insulation; Paper / aluminium foil / cotton wool etc.
• Boss, clamp and stand
• Graph paper
• Stopclock

Chemicals

• Ammonium chloride, NH4Cl [Harmful], 120 g

Health, safety and technical notes

Ammonium nitrate requires very careful handling (R8: Contact with combustible material may cause fire, R9: Explosive when mixed with combustible material, Do not allow the salt to become contaminated with organic matter and do not grind it.)

As a result, ammonium nitrate should NOT be provided to the students under any circumstances.

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