Theory v practice - do they compare?

Examine the reaction of calcium metal with water from both a theoretical and a practical viewpoint. 

Equipment

Materials per group

Fresh calcium metal, 0.5–1.0 g 

Copper gauze to make a cage in which to sink the calcium, 60 mm × 60 mm 

Equipment per group

Beaker (plastic if possible), 2 dm³ 

Thermometer, 0°–100° × 0.1°C 

Safety screen or fume cupboard

Safety glasses

Health, safety and technical notes

• Read our standard health and safety guidance here. 
• Wear eye protection.
• When attempting the practical route, students must have their proposals approved before starting on any experimental work.
• In contact with water calcium releases flammable gases, the use of a safety screen or fume cupboard should be mandatory. 

Commentary

Various versions of the cycle are possible, but if the students invoke the electron affinity of the hydroxide radical, which is given in the brief, they should eventually arrive at the following: 

Energy values ΔH/kJ mol^–1 (from data books and textbooks)

Δ₁H_at = atomisation of Ca(s) = + 193

Δ2HIE = 1^st and 2^nd ionisation energies of Ca = +590 +1150 = + 1740

Δ₃H_hyd = hydration energy of Ca²⁺ = –1650 

Δ₄H_vap = vaporisation of water × 2 = +44 × 2= + 88 

Δ₅H_BE = bond energy O–H × 2 = +463 × 2 = + 926

Δ₆H_EA = electron affinity of OH•(g) = –176.5 × 2= – 353

Δ₇H_BE = bond energy H–H = –436 – 436

Δ₈H_hyd = hydration energy of OH–(aq) = –460 × 2 – 920

                                                                  – 412 kJ mol^–1 

Procedure

The experimental work is straight forward and gives results within 95% of the theoretical value using the following method. 

Weigh about 100.00 g of deionised water in a 2 dm³ beaker. Carefully wipe the inside of the beaker to remove droplets above the water. Stand the beaker in a fume cupboard, insert a thermometer capable of reading to 0.1 °C in the water and leave to obtain a constant value. Weigh out about 0.5 g of calcium metal, choosing large pieces rather than smaller ones. Quickly empty the metal into the water and stir with the thermometer. If the calcium is fresh, the reaction will be complete in ca 90 s and the temperature increase is roughly 1.2 °C. 

Some of the calcium floats, despite its density, because of the rapid evolution of hydrogen, hence some of the heat is lost to the air rather than to the water. To circumvent this problem during trialling, some students designed a copper cage to ensure that the calcium remained submerged. If this is done, a correction factor for the copper will be needed.

The accuracy of the experiment can be improved by using lagging and plotting cooling curves to find the theoretical maximum temperature rise.

Calculation 

The experimental calculation is based on the energy change being related to the product of the mass of water x specific heat capacity of water x temperature rise. 

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.