## Thursday, September 13, 2012

### From Calcium Metal to Enthalpy

Calcium metal is a silver white metal.

It is a little harder than sodium metal. When it reacts with water, it would produce calcium hydroxide , hydrogen gas and a lot of heat. That is because when calcium element during losing two electrons from the atom and form lattice this process, its enthalpy change is very high.

But why the reaction is not exciting as sodium metal that is because the high melting point of calcium metal. If the calcium would melt during the reaction, the surface of calcium to contact with water would become very large, so that the reaction would become exciting.

Ca(OH)2 deposit.

However, compared with sodium,calcium has much stronger covalent bonds between each atoms, so it has higher melting point.

Let's use thermodynamics to check how calcium metal emit heat into the surrounding.

First, we have to write out the reaction formula and figure out the enthalpy change in the reaction.

ΔfHm                         0.0       2 x (-285.8)                             -985.2             0.0            (kJ/mloe)

=>  ΔH = (-985.2+0) - (0.0+ 2 x (-285.8))
=   413.6  kJ/mole

Second, we are going to do an experiment to measure the heat transformation from the reaction.

 Calcium metal weight 0.12 g Molar of calcium 3.00 x 10-3 mole Water weight 20.22 g Origin temperature of water 30.59 ℃

Theoretical ΔT of water:
ΔH = m x s x ΔT

=>ΔT = ΔH / ( m x s)
= (( - (-413.6 x 103 x (0.12/40.078))) / (4.186))  / (20.22 x 1)
= 14.63
=>      T = 30.59 + 14.63
= 45.22

 Theoretical temperature 45.22 ℃ Measured temperature 40.32 ℃ Surrounding temperature 29.44 ℃ Lose heat 98.00 cal Heat capacity of the beaker 2.87 cal/℃

Heat lose in the beaker:
(40.32 – 30.59) x (2.87)
= 27.93 cal
Energy lose in other forms(gas kinetic energy, radiation…):
98.00 – 27.93
= 70.07 cal
= 293.02 J

 Total energy released 1238.32 J Energy in heat form 945.30 J Heat transform efficiency 76.3%

We can know not all the enthalpy change would turn into heat that is why the water is only 40.32℃ in the experiment.