Post-lab part 2 Lime, including calcium oxide (aka quicklime) and calcium (aka s

Question

Post-lab part 2 Lime, including calcium oxide (aka quicklime) and calcium (aka slaked me) is one of the oldest chemicals known and is used as a building material. Slaked lime is prepared by reacting quicklime with water: CaO(s) + H_2O(1) rightarrow Ca(OH)_2(s) delta H_ -65.2 kJ/mol Because slaked lime has a low specific heat (1.20 J/g degree C), storage and transport of quicklime must use containers that are not flammable and resist water. A 500. g sample of water is reacted with an equimolar amount of CaO (both at an initial temperature of 25 degree C). What is the final temperature of the product, Ca(OH)_2? Assume that the product absorbs all of the heat released in the reaction. Is the temperature from this small quantity of reactants enough to ignite wood? For full credit, give the ignition temperature of wood, and a citation of the source of your information.

Explanation / Answer

Ans. Step 1. Moles of H2O = Mass / Molar mass

                                    = 500.0 g/ (18.0 g/ mol)

                                    = 27.778 mol

Moles of CaO used = Moles of H2O taken           ; [equimolar quantity]

                                    = 27.778 mol

Total heat produced during CaO reaction with water =

                                    Molar enthalpy of CaO reaction with water x Moles of CaO

                                    = (- 65.2 kJ/ mol) x 27.778 mol

                                    = -1811.1256 kJ

The -ve sign indicates that heat is released during the process.

Amount of heat absorbed by product Ca(OH)2 = Amount of heat lost during CaO reaction with water = 1811.1256 kJ = 1811125.6 J

Step 2. 1 mol CaO produces 1 mol Ca(OH)2. So, moles of Ca(OH)2 formed = 27.778 mol

Mass of Ca(OH)2 produced = Moles x Molar mass

                                                = 27.778 mol x (74.09268 g/ mol)

                                                = 2058.15 g

Assuming all the heat released during the reaction is absorbed by Ca(OH)2.

Initial temperature of product = 250C = Initial temperature of reaction mixture.

Now, heat absorbed by the product is given by-

q = m x s x dT                      - equation 1   

Where,

m = mass of water in gram

s = specific heat of water = 4.184 J g-10C-1

dT = change in temperature = T1- T1

Putting the values in equation 1-

            1811125.6 J = 2058.15 g x (1.20 J g-10C-1) x (T2 – 250C)

            Or, 1811125.6 J / (2469.78 J 0C-1) = T2 – 250C

            Or, 733.31 0C = T2 – 250C

            Or, T2 = 733.130C – 250C = 708.130C

Therefore, the final temperature of the product = 708.130C

Ans. 2. Ignition temperature of wood = 1900- 2600C

Ref: http://www.tcforensic.com.au/docs/article10.html

We have, the final temperature of the product = 708.130C

Since the final temperature of the product is greater than the ignition temperature of wood, the wood will ignite.

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