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How do I Calculate Hf for CaC2?

Calcium carbide -- CaC2 -- is a colorless solid that has a wide range of industrial applications. Mixed with water, it reacts to form the ripening agent acetylene, while in its plain form it is capable of removing the sulfur from iron. All of these functions depend on the chemical properties of CaC2, one of which is Hf, or the standard enthalpy of formation. Once the standard heat of formation is obtained, it is possible to measure both the enthalpy change that occurred in the compound̵7;s formation, as well as its exothermic or endothermic properties.

Things You'll Need

Calculator
Enthalpy table

Instructions

- 1
  Determine the chemical equation for CaC2. By analyzing the compound, it can be seen that CaC2 contains 1 mole of calcium and 2 moles of carbon. Therefore, Ca + 2C ͛4; CaC2.
- 2
  Obtain the following values from a standard enthalpy table. They are the changes in enthalpy in the formation of calcium hydroxide, calcium oxide and water respectively, with equation 4 used to balance the water.
  1.) CaO(s) + H2O (l) ͛4;Ca(OH)2 (s) ͧ0;H = -65.19kJ
  2.) 2C(s) + O2 (g) ͛4; 2CO(g) ͧ0;H = -221.00kJ
  3.) 2H2 (g) + O2 (g) ͛4; 2H2O(l) ͧ0;H = -571.8kJ
  5.) Ca(s) + 2H2O (l) ͛4; Ca(OH)2 (s) + H2 (g) ͧ0;H = -414.79kJ
  5.) CaO(s) + 3C(s) ͛4; CaC2 (s) + CO (g) ͧ0;H = +462.3kJ
  Use equation 5 as a predetermined value, from the standard procedure of obtaining CaC2 by heating lime with coke.
- 3
  Use Hess's law to obtain Hf for Ca + 2C ͛4; CaC2. Since the ͧ0;H for a reaction that is carried out in a series of steps is equal to the added enthalpy changes of the individual steps, balance the equations so that they cancel out to obtain the desired formula.
- 4
  Imagine the steps as an actual reaction. Using Step 3 as a base, modify the steps and change the ͧ0;H accordingly as follows:
  1.) double coefficients and reverse: ͧ0;H = +130.38 kJ
  2.) reverse products and reactants: ͧ0;H = +221.00kJ
  3.) no change: ͧ0;H = -571.8kJ
  4.) double coefficients: ͧ0;H = -829.58 kJ
  5.) double coefficients: ͧ0;H = +924.6 kJ
- 5
  Cancel out those compounds which appear on both the product and reactant sides of the individual steps and combine the remaining products to obtain the final reaction; 2Ca (s) + 4C (s) ͛4; 2CaC2 (s). Add the ͧ0;H values.
- 6
  Divide the total ͧ0;H of -125.4 kJ to obtain the Hf of one mole of CaC2 instead of two. Therefore, the standard enthalpy of formation of calcium carbide is -62.7 kJ/mol, indicating an exothermic, or energy-producing reaction.

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