Thermochemical Stoicheometry

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Thermochemical Stoicheometry

  1. 1. Thermochemical Stoicheometry Thermochemical Stoicheometry Thermochemical stoicheometry is stoicheometry problems with the energy factor involved. Thermochemical Equations are balanced equations with the energy exchange indicated either within the equation or as a Delta H term on the outside of the equation. For example: 2 Na + 2 H2O(l) -----> 2 NaOH(aq) + H2(g) Delta H = -367.5 kj An alternative way of showing the equation would be: 2 Na + 2 H2O -----> 2 NaOH(aq) + H2 (g) + 367.5 Kj Using the coefficients and the Delta H one can form mole Kj relationships for each component found in the balanced equation. The above thermochemical equation could use any one of the following mole Kj relationships: 2 moles Na = -367.5 2 moles H2O = -367.5 Kj 2 moles NaOH = -367.5 Kj 1 mole H2 = -367.5 Kj Let's use the above thermochemical equation for an example of a thermochemical stoicheometry problem: How much energy will be produced if 18 moles Na are reacted with enough water to consume all the the Na? 1. Determine the mole Kj relationship for the given component using the balanced equation: 2 moles Na = -367.5 Kj http://members.aol.com/profchm/tstoic.html (1 of 3)14/09/2006 15.30.36
  2. 2. Thermochemical Stoicheometry 2. Convert given moles of Na to requested Kj of energy 18 moles Na X -367.5 Kj / 2 moles Na = 9 ( -367.5 Kj) = -3307.5 Kj Another example illustrating a mass-Kj problem How much energy will be produced when 69 grams Na react with excess water. 1. Convert grams Na to moles of Na: 69 grams Na X 1 mole Na / 23 grams Na = 3 moles Na 2. Convert moles Na to Kj using the balanced equation 3 moles Na X -367.5 Kj / 2 moles Na = -551.25 Kj Now it is your turn: Given the following equation: 2 H2 (g) + O2 (g) -----> 2 H2O (l) Delta H = -571.7 Kj How much energy is produced if 100 grams of H2 are converted to water? After you have attempted this problem click here to see how you did. How many grams of H2 would be required to produce 5000 Kj? After you have an answer for this part click here R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College. Send Comments to R.H. Logan: Profchm@aol.com All contents copyrighted (c) 1996 R.H. Logan, Instructor of Chemistry,DCCCD All Rights reserved http://members.aol.com/profchm/tstoic.html (2 of 3)14/09/2006 15.30.36
  3. 3. Thermochemical Stoicheometry Revised: 11/26/96 Original Date of Creation: 11/26/96 Return to Home Page URL:http://members.aol.com/profchm/tstoic. html http://members.aol.com/profchm/tstoic.html (3 of 3)14/09/2006 15.30.36

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