chapter_17-Lecture 5

# chapter_17-Lecture 5 - Predicting Reaction Direction and...

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Predicting Reaction Direction and Calculating Equilibrium Concentrations PROBLEM: The research and development unit of a chemical company is studying the reaction of CH 4 and H 2 S, two components of natural gas. H + 2H ( C S +4H CH 4 ( g ) + 2H 2 S( g ) CS 2 ( g ) + 4H 2 ( g ) In one experiment, 1.00 mol of CH 4 , 1.00 mol of CS 2 , 2.00 mol of H 2 S, and 2.00 mol of H 2 are mixed in a 250-mL vessel at 960 o C. At this temperature, K c = 0.036. (a) In which direction will the reaction proceed to reach equilibrium? (b) If [CH 4 ] = 5.56 M at equilibrium, what are the equilibrium concentrations of the other substances? PLAN: Find the initial molar concentrations of all components and calculate a Q c . Compare Q c to K c , determine the reaction direction, and draw up expressions for equilibrium concentrations. SOLUTION: Step 1) find initial concentrations [CH 4 ] initial = 1.00 mol/0.25 L = 4.0 M [CS 2 ] initial = 1.00 mol/0.25 L = 4.0 M 17-57 [H 2 S] initial = 2.00 mol/0.25 L = 8.0 M [H 2 ] initial = 2.00 mol/0.25 L = 8.0 M

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Predicting Reaction Direction and Calculating Equilibrium Concentrations ontinued ( continued ) [CS 2 ][H 2 ] 4 [4.0][8.0] 4 A Q c of 64 is >> than K c = 0.036 Step 2) Calculate Q c Q c = [CH 4 ][H 2 S] 2 = [4.0][8.0] 2 = 64 The reaction will progress to the left. Step 3) Construct Reaction Table CH 4 ( g ) + 2H 2 S( g ) CS 2 ( g ) + 4H 2 ( g ) Concentration ( M ) initial 4.0 8.0 4.0 8.0 change equilibrium + x +2 x -4 x 4.0 + x 8.0 + 2 x - x 4.0 - x 8.0 - 4 x At equilibrium [CH 4 ] = 5.56 M , so 5.56 = 4.0 + x and x = 1.56 M Therefore - [H 2 S] = 8.0 + 2 x = 11.12 M 17-58 [CS 2 ] = 4.0 - x = 2.44 M [H 2 ] = 8.0 - 4 x = 1.76 M
Lecture 5 17-59

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Steps in solving equilibrium problems. PRELIMINARY SETTING UP WORKING ON A REACTION TABLE 1.Write the balanced equation. 4.When reaction direction is not 2.Write the reaction quotient, Q . 3.Convert all of the amounts into the correct units ( M or atm). known compare Q with K . 5.Construct a reaction table. Check the sign of x , the change OLVING FOR X AND EQUILIBRIUM QUANTITIES in the quantity. SOLVING FOR X AND EQUILIBRIUM QUANTITIES 6. Substitute the quantities into Q . 7. To simplify the math, assume that x is egligible Check that assumption is justified (<5% error). If not, solve quadratic equation for x . negligible. 8. [A] init - x = [A] eq [A] init 9. Solve for x . 10.Find the equilibrium quantities. Check to see that calculated values give the known K . 17-60 qq
Le Le Chatelier’s Chatelier’s Principle Principle hen a chemical system at equilibrium When a chemical system at equilibrium is subjected to a stress, e system will return to equilibrium the system will return to equilibrium by shifting to reduce the stress.

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## chapter_17-Lecture 5 - Predicting Reaction Direction and...

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