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FROM YOUR EARLIER EXPERIENCES IN CHEMISTRY YOU HAVE LEARNED TO RECOGNIZE THAT A
CHEMICAL REACTION CAN BE REPRESENTED BY A CHEMICAL EQUATION. A CHEMICAL EQUATION
CONTAINS THE CHEMICAL FORMULAS OF REACTANTS AND PRODUCTS SEPARATED BY AN ARROW.
WE THEN ADD COEFFICIENTS TO BALANCE THE CHEMICAL REACTION. IN INTRODUCTORY
CHEMISTRY HAVE COME TO RELY VERY HEAVILY ON THE CHEMICAL EQUATION AS A SYMBOLIC
REPRESENTATION OF A CHEMICAL REACTION. SOME EXAMPLES OF CHEMICAL REACTIONS THAT
WE HAVE SEEN IN CLASS ARE;


VIEW THE MOVIE ON THE RIGHT


2H2(G) + O2(G) ---> 2H2O(G)





WE WILL ALL ADMIT THAT IT IS A LITTLE EXCITING TO ACTUALLY SEE SOME OF THESE
REACTIONS OCCUR. ANOTHER REACTION THAT MIGHT BE FUN TO WATCH CAN BE EXPRESSED IN
THE FOLLOWING EQUATION;


VIEW THE MOVIE ON THE RIGHT


2AL(S) + 3BR2(L) ---> 2ALBR3(S)


THESE TWO REACTIONS ARE EXAMPLES OF REACTIONS THAT GO TOO COMPLETION. A REACTION
THAT PROCEEDS FROM LEFT TO RIGHT UNTIL ONE OF THE REACTANTS IS COMPLETE USED UP.
IT IS VERY DIFFICULT FOR ME TO BE ABLE TO SHOW YOU THE REVERSE REACTION OF ANY
OF THESE TWO SYSTEMS.


IN CHAPTER 17 WE BEGIN TO LOOK AT CHEMICAL REACTIONS IN MORE DETAIL. TO AFFORD
THIS TYPE OF SCRUTINY WE WILL BE STUDYING A PARTICULAR TYPE OF REACTION REFERRED
TO AS AN REVERSIBLE/EQUILIBRIUM REACTION. THE TERM EQUILIBRIUM IS NOT UNFAMILIAR
TO US BECAUSE WE USED IT IN CHAPTER 13 AND 14 TO DESCRIBE VAPOR PRESSURE. THE
EQUILIBRIUM VAPOR PRESSURE FOR A LIQUID WAS ATTAINED WHEN THE RATE OF
CONDENSATION OF A LIQUID WAS EQUAL TO THE RATE OF EVAPORATION OF THE LIQUID.
THIS EQUILIBRIUM VAPOR PRESSURE WAS DEPENDENT ON THE TEMPERATURE OF THE SYSTEM.
WE WILL STUDY THE IDEA OF CHEMICAL EQUILIBRIUM IN MORE DETAIL IN THIS CHAPTER
AND RELATE IT TO CHEMICAL SYSTEMS. LET'S BEGIN BY LOOKING AT AN EXAMPLE OF A
CHEMICAL REACTION THAT WE KNOW TO EXHIBIT SOME OF THE OBSERVABLE BEHAVIOR THAT
WE ASSOCIATE WITH CHEMICAL EQUILIBRIUM. I'M NOT PROVIDING YOU WITH A DEFINITION
OF CHEMICAL EQUILIBRIUM JUST YET, RATHER I WOULD LIKE TO SHOW YOU SOME EXAMPLES
OF CHEMICAL SYSTEMS WHICH EXHIBIT THE BEHAVIOR THAT WE ASSOCIATE WITH CHEMICAL
EQUILIBRIA.


THE REACTION WE WILL STUDY IS;


FE3+(AQ) + SCN-(AQ) ---> FESCN2+(AQ)


VIEW THE MOVIE ON THE RIGHT AND ANSWER THE FOLLOWING THREE QUESTIONS:


1. IDENTIFY THE CHEMICAL SPECIES PRESENT IN THE SOLUTION POURED INTO THE BEAKER
ON THE LEFT IN THE MOVIE. ANSWER


2. IDENTIFY THE CHEMICAL SPECIES PRESENT IN THE SOLUTION POURED INTO THE BEAKER
ON THE RIGHT IN THE MOVIE. ANSWER


3. WHAT EVIDENCE IS THERE FOR A CHEMICAL REACTION WHEN THE TWO SOLUTIONS ARE
MIXED? ANSWER


IN THE REACTION THE COLOR OF EACH SPECIES IS;


FE3+(AQ) + SCN-(AQ) ---> FESCN2+(AQ)


YELLOWISH


CLEAR


REDDISH-BROWN


 


IN THE MOVIE EQUAL SAMPLES OF THE REACTION MIXTURE ARE ADDED TO TWO PETRI
DISHES.


TO THE SOLUTION IN THE PETRI DISH ON THE LEFT SEVERAL DROPS OF FE3+ ARE ADDED.


DESCRIBE WHAT YOU OBSERVE WHEN THE FE3+ IS ADDED TO THE SOLUTION IN THE PETRI
DISH.


WHAT CHEMICAL SPECIES MUST HAVE BEEN FORMED TO ACCOUNT FOR YOUR OBSERVATION?


WHAT CHEMICAL SPECIES DID THE FE3+, THAT WAS ADDED TO THE SOLUTION IN THE PETRI
DISH, REACT WITH TO PRODUCE THE CHANGE YOU OBSERVED?


ANSWER


 


 


IN THIS MOVIE EQUAL SAMPLES OF THE REACTION MIXTURE WERE ADDED TO TWO PETRI
DISHES. TO THE PETRI DISH ON THE LEFT SEVERAL DROPS OF FE3+ HAVE BEEN ADDED.


TO THE SOLUTION IN THE PETRI DISH ON THE RIGHT SEVERAL DROPS OF SCN- ARE ADDED.


DESCRIBE WHAT YOU OBSERVE WHEN THE SCN- IS ADDED TO THE SOLUTION IN THE PETRI
DISH.


WHAT CHEMICAL SPECIES MUST HAVE BEEN FORMED TO ACCOUNT FOR YOUR OBSERVATION?


WHAT CHEMICAL SPECIES DID THE SCN-, THAT WAS ADDED TO THE SOLUTION IN THE PETRI
DISH, REACT WITH TO PRODUCE THE CHANGE YOU OBSERVED?


ANSWER


 


WHAT IS REALLY INTERESTING ABOUT ADDING FE3+(AQ) TO THE PETRI DISH ON THE LEFT
AND SCN-(AQ) TO THE PETRI DISH ON THE RIGHT ARE THE RESULTS WE OBSERVE. ADDING
SOME FE3+(AQ) AND OBSERVING THE SOLUTION GET DARKER IMPLIES THAT THERE MUST BE
SOME UNREACTED SCN-(AQ) IN THE PETRI DISH ON THE LEFT. SO WHEN THE ORIGINAL
SAMPLE OF FE3+(AQ) AND SCN-(AQ) WERE MIXED THERE IS UNREACTED SCN-(AQ) AVAILABLE
TO REACT WITH ANY ADDITIONAL FE3+(AQ) .


BUT WAIT, WHEN SOME SCN-(AQ) IS ADDED TO PETRI DISH ON THE RIGHT THE COLOR GETS
DARKER IMPLYING THERE IS UNREACTED FE3+(AQ) IN THAT SOLUTION. SO WHEN THE
ORIGINAL SAMPLE OF FE3+(AQ) AND SCN-(AQ) WERE MIXED THERE IS UNREACTED FE3+(AQ)
AVAILABLE TO REACT WITH ANY ADDITIONAL SCN-(AQ).


HOW CAN THAT BE!!! HOW CAN IT BE POSSIBLE TO MIX THE ORIGINAL SAMPLES OF
FE3+(AQ) AND SCN-(AQ) SPLIT THE RESULTING MIXTURE AND FIND THERE IS STILL SOME
UNREACTED FE3+(AQ) AND SCN-(AQ) IN BOTH SOLUTIONS?!


THE ONLY WAY THIS COULD HAPPEN IS IF WHEN THE ORIGINAL SOLUTIONS OF FE3+(AQ) AND
SCN-(AQ) WERE MIXED, SOME PRODUCT, FESCN2+(AQ) WAS FORMED, BUT THE REACTION DID
NOT COMPLETELY USE UP EITHER OF THE REACTANTS. SOME FE3+(AQ) AND SCN-(AQ)
REMAINED UNREACTED. SO WHEN THE SAMPLE WAS SPLIT INTO TWO BEAKERS THE ADDITION
OF MORE REACTANT RESULTED IN MORE REACTION.


WHAT IS EVEN MORE INTERESTING IS WHAT HAPPENS WHEN F-(AQ) IS ADDED TO A FRESH
SAMPLE OF FESCN2+(AQ). BEFORE DESCRIBING WHAT IS OBSERVED IT IS IMPORTANT TO
KNOW THAT MIXING F-(AQ) WITH FE3+(AQ) PRODUCES THE COLORLESS COMPOUND FEF63-. SO
ADDING F-(AQ) WILL REACT WITH ANY FE3+(AQ) IN THE SOLUTION.


DESCRIBE WHAT YOU OBSERVE WHEN THE F- IS ADDED TO THE SOLUTION IN THE BEAKER.


EXPLAIN WHY THE SOLUTION CHANGES COLOR.


ANSWER


 


 


 


 


AMAZING, IT IS AS THOUGH THE REACTION


FE3+(AQ) + SCN-(AQ) ---> FESCN2+(AQ)


CAN ALSO GO BACKWARDS;


FESCN2+(AQ) ---> FE3+(AQ) + SCN-(AQ)


THIS IS UNLIKE ANY REACTION WE HAVE INVESTIGATED BEFORE.


ACTUALLY THIS IS NOT THAT UNIQUE, AND WE DESCRIBE SUCH A REACTION AS AN
EQUILIBRIUM REACTION. THE EQUATION IS WRITTEN;


FE3+(AQ) + SCN-(AQ) FESCN2+(AQ)


WHERE THE SYMBOL REPRESENTS A REACTION THAT IS CAPABLE OF GOING IN BOTH
DIRECTIONS.


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