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L L AABB 131 3:: RR ATATEESS && EE QQUUIILLIIBBRRIIUUMM ::
R R EEAACCTTIIOONN RR AATTEESS AANNDD LL EE CC HAHATTEELLIIEERR ’’ SS PP RIRINNCCIIPPLLEE
P P UURRPPOOSSEE :: To observe and describe factors that affect the rate of a chemical reaction.
To observe the concentration effects on a system at equilibrium.
S S AAFFEETTYY CC OONNCCEERRNNSS :: Always wear safety goggles. Handle and dispose of broken glass safely.
Acids and bases are corrosive and can severely damage tissue. If acid or base is spilled, flood the area with water and notify the instructor immediately. If acid or base contacts the skin, remove any clothing covering the skin and flush with copious amounts of water. If acid or base contacts the eyes, flush with water for 20 minutes. Wash your hands after handling acids and bases. Solutions of hydrogen peroxide, especially greater than 3%, should be handled with great care. If spilled, immediately flood the area with water and wash the liquid down the drain. Be careful not to spill any on your person, books, or clothing. Concentrations greater than 3% may cause white patches or blisters and chemical burns on the skin.
R R EEAACCTTIIOONN RR ATATEESS ::
The rate of a chemical reaction is the speed at which a reaction occurs. Some reactions, like the rusting of an iron nail, take years to complete. Other reactions, like the explosion of dynamite, are over in a fraction of a second.
Reaction Energy:
In order to react, two molecules must physically collide with each other; the more frequent the collisions, the faster the reaction. The molecules must collide with enough energy (the activation energy ) to overcome the repelling forces between the molecules. If molecules do not collide with energy at least as large as the activation energy, they will not react. The greater the number of collisions meeting or exceeding the activation energy, the faster the reaction.
You will investigate several factors that affect the rate of a chemical reaction: the concentration of the reactants, the temperature of the reactants, and the effect of adding a catalyst. These factors determine the frequency and energy of reactant collisions.
Concentration:
In this experiment you will study the reaction rates of acids with metals under different conditions. You will vary the concentration of reactants by using acids of differing molarity ( M ). If two solutions have equal volumes, the one having a greater molarity, M, will have a greater number of solute molecules.
When a metal reacts with an acid, vigorous bubbling, due to the liberation of hydrogen gas, takes place at the surface of the metal. The existence of hydrogen gas bubbles is evidence that a reaction is occurring. The presence of hydrogen can be verified by placing a burning wooden splint above the surface of reactants. A whistling or popping sound indicates the presence of hydrogen. The lighted splint furnishes the activation energy needed to initiate the reaction between hydrogen and oxygen.
2H2(g) + O2(g) Æ 2H 2 O (g)
hydrogen oxygen water
The intensity of the sound is determined by the concentrations of oxygen and hydrogen. Pure hydrogen will produce a muffled "whoof," while the right mixture of hydrogen and air gives a loud "pop." Thus, the intensity of the sound of the reaction between hydrogen and oxygen cannot be used to judge the rate of the reaction between the metal and hydrochloric acid. The rate of reaction between metal and hydrochloric acid can be qualitatively judged by observing how rapidly the metal produces bubbles of gas. A more quantitative determination can be made by timing the number of minutes before the metal disappears.
C C AATTAALLYYSSIISS :: A catalyst is a substance that increases the rate of a chemical reaction, not by increasing the number or energy of molecular collisions, but by lowering the activation energy required for the reaction. The catalyst interacts with the reactants to create an alternative path or route for the reaction--a route with lower activation energy. Reactant molecules are more likely to collide with sufficient energy and the reaction rate is greater in the presence of a catalyst.
The reaction under study in this experiment is the decomposition (breaking apart) of hydrogen peroxide, H 2 O 2. Hydrogen peroxide slowly decomposes as it sits in its bottle, gradually turning to water and oxygen gas. The unbalanced equation:
H2O2(aq) Æ H2O(l) + O2(g)
This reaction rate is accelerated by a catalyst. The presence of oxygen, which indicates that decomposition has taken place, can be easily verified. A glowing wooden splint is slowly burning in the oxygen of air. If the glowing splint is placed in a higher concentration of oxygen, the rate of combustion will increase and the wood splint will burst into flames.
Hydrogen peroxide is an oxidizing agent --it removes electrons from other substances. It destroys organic compounds that come in contact with it. A 3% solution of hydrogen peroxide is used as an antiseptic for small cuts of the skin--it oxidizes germs to death. When placed on an open wound, the hydrogen peroxide bubbles; the bubbles are oxygen gas. The exposed blood contains an enzyme (peroxidase) that catalyzes the decomposition of hydrogen peroxide. The oxygen bubbles come from the hydrogen peroxide, not from the catalyst. A catalyst is neither used up in a reaction nor irreversibly altered; a catalyst is a condition of the reaction and, if included in an equation, is written above the equation arrow. The unbalanced equation:
Peroxidase cat
H2O2(aq) Æ H2O(l) + O2(g)
When a red spot is suspected of being blood, forensic chemists can use this reaction in a presumptive test for blood. The release of oxygen from hydrogen peroxide by the peroxidase activity causes an indicator to change color. This is not a definitive test for blood since other substances can also catalyze the decomposition of hydrogen peroxide.
Hydrogen peroxide is used in higher concentrations (6%) to bleach (oxidize) hair pigments in many hair-dyeing systems. Still higher concentrations (90%) are sometimes used as oxidizers in rocket propulsion.
In this experiment you will examine the following equilibrium system:
FeCl 3(aq) + 3 NH 4SCN(aq) R Fe(SCN) 3(aq) + 3 NH 4Cl (aq)
iron(III) chloride ammonium thiocyanate iron(III) thiocyanate ammonium chloride (pale yellow) (colorless) (deep red) (colorless)
The shifts in equilibrium can be easily followed in each case because the color of reactants is different than the color of products.
P P ROROCCEEDDUURREESS ::
ACTIONS :
I. R EACTION RATES:
A. Concentration Effects:
- Obtain 3 pieces of Magnesium ribbon (Mg) of equal length, about 2-3 cm long.^1 Clean and polish the Mg with steel wool to remove any oxide coating. 2
- Obtain 3 clean test tubes with rubber stoppers and label them #1-3. Pour acids into the tubes as follows: Tube #1: 10 mL of 1.0 M HCl Tube #2: 10 mL of 2.0 M HCl Tube #3: 10 mL of 3.0 M HCl
- Have a clock with a second hand or a stop watch available to time the reactions of Mg and acids that you are about to perform.
- As close to the same time as possible, place the cleaned pieces of Mg into each of the tubes #1-3, stopper loosely^3 , and note the start time of the reactions.
- With the clock still running carefully bring a burning wood splint to the mouth of one of the tubes. Quickly remove the stopper and test the flammability and verify the identity of the gas.
- Record the amount of time for each sample of Mg to completely react and disappear. 4
- At the end of the reaction determine whether the reaction is exothermic or endothermic by touching the bottom of the test tube.
- Rank the rates of reactions from slowest to fastest.
NOTES :
(^1) The exact length of the Mg ribbon is not important; however you want all 3 pieces to be of equal length for comparison purposes. (^2) Metals slowly react with oxygen in the air to form oxides. Iron reacts with oxygen over time to form rust, iron(III) oxide; 4Fe + 3O 2 Æ 2Fe (^) 2O (^3) Likewise, magnesium over time may acquire a coating of magnesium oxide that can interfere with our experiment. 2Mg + O 2 Æ 2MgO (^3) The reaction of hydrochloric acid with magnesium produces hydrogen gas. If stoppers are on too tight then pressure will build and they will be popped off.
(^4) Magnesium, Mg, reacts with hydrochloric acid, HCl, to produce Magnesium chloride, MgCl2, and Hydrogen gas, H 2.
2 M HCl
3 M HCl
1 M HCl
B. Temperature Effects:
- Prepare a hot water bath by filling a 250 mL beaker about ½ full of water and heating on a hot plate. Do not allow it to boil.
- Prepare an ice water bath by filling a 250 mL beaker about ½ full of ice and water.
- Place 10 mL of vinegar in each of two large test tubes one labeled H (for hot) and the other C (for cold).
- Place tube H into the hot water bath and tube C into the ice bath and allow them to stay long enough to insure that each vinegar solution has reached the bath temperature. 5
- Obtain 2 clean dry test tubes and into each place equal amounts (1 scoop or 2 spatula tips) of sodium bicarbonate, NaHCO 3.
- Remove test tubes H and C from their temperature baths and place them in a test tube rack. Measure the temperature in each test tube.
- At the same time, add a sample of NaHCO 3 to each tube H and C, and observe and compare the fizzing (bubbles) in each. 6
- Determine which reaction ( H or C ) clears first.
C. Catalyst Effects:
- Pour 10 mLs of 3% hydrogen peroxide, H 2 O 2 into a test tube and loosely stopper. Take and record the temperature of the H 2 O 2 .solution
- Check for the presence of oxygen by inserting the glowing 7 (not burning) splint deep into the test tube of H 2 O 2. Observe if the splint bursts into flames. Remove the splint after a few seconds so that the test tube does not fill with smoke.
- Obtain a skinless rectangle of raw potato^8 (about 1cm x 1 cm x 3 cm) and dice it into tiny bits.
- Add the diced potato to the tube of H 2 O 2 and loosely stopper. Observe any reaction.
- After about 5-10 minutes test for the presence of oxygen, O 2 , with a glowing splint. 7
- Record the final temperature of the reaction.
(^5) You might use a thermometer to verify the temperatures.
(^6) Caution : Move your papers out of the way as this reaction may get messy. ☺ Acetic Acid, HC 2 H (^) 3O (^) 2, reacts with sodium bicarbonate, NaHCO (^) 3, to give sodium acetate, NaC2H (^) 3O 2 , and carbonic acid, H (^) 2CO (^) 3. The carbonic acid immediately falls apart to give carbon dioxide, CO (^2) and water, H 2 O. (^7) Use a match to light the end of a wooden splint. Allow the fire to burn a few seconds, then blow out the flames, leaving glowing embers. A glowing splint will flicker into a flame in the presence of oxygen, O (^) 2, gas. (^8) When a slice of potato is placed into a solution of hydrogen peroxide, bubbles of oxygen form on the potato surface and rises to the top of the solution. Potatoes contain catalase , an enzyme that breaks down hydrogen peroxide into water and oxygen.
C Cold HC 2 H 3 O 2
H Hot HC 2 H 3 O 2
NaHCO 3
Potato Catalase In H 2 O 2
L L ABAB 131 3:: RR ATATEESS && (^) EE (^) QQUUIILLIIBBRRIIUUMM NN AMAMEE (^) __________________________
PP (^) RREE LL (^) AABB EE (^) XXEERRCCIISSEESS (^) :: DD ATATEE (^) ____________________________
1.___ If concentrated acids or bases contact your skin you should A. remove any clothing covering the skin affected. D. wash with soap. B. flush with copious amounts of water. E. all of these. C. inform your instructor
2.___ The word copious means _____ A. a very little bit. B. a large quantity. C. hot. D. ice cold.
3.___ The rate of a chemical reaction may be increased by A. increasing the frequency of molecular collisions B. increasing the energy of molecular collisions C. lowering the activation energy of the reaction D. all of these
- Write the complete and balanced equation for the equilibrium reaction between Iron (III) chloride and Ammonium thiocyanate:
- Write the equilibrium expression (Keq ) for the net ionic equation of this reaction: Fe 3+(aq) + 2SCN1-(aq) R Fe(SCN) 2 1-(aq)
- Given the following equilibrium A + B R C + D, what will be the effect on the
equilibrium if each of the following takes place?
Will equilibrium Shift Å or Æ or none (N)?
Will the concentration of these substances increase (K ) or decrease ( L) or be unaffected (U)? A. Increase [C] A and B
B. Decrease [A] C and D
C. Increase [B] C and D
D. Decrease [D] A and B
C. Catalyst Effects:
Complete and Balance:
Equation= _____H 2 O2(aq) Æ Temp Reaction Rate (#1 fastest )
Observations Include verification for identity of gas
Hydrogen Peroxide H 2 O 2 Hydrogen Peroxide H 2 O 2 w/ potato catalase
Summary and Conclusions:
1.___ According to our experimental data, adding a catalyst ______ the rate of a reaction. A. increases B. decreases C. has no effect on D. has variable effects on
2.___ According to our experimental data, decomposition of H 2 O 2 is ______. A. endothermic B. exothermic C. neither endo nor exothermic
Error Analysis:
- Commercial hydrogen peroxide, H 2 O 2 , is stored in dark bottles in which a trace of the inhibitor acetanilide has been added. Both the dark bottle and the acetanilide help prevent the decomposition of the hydrogen peroxide. An inhibitor acts the opposite of a catalyst and so increases the activation energy of the reaction.
Complete the given potential energy diagram using a
- solid line ( ___ ) to represent the decomposition of hydrogen peroxide, H 2 O 2.
- dotted line ( ……. ) to represent the decomposition of H 2 O 2 in the presence of raw potato containing the enzyme catalase.
- starred line (*****) to represent the decomposition of H 2 O 2 in the presence of acetanilide.
Energy
Reaction Progress
4.___ The purpose of storing commercial H 2 O 2 in a dark bottle is ___ A. To keep the hydrogen peroxide cool. B. To increase the Eact and so prevent hydrogen peroxide from decomposing. C. to prevent light from entering the bottle because light can act as a catalyst and so lower the E (^) act enabling hydrogen peroxide to decompose at room temperature. D. To prevent light from entering the bottle because light can provide the Eact needed to cause hydrogen peroxide to decompose.
II II.. (^) EE QUQUIILLIIBBRRIIUUMM
Fe 3+(aq) + 2SCN1-(aq) R Fe(SCN) 2 1-(aq) iron(III) ion thiocyanate ion iron(III) thiocyanate complex (pale yellow) (colorless) (deep red) Stress Applied
Observation (resulting color)
[Fe3+(aq) ] K or L
[SCN1-(aq) ] K or L
Shift Å or Æor N
[Fe(SCN) 2 1-(aq) ] K or L Original color Original color
A. Control Original color of mix
B. Add [Fe3+^ ]
Increase
C. Add [SCN 1-^ ]
Increase
D. Add [Cl1-^ ]
E. Add heat (Increase Temp)
F. Remove heat ( Decrease Temp)
Summary and Conclusions:
Give an explanation for the effect observed when HCl was added:
Give an explanation for the effect of heat and cold on the reaction.
1.___ According to our experimental data, this reaction is ______. A. endothermic B. exothermic C. neither endo nor exothermic
Error Analysis:
R R ELELAATTEEDD EE XEXERRCCIISSEESS ::
1.___ The fact that chewed food digests faster than unchewed food can best be explained by __ A. increased surface area in chewed food than in unchewed. B. increased temperature of chewed food than of unchewed. C. greater concentration of food in chewed than in unchewed. D. more catalyst present in chewed food than in unchewed.
