Journal of Chemical Education, Chemistry Comes Alive! Videos

Oxidation of alcohol by Mn₂O₇:

• This demonstration is Potentially Dangerous! Try adding only a few crystals at first (you can always add more if the reaction slows) and do NOT try to stop the reaction!
• Fasten a large test tube to a ringstand with the clamp at the top of the tube.
• Pour concentrated sulfuric acid to fill one-fourth of the tube.
• Slowly add an equal amount of ethyl alcohol. Pour it down the side of the tube with a pipet so that the two liquids do not mix.
• As a safety precaution, immerse the tube in a large beaker of water to cover at least half of the acid layer.
• Add a few crystals of potassium permanganate to the tube.
• In a couple of minutes bubbles of gas begin and a green color forms at the interface of the two liquids. Then sparks are produced that pop and flash.
• Some comments:
  • Green Mn₂O₇ forms at the interface.
  • It is a powerful oxidizing agent that oxidizes the alcohol at the interface.
  • In concentrated form, this anhydride of permanganic acid is highly explosive!

• The reaction complex:
  1. KMnO_{4 (s)} + 3H₂SO_{4 (aq)} → K⁺ _(aq) + MnO₃⁺ _(aq) + H₃O⁺ _(aq) + 3HSO₄^- _(aq)

  2. MnO₃⁺ _(aq) + MnO₄^- _(aq) → Mn₂O_{7 (s)}

  3. Mn₂O_{7 (s)} + CH₃CH₂OH _(aq) → 2CO_{2 (g)} + 3H₂O _(l) + 4MnO_{2 (aq)}

 

NI₃, an unstable compound:

• I have seen the explosion from a pile about the size of a quarter shatter the end of a meter stick!
• Place about 5 g of iodine crystals in a beaker.
  
• Add about 20 ml of concentrated ammonium hydroxide and stir.
  
• Filter the solution.
  
• The compound on the filter paper is nitrogen triiodide. It SHOULD be stable while wet. After it drys, it WILL EXPLODE loudly when touched or disturbed in any way.
• The compound is too unstable to be stored.
• Time your preparation so the compound will be just dry when needed.
• The reactions:
  • Production of nitrogen triiodide - 3I₂ + NH₃ → NI₃ + 3 HI
  • Decomposition of nitrogen triiodide - 2NI_3(s) → N_2(g) + 3I_2(g)
  • The energy released by detonating NI₃ exceeds that required to form the compound, which is the definition of a high yield explosive.
• A video

 

Dehydration of sugar by H₂SO₄:

• Fill a small beaker about one-third with table sugar.
• Slowly pour about 10 ml of concentrated sulfuric acid over the sugar.
• Steam and a column of black carbon will begin to rise from the beaker.
• This reaction produces noxious fumes and should be well vented.
• The overall reaction:

  C₁₂H₂₂O_{11 (s)} + 11H₂SO_{4 (aq)} → 12C _(s) + 11H₂SO₄ ^. H₂O _(g)

 

Rapid oxidation:

• Do this in a hood.
• Place a spoonful of potassium permanganate in an evaporating dish.
• Pour about 10 ml of glycerin onto the pile.
• Smoke and Fire in about 15 seconds.
• Video

 

Popcorn mass:

• Lightly coat the inside of a 500 ml beaker with cooking oil. Record the weight of the beaker.
• Place about 30 kernels of popcorn in the beaker. Record the weight of the beaker and contents.
• Cover the beaker with a wire screen and heat over a burner flame. Be careful not to burn the popped kernels.
• After the beaker cools, remove the wire screen and record the mass of the beaker and contents.
• Compare the mass before and after popping.

  Questions:

• Does the popcorn gain or lose mass during heating?
• What is the percentage of mass change?
• What substance accounts for the change in mass?

 

Change copper to gold - making brass alloy:

• Place about 5 grams of zinc dust in an evaporating dish.
• Add enough 6M NaOH to cover the zinc and fill the dish one-third full. Six molar NaHO is 240 g per liter.
• Heat the solution to near boiling.
• Carefully place a cleaned penny into the mixture.
• It will take 3 to 4 minutes for the zinc plating to form.
• Remove the penny, wash it and blot it dry. The copper has now been changed into "silver".
• Using tweezers, hold the coated penny in a burner flame. The gold color begins to form immediately. After about 5 seconds remove the penny, wash it, and dry it.
• Zinc reacts with sodium hydroxide to produce sodium zincate, Na₂ZnO₂, which is then reduced by the copper to metallic zinc. Heating the penny causes a fusion of the zinc and copper.
• Brass is 60 to 82% Cu and 18 to 40% Zn.

 

Oxygen gas from laundry bleach:

• Set up a gas-collecting apparatus. A filtering flask with a hose connected to the arm is a quick one to use.
• Put 100 ml of bleach in the flask.
• Add about 5 g of cobalt (II) chloride.
• Quickly stopper the flask and swirl the contents.
• Displace all the air in the flask and collecting tube before collecting the gas by water displacement.
• The reaction: 2ClO ⁻_(aq) → CoCl₂ catalyst → O_{2 (g)} + 2Cl ⁻_(aq)

 

Chlorine gas from laundry bleach:

• Set up a gas-collecting apparatus.
• Put 30 ml of bleach in the flask.
• Add 5 ml of 1M HCl and stopper the flask. (stronger acid will generate gas more quickly)
• Collect the gas by upward displacement of air.
• The reaction: ClO ⁻_(aq) + Cl ⁻_(aq) + 2H⁺_(aq) → Cl_{2 (g)} + H₂O_(l)
• Do not inhale the chlorine gas! It can be identified by its color.

• Why were the two gases (oxygen and chlorine) collected differently?

 

Blue precipitate:

• Start with about 200 ml of clear, saturated limewater solution.
• Add a few drops of cobalt chloride solution.
• Immediately a blue precipitate forms. CoCl_{2 (aq)} + Ca(OH)_{2 (aq)} → Ca⁺²_(aq) + 2Cl⁻¹_(aq) + Co(OH)_{2 (s)}

 

How much is a mole?

• Place 12 grams of animal charcoal (carbon) in a container.
• "How many molecules are in this container?"

 

Displacement of Tin by Zinc:

• Make a solution of SnCl₂ by dissolving 20 g in 200 ml of water in a 400 ml beaker.
• Add 40 ml of concentrated HCl and mix the solution.
• Cover the bottom of the beaker with mossy zinc.
• Spongy tin will immediately form and soon rise to the surface.
• The two reactions involved:
  1. Zn _(s) + SnCl_{2 (aq)} → Sn _(s) + ZnCl_{2 (aq)}
  2. Zn _(s) + 2HCl _(aq) → H_{2 (g)} + ZnCl_{2 (aq)}

 

Double Displacement between two solids:

• Place equal amounts of lead (II) nitrate and potassium iodide in a small flask.
• Stopper the flask and swirl.
• The two white solids produce a yellow one.
• The reaction: Pb(NO₃)_{2 (s)} + 2KI _(s) → PbI_{2 (s)} + 2KNO_{3 (s)}
• Lead (II) iodide is the yellow solid.

 

Equilibrium in Gases:

• Place 10 ml of of concentrated nitric acid in a flask.
• Drop a penny (pre-1982) in the flask.
• The deep red gas is NO₂. When the gas has filled the flask, "pour" two test tubes of the gas and stopper them.
• Stop the rection in the flask by filling the flask with water.
• Place one tube in a beaker of boiling water. The color gets deep brown.
• Place the other tube in an ice bath. The gas becomes almost colorless.
• Remove both tubes and bring them to room temperature. The original color returns to both tubes.
• The equation for the gas production is: Cu_(s) + 4H⁺_(aq) + 2 NO₃^-_(aq) → Cu⁺²_(aq) + 2NO_2(g) + 2H₂O_(l)
• The equilibrium gases are:
  2NO_{2(g) RED} N₂O_{4(g) COLORLESS}

 

Graham's Law of Gas Diffusion:

• Clean and dry two 100 ml graduated cylinders.
• Push a small, moist piece of blue litmus paper to the bottom of the cylinder.
• Lay the cylinder on its side, making sure it is level.
• Dip a small piece of cottom in concentrated HCl and place it just inside the mouth of the cylinder.
• Immediately seal the mouth of the cylinder with plastic wrap and begin timing the diffusion.
• When the HCl gas reaches the bottom of the cylinder, the litmus paper will turn red.
• Record the time required for the gas to travel the length of the cylinder.
• Repeat the procedure with ammonium hydroxide and red litmus paper. Graham's law states that the rate of diffusion of two gases is inversely proportional to the square roots of their molecular masses.
  1. Determine the rate by dividing the distance the gas traveled, in centimetes, by time, in seconds.
  2. Determine the ratio by dividing the rate of diffusion of HCl gas by the rate of diffusion of NH₃ gas.
  3. The theoretical ratio is:
     (NH₃ diffusion rate) / (HCl diffusion rate) = square root of [(HCl molecular mass) / (NH₃ molecular mass)] = 1.46

 

Decomposing hydrogen peroxide in a catalyst:

• Place a LARGE graduated cylinder on a tray.
• Pour about 50 ml of 30% hydrogen peroxide into the cylinder.
• Add a squirt of dishwashing detergent.
• Finally add about one-fourth of a spoonful of solid KI.
• The rapid production of oxygen causes boam to fill, and probably overflow the cylinder.
• The two-step decomposition of H₂O₂ is:
  1. H₂O_2aq + I⁻_aq → H₂O_l + OI⁻_aq
  2. H₂O_2aq + OI⁻_aq → H₂O_l + O_2g + I⁻_aq
• Reaction "a" is the rate-determing reaction.

 

A Reversible Reaction:

• Dissolve 14 grams of NaOH in 500ml of water in a 1000ml flask.
• Add 14 grams of dextrose and a few drops of methylene blue indicator to the flask.
• Stopper the flask tightly and shake vigorously until the solution is blue in color.
  This caustic solution damages skin and eyes. Be careful that the flask does not leak.
• Allow the solution to sit and observe the color change as the reaction reverses.
• Will shaking the flask again cause the color to change back?

 

Does Temperature Affect Equilibrium?

• Dissolve 6 g of cobalt chloride in 250 ml of ethanol.
• Add water drop by drop while stirring until pink.
• Add 12M hydrochloric acid drop by drop while stirring until purple.
• Place 80 ml of this solution in each of 3 graduated cylinders.
• Place one in ice, one in hot water, and leave one at room temperature.
• Observe the color after 10 minutes.
• The reaction: CoCl₄ ⁻²_(aq) (BLUE) + 6H₂O_(l) Co(H₂O)₆ ⁺² (PINK) + 4Cl ⁻²_(aq)

 

Preparation for solutions with pH from 1 to 13:

• Standard Acidic Solutions:
  • Mix 2.1 cm³ of 12M HCl with 248 cm³ of distilled water to make a 0.1M HCl solution with a pH of 1.
  • Add 5 cm³ of the 0.1M HCl to 45 cm³ of distilled water to make a solution with a pH of 2.
  • Repeat using 5 cm³ of each previously diluted HCl solution and 45 cm³ of distilled water to make solutions with pH 3-6.
• Standard Neutral Solution:
  • Boiled distilled water will have a pH of 7.
• Standard Basic Solutions:
  • Add 1.05 g of NaOH to 249 cm³ of distilled water to make a 0.1M NaOH solution with a pH of 13.
  • Add 5 cm³ of the 0.1M NaOH to 45 cm³ of distilled water to make a solution with a pH of 12.
  • Repeat using 5 cm³ of each previously diluted NaOH solution and 45 cm³ of distilled water to make solutions with pH 11-8.

 

Formation of Metals by Displacement:

• Solutions:
  • 2% AgNO₃ - Dissolve 4 g in 200 ml of distilled water.
  • 5% SnCl₂ - Dissolve 10 g in 200 ml of water.
  • 5% Pb(CH₃COO)₂ - Dissolve 10 g in 200 ml of water.
• Place a heavy, coiled copper wire in the AgNO₃ solution.
• Place an iron nail, or coil of heavy iron wire in the SnCl₂ solution.
• Place a strip of zinc in the lead acetate solution.
• Each of these displacements becomes apparent in a few minutes, but complete crystal formation may take several hours.
• The reactions involved:
  • Ag⁺²_(aq) + 2NO₃^-_(aq) + Cu_(s) → Ag_(s) + Cu⁺²_(aq) + 2NO₃⁻_(aq)
  • Sn⁺²_(aq) + 2Cl⁻_(aq) + Fe_(s) → Sn_(s) + Fe⁺²_(aq) + 2Cl⁻_(aq)
  • Pb⁺²_(aq) + (CH₃COO⁻)_2(aq) + Zn_(s) → Pb_(s) + Zn⁺²_(aq) + (OCH₃CO⁻)_2(aq)

 

Oxidation of glycerin by permanganate:

• Place a small pile of granular potassium permanganate, a tablespoonful, in an evaporating dish.
• Pour about 10 ml of glycerin, glycerol, on top of the pile.
• In 10 to 20 seconds, rapid oxidation will occur!!
• The reaction is:
  14KMnO_{4 (g)} + 4C₃H₅(OH)_{3 (l)} → 7 K₂CO_{3 (s)} + 7Mn₂O_{3 (s)} + 5CO_{2 (g)} + 16H₂O _(l)

• Is this a Redox reaction?

 

Electrolysis:

• Disolve 2g KI in 50 cm³ of distilled water.
• Place a small amount of the solution in a small beaker and add a couple of drops of phenolphthalein.
• Connect two pieces of copper wire (electrodes) to a 9-volt battery.
• Dip the electrodes into the KI soultion and observe.

  - Bubbling occurs at the cathode as H₂ .
  - The solution around the cathode turns pink as OH ^- is produced.
  - The solution around the anode turns brown as I₂ is produced and combines with I ⁻ to form I₃ ⁻, which is brown.
  - The reaction is: 2H₂O + 3I ⁻ → H₂ + I₃⁻ + 2OH ⁻

 

An Endothermic Reaction of Two Solids:

• Place 20 g of barium hydroxide in a small beaker.
• Add 20 g of ammonium thiocyanate.
• Stir with a wooden splint.
• This reaction will freeze water is the beaker is placed in a small pool.

 

An Exothermic Reaction of Two Solutions:

• Prepare a 0.5M solution of Na₂SO₃ by adding 6.3 g to 100 ml of water.
• Pour 50 ml of bleach into a beaker and record the temperature.
• Add 50 ml of the sodium sulfite solution.
• Note the increase in temperature is approximately 20 C^o.

 

Spontaneous Combustion of Acetylene by Chlorine Gas:

• Place 25 ml of bleach (sodium hypochlorite) in a 500 ml beaker.
• Add 10 ml of 6M HCl.
• Cover the beaker with a piece of cardboard and watch the gas produced.
• After a couple of minutes, quickly drop a few small lumps of calcium carbide into the beaker and recover.
• The bubbles of acetylene will immediately burst into flame.
• The reactions involved:
  • Chlorine gas: ClO⁻_(aq) + Cl⁻_(aq) + 2H⁺_(aq) → Cl_2(g) + H₂O
  • Acetylene gas: CaC_2(s) + 2H₂O_(l) → C₂H₂ + Ca⁺²_(aq) + 2OH⁻_(aq)
  • The tendency of Cl_2(g) to remove hydrogen is so great that it reacts spontaneously with C₂H_2(g) to produce the highly exothermic reaction: C₂H_2(g) + Cl_2(g) → 2HCl + 2C + heat