110.ReactionsIntro.doc1 of 1

04/07/14

Resources:Cu + HNO3 (in a jar) demo

Homework for 04/08:

Bell Work 4/7:Acid in Eye (safety reminder) < do if time

Bell Work 4/8:GYOH

Cannon & TOE Tie-in

  • STILL going to re-arrange energy stored in chemical bonds
  • Energy can be released as in cannon …
  • … by way of reaction C2H2 + O2 -> ???
  • When do rxns happen? Why do rxns happen? Where does energy go when rxns happen?
  1. Reactions Rule / Reaction Rules
  2. Rules: Reactions always Re-arrange (= move around but never gain or lose!)
  3. Re-arrange electrons

1)Electrons cannot be created or destroyed, only re-arranged

2)Old bonds are broken and new bonds are made …

3)So, new substance must be made (chem change)

4)Ex: Zn + HCl  ZnCl2 + H2

a)label each chemical with bond type (metallic, covalent, ionic, covalent)

b)Have to break e.g. metallic zinc bond so Cl can steal an electron and form ionic bond

c)Zn loses e, Cl gains e, H stays the same

d)electrons are re-arranged to form new compounds

  1. Re-arrange atoms

1)Atoms cannot be created or destroyed, only re-arranged

2)Same number & identity before an after do MASS must be conserved

3)Re-write Zn + HCl equation, then create “before” and “after” atom tables. Use to balance equation

  1. Re-arrange energy

1)Energy cannot be created or destroyed only transformed or transferred

2)Options (refer to TOE)

a)Chem PE (in bonds) transformed to KE (thermal, electrical, radiant, mechanical)

b)KE transformed into chem PE

3)Do reduced version of Zn & HCl: one zinc pellet in test tube plus a few ml of HCl. While bubbling discuss where energy is stored and where it goes

4)Return to reaction, have Ss feel test tube (warm!) & decide if reaction followed Option 1 or 2

  1. “Re-arrange” collisions

1)All reactions require a certain number of collisions between molecules and atoms – cannot change the NUMBER of collisions for a given reaction

2)RATE of collisions can be changed by 4 factors: temp, concentration, surface area & catalyst

  1. Following the Rules: Cu + HNO3
  2. Express chemical reaction using a chemical equation

1)“Reactants  Products”

2)Names: Copper (solid) + Nitric Acid (aqueous)  Copper Nitrate + Nitrogen Dioxide (gas) + Water (liquid)

3)DO DEMO HERE:

a)1 penny in bottom of 500-ml round flask / beral pipette in 1-hole stopper

b)Draw 1 ml of nitric into pipette / put pipette into flask

c)squeeze nitric onto pennies / swirl / observe gas & heat (CAREFULLY hold flask and move among tables, allow a couple Ss to feel bottom of flask)

d)NOTE: rxn will only produce 70 ml of NO2 with new penny

e)Once flask is shown to Ss, take to sink, open stopper and quickly pour in 100-200 ml water. Dilute HNO3 pushes rxn to NO not NO2. Flask can then be opened and poured into sink.

4)Formulas (leave off product coefficients for now)
Cu (s) + 4 HNO3 (aq) → Cu(NO3)2 (aq) + __ NO2 (g) + __ H2O (l)

  1. Rule 1 — Re-arrange electrons (break old, make new bonds)

1)What indications suggest electrons re-arranged?

2)H-NO3 bond broken, H2O bond forms etc.

3)Rule 1 √

  1. Rule 2 — Re-arrange atoms (same number & identity)

1)Count atoms on both sides

2)Figure out missing coefficients on product side

3)Solution: Cu(NO3)2 (aq) + 2 NO2 (g) + 2 H2O (l)

4)Rule 2 √

  1. Rule 3 — Re-arrange energy

1)Two options —which one happened based on observation that flask got warm?

a)Surroundings lost KE and chemicals gained PE?

b)Chemicals lost PE and surroundings gained KE?

  1. Rule 4: How could we accelerate reaction?

1)higher temp

2)higher concentration

3)use powdered or shredded copper

A Historical Sidelight: Ira Remsen on Copper and Nitric Acid

Ira Remsen (1846-1927) founded the chemistry department at Johns Hopkins University, and founded one of the first centers for chemical research in the United States; saccharin was discovered in his research lab in 1879. Like many chemists, he had a vivid "learning experience," which led to a heightened interest in laboratory work:

While reading a textbook of chemistry I came upon the statement, "nitric acid acts upon copper." I was getting tired of reading such absurd stuff and I was determined to see what this meant. Copper was more or less familiar to me, for copper cents were then in use. I had seen a bottle marked nitric acid on a table in the doctor's office where I was then "doing time." I did not know its peculiarities, but the spirit of adventure was upon me. Having nitric acid and copper, I had only to learn what the words "act upon" meant. The statement "nitric acid acts upon copper" would be something more than mere words. All was still. In the interest of knowledge I was even willing to sacrifice one of the few copper cents then in my possession. I put one of them on the table, opened the bottle marked nitric acid, poured some of the liquid on the copper and prepared to make an observation. But what was this wonderful thing which I beheld? The cent was already changed and it was no small change either. A green-blue liquid foamed and fumed over the cent and over the table. The air in the neighborhood of the performance became colored dark red. A great colored cloud arose. This was disagreeable and suffocating. How should I stop this? I tried to get rid of the objectionable mess by picking it up and throwing it out of the window. I learned another fact. Nitric acid not only acts upon copper, but it acts upon fingers. The pain led to another unpremeditated experiment. I drew my fingers across my trousers and another fact was discovered. Nitric acid acts upon trousers. Taking everything into consideration, that was the most impressive experiment and relatively probably the most costly experiment I have ever performed. . . . It was a revelation to me. It resulted in a desire on my part to learn more about that remarkable kind of action. Plainly, the only way to learn about it was to see its results, to experiment, to work in a laboratory.

from F. H. Getman, "The Life of Ira Remsen"; Journal of Chemical Education: Easton, Pennsylvania, 1940; pp 9-10; quoted in Richard W. Ramette, "Exocharmic Reactions" in Bassam Z. Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 1. Madison: The University of Wisconsin Press, 1983, p. xiv: