Tro3 lecture 07

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Transcript of Tro3 lecture 07

  • 1. Experiencing Chemical Change
    • Chemical reactions are happening both around you and in you all the time.
  • Some are very simple, others are complex.
    • In terms of the pieceseven the simple ones have a lot of interesting principles to learn from.
  • Chemical reactions involve changes in the structures of the molecules, and many times we can experience the effects of those changes.
  • What are some examples of chemical reactions you experience?

Tro's "Introductory Chemistry", Chapter 7 2. Chemical Reactions

  • Reactions involve chemical changes in matter resulting in new substances.
  • Reactions involve rearrangement and exchange of atoms to produce new molecules.
    • Elements are not transmuted during a reaction.
    • Atoms of different elements can combine to make new compounds.
    • Molecules can combine to make bigger molecules.
    • Molecules can decompose into smaller molecules or atoms.
    • Atoms can be exchanged between molecules or transferred to another molecule.
    • Atoms can gain or lose electrons, turning them into ions.
      • Or changing the charge on ions that are already there.

Tro's "Introductory Chemistry", Chapter 7 3. Combustion Reactions

  • Reactions in which O 2is consumed by combining with another substance are calledcombustion reactions .
    • Always release heat and/or other forms of energy.
    • Produce one or more oxygen-containing compounds.
  • Combustion reactions are a subclass ofoxidation reductionreactions.
    • aka redox reactions.
    • Involve the transfer of electrons between atoms.

Reactants Products 4. Evidence of Chemical Reactions

  • Look for evidence of a new substance.
  • Visual clues (permanent).
    • Color change.
    • Precipitate formation.
      • Solid that forms when liquid solutions are mixed.
    • Gas bubbles.
    • Large energy changes.
      • Container becomes very hot or cold.
      • Emission of light.
  • Other clues.
    • New odor.
    • Whooshing sound from a tube.
    • Permanent new state.

Tro's "Introductory Chemistry", Chapter 7 5. Evidence of Chemical Change Tro's "Introductory Chemistry", Chapter 7 Color Change Formation of Solid Precipitate Formation of a Gas Emission of Light Release or Absorption of Heat 6. Conservation of Mass

  • Matter cannot be created or destroyed.
    • Therefore, the total mass cannot change.
    • And the total mass of the reactants will be the same as the total mass of the products.
  • In a chemical reaction, all the atoms present at the beginning are still present at the end.
    • If all the atoms are still there, then the mass will not change.

Tro's "Introductory Chemistry", Chapter 7 7. Chemical Equations

  • CH 4and O 2are the reactants, and CO 2and H 2 O are the products.
  • The ( g ) after the formulas tells us the state of the chemical.
  • The number in front of each substance tells us the numbers of those molecules in the reaction.
    • Called thecoefficients .

Tro's "Introductory Chemistry", Chapter 7 CH 4 ( g ) +2O 2 ( g )CO 2 ( g ) +2H 2 O( g ) 8. Example

  • When magnesium metal burns in air, it produces a white, powdery compound magnesium oxide.
  • Write a skeletal equation
    • Mg( s ) + O 2 ( g )MgO( s )
  • Count the number of atoms on each side.
  • Mg( s ) + O 2 ( g ) MgO( s )
  • 1Mg 1
  • 2O1

Tro's "Introductory Chemistry", Chapter 7 9. Example, Continued

  • When magnesium metal burns in air, it produces a white, powdery compound magnesium oxide.
    • Mg( s ) + O 2 ( g ) MgO( s )
  • Use factors as coefficients infrontof the compound containing the element.
    • We do not write1as a coefficient, its understood.
  • Mg( s ) + O 2 ( g ) MgO( s )
  • 1Mg 1
  • 1 x 2O1 x 2

Tro's "Introductory Chemistry", Chapter 7 2 10. Another Example

  • Under appropriate conditions at 1000C, ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and steam
  • write the skeletal equation
  • first in words
    • identify the state of each chemical
  • ammonia(g) + oxygen(g) nitrogen monoxide(g) + water(g)
  • then write the equation in formulas
    • identify diatomic elements
    • identify polyatomic ions
    • determine formulas
  • NH 3 (g) + O 2 (g)NO(g) + H 2 O(g)

Tro's "Introductory Chemistry", Chapter 7 11.

  • Under appropriate conditions at 1000C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and steam
  • NH 3 (g) + O 2 (g) NO(g) + H 2 O(g)
  • count the number of atoms of on each side
  • NH 3 (g) + O 2 (g) NO(g) + H 2 O(g)
  • 1N 1
  • 3H2
  • 2O1 + 1

Another Example, Continued Tro's "Introductory Chemistry", Chapter 7 12.

  • Under appropriate conditions at 1000C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous steam
  • NH 3 (g) + O 2 (g) NO(g) + H 2 O(g)
  • pick an element to balance - H
    • avoid element in multiple compounds on same side - O
  • find least common multiple of both sides (6)
  • multiply each side by factor so it equals LCM
  • NH 3 (g) + O 2 (g) NO(g) + H 2 O(g)
  • 1N 1
  • 3H2
  • 2O1 + 1

Another Example, Continued Tro's "Introductory Chemistry", Chapter 7 2 x x 3 13.

  • Under appropriate conditions at 1000C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water
  • NH 3 (g) + O 2 (g) NO(g) + H 2 O(g)
  • use factors as coefficients infrontof compound containing the element
  • NH 3 (g) + O 2 (g) NO(g) +H 2 O(g)
  • 1N 1
  • 2x 3H2 x3
  • 2O1 + 1

Another Example, Continued Tro's "Introductory Chemistry", Chapter 7 2 3 14.

  • RecountN & O not balanced
  • 2 NH 3 (g) + O 2 (g) NO(g) + 3 H 2 O(g)
  • 2N 1
  • 6H6
  • 2O1 + 3
  • and Repeat attack the N
  • 2 NH 3 (g) + O 2 (g) 2 NO(g) + 3 H 2 O(g)
  • 2N 1x 2
  • 6H6
  • 2O1 + 3

Another Example, Continued Tro's "Introductory Chemistry", Chapter 7 15.

  • Recount Again Still not balanced and the only element left is O!
  • 2 NH 3 (g) + O 2 (g) 2 NO(g) + 3 H 2 O(g)
  • 2N 2
  • 6H6
  • 2O2 + 3

Another Example, Continued Tro's "Introductory Chemistry", Chapter 7 16.

  • and Repeat Again
    • A trick of the trade - when you are forced to attack an element that is in 3 or more compounds find where it is uncombined.You can find a factor to make it any amount you want, even if that factor is a fraction!
    • 2 NH 3 (g) +?O 2 (g) 2 NO(g) + 3 H 2 O(g)
  • 2N 2
  • 6H6
  • 2O2 + 3
    • We want to make the O on the left equal 5, therefore we will multiply it by 2.5
  • 2 NH 3 (g) +2.5O 2 (g) 2 NO(g) + 3 H 2 O(g)
  • 2N 2
  • 6H6
  • 2.5 x2O2 + 3

Another Example, Continued 17.

  • You cant have a coefficient that isnt a whole number.Multiply all the coefficients by a number to eliminate fractions
    • If ?.5, then multiply by 2; if ?.33, then3; if ?.25, then 4
  • {2 NH 3 (g) + 2.5 O 2 (g) 2 NO(g) + 3 H 2 O(g)}x 2
  • 4NH 3 (g) +5O 2 (g) 4 NO(g) +6H 2 O(g)
  • 4N 4
  • 12H12
  • 10O4 + 6

Another Example, Continued Tro's "Introductory Chemistry", Chapter 7 18. Practice #1

  • When aluminum metal reacts with air, it produces a white, powdery compound called aluminum oxide.
    • Reacting with air means reacting with O 2 :
  • Aluminum( s )