CHEMICAL REACTIONS

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CHEMICAL REACTIONS
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PHYSICAL VERSUS CHEMICAL CHANGEA REFRESHER
Physical Change A change that alters the form or appearance of a material but DOES NOT convert the material into a new substance.
Chemical Change A change in matter that forms one or more new substances. The new substances) are made of the same elements as the original substance, but are now in different combinations.
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ATOMS AND MOLECULES

• Atom is the smallest particle of an element.
• All atoms of an element have the same chemical
  properties.
• A molecule is a particle made of two or more
  atoms bonded together.
• Some molecules are made of the same element
  (example O2 the form of oxygen we breathe)
• MOST molecules are made of more than one type of
  atom. (example H20 )
• Try to count the atoms how many total atoms are
  there in 1 molecule of water?

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MORE ABOUT MOLECULES

• All compounds are molecules.
• but NOT all molecules are compounds.
• Think about it.
• A compound is a pure substance made up of two or
  more elements.
• A molecule is two or more atoms.

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OBSERVING CHEMICAL REACTIONS

• Chemical Reaction The process in which
  substances undergo chemical changes that results
  in the formation of new substance.
• How can you tell if a chemical reaction has taken
  place?
• Evidence for chemical reactions
• Color change
• A Precipitate Forms
• Precipitate is a solid that forms from solution
  during a chemical reaction.
• Gas is produced (like when you mix baking soda
  and vinegar)
• Changes in temperature
• Changes in properties

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CHEMICAL REACTION VOCABULARY

• Look up the definitions for the following terms
  and add to your notes (Page 24-25 or glossary)
• Symbol, Chemical Formula, Chemical Equation,
  Subscript
• Symbol In chemistry, a one-letter or two-letter
  set of characters used to identify an element.
• O, Fe, He, H
• Chemical formula A combination of symbols that
  represent the elements in a compound.
• NaCl, CO, CO2
• Chemical Equation Uses symbols and formulas to
  show the reactants and the products of a chemical
  reaction.
• Subscripts Show the number of atoms of an
  element in a molecule or the ratio of elements in
  a compound.
• If a letter doesnt have a subscript the number 1
  is understood to be there.
• For example, CO2 has 1 atom of Carbon and 2
  atoms of Oxygen, for a total of 3 atoms.

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STRUCTURE OF A CHEMICAL EQUATION

• A chemical equation summarizes a chemical
  reaction.
• The REACTANTS are the materials you have at the
  beginning of the reaction.
• The PRODUCTS are the new materials that you have
  when the reaction is complete.
• Chemical equations have a definite structure.
• The reactants are always on the LEFT, followed by
  an arrow (yields).
• The products are always on the RIGHT.
• Reactant Reactant Product
  Product

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CHEMICAL BONDS AND REACTIONS

• A Chemical Bond is the force that holds atoms
  together.
• Think of it like the glue that makes atoms
  stick together.
• Chemical Reactions occur when chemical bonds
  break or when new bonds form.

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LAW OF CONSERVATION OF MASS

• The Law of Conservation of Mass states that the
  amount of matter in a chemical reaction does not
  change. Therefore, the total mass of the
  reactants MUST EQUAL the total mass of the
  products.

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COUNTING ATOMS

• Make a 2-column list of the different elements in
  each compound.
• Write down how many atoms of each element.
• Add all of the atoms together for a total.
• Example NaOH
• Now You Try MgO
• Did you say Mg 1 and O 1 for 2 Total?

Element of Atoms
Na 1
O 1
H 1 3 Total
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COUNTING ATOMS WITH SUBSCRIPTS

• MgCl2
• Now You Try!
• C6H12O6

Subscript
Mg 1
Cl 2 3 Total
C 6
H 12
O 6 24 Total
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COUNTING ATOMS WITH COEFFICIENTS AND SUBSCRIPTS
H 4
N 4
O 12 2o total

• 4HNO3

• Coefficient number placed in front of a chemical
  formula in an equation. It tells you how many
  atoms or molecules of each reactant and product
  take place in the reaction.
• Count Just Like Before
• List each element.
• Multiply each element by the coefficient.
• If the element also has a subscript, multiply the
  coefficient by the subscript.
• Now You Try!
• 6 PO4

P 6
O 24
30 total
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COUNTING ATOMS WITH COEFFICIENTS, SUBSCRIPTS AND
PARENTHESIS

• Ca (C2H2O2)2
• Just like before make a chart of all elements in
  the chemical formula.
• Start with the parenthesis.
• Count each atom of the elements.
• Multiple each element by the parenthesis
  subscript.
• Now You Try Mg(C2H3O2)2
• What happens if we add a coefficient of 2 in
  front of the entire formula?

Ca 1
C 2 X 2 4
H 2 X 2 4
O 2 X 2 4
TOTAL OF 13 Atoms TOTAL OF 13 Atoms TOTAL OF 13 Atoms TOTAL OF 13 Atoms
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BALANCING CHEMICAL EQUATIONS
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TWO SIDES TO A CHEMICAL EQUATION

• REACTANTS PRODUCTS
• Reactants what goes into the chemical rxn
• Products what comes out of the chemical rxn

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• In chemical reactions, new things are made

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BASIC EQUATION

• Chemical Equation
• CO2 H2O C6H12O6 O2
  H2O

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LAW OF CONSERVATION OF MATTER????

• Matter cannot be created or destroyed
• -Look at the equation again, is there the same
  amount of matter on the REACTANTS side as there
  is on the PRODUCTS side? Count the number of
  atoms for each element in the Reactant side and
  the Product side.
• CO2 H2O C6H12O6 O2
  H2O

C 1
O 3
H 2
C 6
O 14
H 9
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COUNT THE ATOMS NOW REMEMBER TO MULTIPLY WITH
THE COEFFICIENT!
C 6
O 24
H 24
C 6
O 24
H 24
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HOW TO BALANCE A CHEMICAL EQUATION

• Create a list of the atoms of each element just
  as if you are counting atoms.
• Pick an element that is not equal on both sides
  of the equation.
• Add a coefficient in front of the formula with
  that element and adjust your original counts.
• Continue adding coefficients until you get the
  same number of atoms of each element on both
  sides.
• Update your list (inventory) after every move.
• Save Oxygen and Hydrogen for last
• PRACTICE!
• H2 O2 H2O
• C Cl2 CCl4

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DEMONSTRATION - CONSERVATION OF MASS

• Vinegar baking soda reaction
• We need to measure the mass of all reactants
  BEFORE the reaction.
• Vinegar and baking soda
• After we measure the mass of the reactants you
  should be able to calculate the total mass of the
  product. What would it be?
• We also need to find the mass of the bag and
  cups. Why?
• We then need to close the bag to created a closed
  system.
• Why do you think we need to create a closed
  system by closing the bag? What might be lost if
  we did not close the bag?

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MORE PRACTICE

• Complete the front of the balancing act
  worksheet.
• If you finish, try these

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LAW OF CONSERVATION OF MASS

• Open system versus Closed System
• If a chemical reaction takes place in a closed
  system, no matter can enter or leave the
  reaction.
• - the reaction takes place in a sealed container
  or in a vacuum.
• If a chemical reaction takes place in an open
  system, matter can enter or leave during the
  reaction.
• The reaction DOES NOT take place in a sealed
  container.