Chemical Changes and Conservation of Matter

1
Chemical Changes and Conservation of Matter

• What happens during a chemical change?

2
Chemical change means Molecules change

• Molecules are made of elements.
• Example Water (H2O) is made of Hydrogen (H) and
  Oxygen (O).
• During a chemical change the elements of at least
  one molecule are rearranged to make a molecule
  that was not present before the change.
• During a physical change, the molecules stay the
  same afterwards.

3
Specific Chemical Changes

• Synthesis?reactant reactant ? product
• Decomposition?reactant ? product product
• Single Replacement?reactant reactant ? product
  product
• Double Replacement?reactant reactant ? product
  product
• Combustion?fuel oxygen ? water carbon dioxide

4
Synthesis

• Happens when 2 molecules combine to form one
  molecule
• Example Rust (Oxidization)
• Iron Oxygen Water ? Iron Oxide (Iron oxide
  is the brown stuff we call rust.)

?
5
Decomposition

• Happens when a molecule breaks apart to form two
  or more molecules
• Example Carbonic Acid (pop fizz)
• Carbonic Acid ? Water Carbon Dioxide

?

6
Single Replacement

• Happens when an element is exchanged from one
  molecule to another
• Example Hydrogen and Magnesium
• Magnesium Hydrochloric Acid ?
  Magnesium Chloride Hydrogen

?

7
Double Replacement

• Happens when two elements from a pair of
  molecules switch places
• Example Neutralization (Acid Base)
• Sodium Hydroxide Hydrochloric Acid ? Water
  Sodium Chloride (Salt)

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8
Combustion

• Happens when a hydrocarbon (a molecule made from
  hydrogen and carbon) is burned in the presence of
  oxygen
• Example Burning gas
• Gas Oxygen Flame ? Water Carbon Dioxide
  Heat

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9
Quiz! Name the type of Reaction

• 1.When wood is burned, it combines with oxygen to
  make water and carbon dioxide.
• 2.When electricity is passed through water, it
  releases hydrogen and oxygen.
• 3.When iron and sulfur are combined it makes iron
  (II) sulfide.
• 4.When zinc is combined with hydrochloric acid,
  it makes zinc chloride and hydrogen.
• 5.When silver nitrate and sodium chloride are
  mixed, silver chloride and sodium nitrate are
  produced.

10
Exothermic vs. Endothermic

• If a reaction either absorbs heat or gives off
  heat, that is an indication that a chemical
  change has happened.
• Thermic means heat (think thermos).
• Exo means it gives off heat (think exit).
• Endo means it absorbs heat (think enter).
• Examples Combustion is exothermic and
  Photosynthesis is endothermic.

11
Conservation of Matter

• In any of the examples seen so far, did an
  element ever go missing or disappear?
• IDEA In any chemical change, the same elements
  present beforehand are present after the
  reaction. That way, matter is conserved.
• The total mass of the reactants will always equal
  the total mass of the products.

12
Quiz 2! Find the Missing Masses

• 1. A solid has a mass of 35 g. When it is mixed
  with a solution, a chemical reaction occurs. If
  the final total mass of products is 85 g, what
  was the mass of the solution?
• 2. Solution A has a mass of 60 g. Solution B has
  a mass of 40 g. When they are mixed, a chemical
  reaction occurs in which gas is produced. If the
  mass of the final mixture is 85 g, what mass of
  gas was produced?

13
Overview

• Chemical changes involve rearranging elements to
  form new molecules.
• We looked at 5 types synthesis, decomposition,
  single replacement, double replacement and
  combustion.
• Many chemical changes either give off heat
  (exothermic) or absorb heat (endothermic).
• In all chemical changes, mass is conserved. This
  means the total mass of the reactants is equal to
  the total mass of the products.

14
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