Science 9: Unit B

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Science 9 Unit B Matter and Chemical Change

• Topic 7 Writing Chemical Equations

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Background

• All compounds are created, changed, or broken
  apart through chemical reactions.
• Signs that a chemical change have occurred
  include a color change, bubbling, a PRECIPITATE
  (solid) forming in a liquid, and heat and light
  being produced or absorbed.
• In a chemical reaction, the BONDS between atoms
  are broken and new bonds are formed so that atoms
  are rearranged to form new compounds.
• Eg. hydrogen oxygen water
• 2H2 O2 ? 2H2O
• or H-H H-H O-O H-O-H H-O-H

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Reactants and Products

• In chemical reactions, the chemicals that undergo
  the reaction are called the reactants. These are
  your original materials. In the above example
  hydrogen and oxygen are your reactants.
• The chemicals that come out of a chemical
  reaction are called the products. In the above
  example, two water molecules are the product.
• Note that when we write down a chemical reaction,
  we are writing down the lowest RATIO of molecules
  that will react together. In reality, billions of
  molecules are reacting with each other.

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Signs of a Chemical Reaction

• Products are new chemicals with different
  chemical properties than the reactants that make
  them. Water is a liquid at room temperature
  hydrogen and oxygen are gases at room
  temperature.
• There are signs that indicate a chemical reaction
  has taken place color change, bubbles forming,
  heat absorbed or given off, and a new material
  formed all point towards a chemical reaction.

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Writing Chemical Reactions

• To describe what occurs during a chemical
  reaction, scientists use chemical equations.
• Chemical equations involve the following parts

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The Parts of a Chemical Reaction

• 1. The reactants on the left side, using the
  symbols of elements for each atom. These symbols
  are the same ones from the periodic table. For
  each molecule indicate in brackets behind it
  whether it is a solid (s), liquid (l), or gas
  (g). An is used to separate the different
  reactants.
• 2. An arrow pointing to the right separating the
  reactants from the products.
• 3. The products on the right side of the equation
  are labeled in the same manner as the reactants.
• Eg. CH4(g) O2(g) ? CO2(g) H2O(l)

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Exothermic and Endothermic Reactions

• Exothermic reactions RELEASE energy when
  reacting.
• Reactants ? Products Energy
• Endothermic reactions ABSORB energy from the
  surrounding environment.
• Reactants Energy ? Products
• An exothermic example is the starting of a
  barbecue involves the igniting of propane and
  oxygen. This releases a large amount of energy.
• C3H8(l) 5O2(g) ? 3CO2(g) 4H2O(g)
  Energy
• An endothermic reaction example is the splitting
  up of water into hydrogen and oxygen molecules
  using an electric current.
• 2H2O(l) Energy ? 2H2(g) O2(g)

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An Important Note about Endothermic and
Exothermic Reactions

• An important note is that if a reaction is
  exothermic then the products are MORE stable than
  the reactants. If the reaction is endothermic the
  products are LESS stable than the reactants.
  Since all elements are trying to become more
  stable exothermic reactions are more natural
  while endothermic reactions need an outside
  energy source.