Chemical Reactions

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Chemical Reactions

• TEKS Objectives
• Demonstrate that substance may react
  chemically to form new substances.
• Recognize the importance of formulas and
  equations to express what happens in a chemical
  reaction.

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Chemical Bonds

• The joining of atoms to form new substances
• The force of attraction that holds two or more
  atoms together
• The tendency of an element to form bonds
  (reactivity) depends on the number of valence
  electrons electrons on the outermost electron
  cloud
• Elements with 8 valence electrons are unreactive
  and do not bond with other elements

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Why do elements bond?

• 8 is the magic number! Most elements, with the
  exception of Hydrogen and Helium (their magic
  number is 2), will bond in order to have a full
  outer shell with 8 valence electrons
• Protons and neutrons are stuck in the nucleus,
  but electrons can move around and interact with
  other atoms
• Electrons can either leave their atom and join
  another atom (creating an ion) OR they can join
  forces with electrons of other atoms and be
  shared among the atomswell get to this in the
  next slide

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Two Types of Chemical Bonding

• Ionic Bonding the force of attraction between
  two oppositely charged ions.
• Covalent Bonding the force of attraction
  between the nuclei of atoms and the electrons
  shared by the atoms.

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Covalent vs. Ionic Bonds
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Ionic Bonds

• Form ions charged particles that form during
  chemical reactions.
• One or more valence electrons are transferred
  from one atom to another.
• Metals lose electrons and become positive ions.
• Non-metals gain electrons and become negative
  ions.

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NaCl - Sodium ChlorideIonic Bond

• Na loses its one valence electron becomes a
  positive ion.
• Cl gains the electron becomes a negative ion.
• Na and Cl form the ionic compound NaCl.

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Covalent Bonds

• Form Molecules - neutral compounds of atoms that
  share their electrons.
• Bonds that form between Non-metals.
• Valence electrons are shared.

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H2O Dihydrogen MonoxideCovalent Bond
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Ionic Bonds vs. Covalent Bonds
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Chemical Reactions

• The process by which one or more substances
  change to produce one or more different
  substances.
• Bonds between elements are broken and/or new
  bonds are formed.
• Reactants starting materials in a chemical
  reaction.
• Products substances formed in a chemical
  reaction.

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Intro to chemical reactions
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Signs of a chemical reaction

• Formation of a gas
• Bubbles or smoke
• Color Change
• Formation of a solid
• Called a precipitate
• Change in heat or energy
• Flame, light, change in temperature

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Representing a chemical reaction
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Chemical Equations

• Use chemical formulas as a shorthand description
  of a chemical reaction.
• Charcoal is used to cook food on a barbecue. When
  the carbon in charcoal reacts with oxygen in the
  air, the primary product is carbon dioxide
• Here is what the chemical equation would look
  like
• C O2 CO2
• Reactants Products

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Chemical Formulas

• Shorthand notations that uses chemical symbols
  and numbers to represent a substance.
• NaCl Sodium chloride (salt)
• H2O Dihydrogen monoxide (water)
• CO2 Carbon dioxide

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Subscriptssubscripts

• Numbers written below and to the right of a
  chemical symbol in a formula.
• Tells how many atoms of an element are in each
  molecule.
• If there is no subscript next to the element,
  then there is one atom present
• Ex H2SO4 contains 2 hydrogen atoms, 1 sulfur
  atom, 4 oxygen atoms
• Extension Can you count the elements and atoms
  in this compound? NH4NO3

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Different types of Reactions (and their equations)
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Law of Conservation of Mass

• The mass of atoms and molecules is neither
  created or destroyed in chemical reactions.
• The of atoms in the reactants must equal the
  of atoms in the products in a chemical reaction.

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Balancing Chemical Equations

• Coefficients used to balance chemical
  equations.
• placed in front of a chemical symbol or formula
  in an equation to balance the equation and follow
  the law of conservation of mass.
• Shows how many atoms or molecules are reacting or
  formed in the equation
• Ex 4 sodium atoms combine with 2 oxygen atoms to
  form 2 molecules of sodium oxide
• 4Na O2 ? 2Na2O

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Balanced or unbalanced? To determine whether
or not an equation is balanced, count the atoms
for each element on the product and reactant
sides of the equation. They must be equal

• Balanced equation

• Unbalanced equation

• 2Mg O2 ? 2MgO

• Cu O2 ----gt Cu2O

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Energy is also Conserved

• Law of Conservation of Energy -states that energy
  can be neither created or destroyed in a chemical
  reaction.
• Chemical Reactions can be
• Exothermic-energy is released
• exo-out of therm- heat
• OR
• Endothermic-energy is absorbed
• endo-into

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Exothermic Reactions

• Energy is given off (released or lost) in the
  reaction.
• Heat, light, electricity
• Chemical energy of the reactants is GREATER than
  the energy of the products.
• Examples - burning a candle, combustion, hot
  packs, cellular respiration.
• Equation heat or energy is a product and is on
  the right side of the equation
• 2Na Cl2 2NaCl energy

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Endothermic Reactions

• Energy is absorbed (gained) in a chemical
  reaction.
• Energy of the products in greater than the energy
  of the reactants.
• Examples - Cold packs, photosynthesis,
  photography, setting of plaster of paris.
• Equation - heat or energy is a reactant and is on
  the left side
• 2H2O energy 2H2 O2