CHEMICAL REACTIONS

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CHAPTER 7

• CHEMICAL REACTIONS

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Conservation of Mass

• The law of conservation of mass states that mass
  is neither created nor destroyed in a chemical
  reaction.

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• During chemical reactions, the mass of the
  products is always equal to the mass of the
  reactants. This principle is the law of
  conservation of mass.
• When charcoal burns, the mass of the carbon
  dioxide produced is equal to the mass of the
  charcoal and oxygen that reacted.

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Describing Reactions

• When a substance undergoes a chemical change a
  reaction takes place.
• Chemical Equations are used to describe what is
  present before and after the change
• Reactants substances that undergo a change.
• Products new substances formed after the
  change.
• Reactants --gt Products
• Ex Carbon Oxygen -gt Carbon dioxide
• Or
• C O2 -gt CO2

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Balancing Equations

• In order to show that mass is conserved during a
  reaction a chemical equation must be balanced.
• Ex. N2H4 O2 ? N2 H20
• You balance an equation by changing the
  coefficients, the numbers that appear before the
  formula ex. 2H20
• You can never change the subscript because that
  changes the formula!

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Example
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Counting With Moles

• Because chemical reactions often involve large
  numbers of particles, chemists use a counting
  unit called the mole to measure the amount of a
  substance.
• A mole is an amount of a substance that contains
  approximately 6.02x1023 particles of that
  substance. It is also known as Avogadros number.
• Ex. A mole of carbon 6.02 x 1023 atoms of
  carbon.

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Molar Mass

• The mass of one mole of a substance is called a
  molar mass. For an element, the molar mass is the
  same as its atomic mass expressed in grams.
• The molar mass of carbon is 12.0 grams.
• The molar mass of sulfur is 32.1 grams.

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Mole-Mass Conversions

• Once you know the molar mass of a substance, you
  can convert moles of that substance into mass, or
  a mass of that substance into moles.
• The molar mass of CO2 is 44.0 grams, which means
  that one mole of CO2 has a mass of 44.0 grams.
  This relationship yields the following conversion
  factors.

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Mole Mass Conversions

• Suppose you have 55.0 grams of CO2. To calculate
  how many moles of CO2 you have, multiply the mass
  by a conversion factor.

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Classifying Reactions

• Reactions are classified by the type of reactant
  and the number of reactants and products. There
  are five different types.
• Synthesis
• Decomposition
• Single-replacement
• Double-replacement
• Combustion reactions.

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Synthesis Reactions

• Synthesis reaction a reaction where two or more
  substances form a single substance A B -gt AB.
• Ex. 2Na Cl2 -gt 2NaCl

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Decomposition Reactions

• Decomposition reaction is the opposite of
  synthesis. Its a reaction where a compound
  breaks down into two or more simpler substances
• AB --gt A B
• Ex. 2H2O ? 2H2 O2

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Single Replacement Reactions

• Single Replacement reaction is a reaction in
  which one element takes the place of another
  element in a compound. A BC ? B AC
• Ex. Cu 2AgNO3 ? Cu(NO3)2 2Ag

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Double Replacement Reactions

• Double Replacement reaction is one in which two
  different compounds exchange positive ions and
  form two new compounds. AB CD ? AD CB
• Ex. CaCO3 2HCl ? CaCl2 H2CO3

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Combustion Reactions

• Combustion reaction is one in which a substance
  reacts rapidly with oxygen, often producing heat
  and light.
• Ex. CH4 2O2 ? CO2 2H2O

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Reactions

• A reaction in which electrons are transferred
  from one reactant to another is called an
  oxidation-reduction reaction, or redox reaction.
• Oxidation any process that loses electrons
  during a chemical reaction
• Reduction the process in which an element gains
  electrons during a chemical reaction
• Oxidation and reduction always occur together.
• When one element loses electrons, another element
  must gain electrons.

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Memory trick

• LEO the lion says GER.
• LEO Loss of Electrons is OxidationGER Gain
  of Electrons is Reduction

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Chemical Bonds Energy

• Chemical energy- is the energy stored in the
  chemical bonds of a substance.
• Energy changes in a chemical reaction are
  determined by changes that occur in chemical
  bonding
• Chemical reactions involve the breaking of
  chemical bonds in the reactants and the formation
  of chemical bonds in the products

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Chem. Bonds Energy cont.

• Breaking bonds breaking chemical bonds requires
  energy
• Forming bonds forming bonds releases energy

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Exothermic and Endothermic Reactions

• During chemical reactions energy is either
  released or absorbed.
• Exothermic reaction releases energy to its
  surroundings. Energy released as the products
  form is greater than the energy required to break
  the bonds of the reactants.

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Exothermic and Endothermic Reactions cont.

• Endothermic Reactions absorbs energy from its
  surroundings. In an endothermic reaction, more
  energy is required to break the bonds in the
  reactants than is released by the formation of
  the products

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• In both endothermic and exothermic reactions the
  total amount of energy before and after the
  reaction is the same.
• Law of Conservation of Energy states that energy
  is not created or destroyed, but only changes
  form.

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Reaction Rates

• Reaction rates tell you how fast a reaction is
  going.
• Reaction rates can be affected by various factors

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• Temperature ? temp ? reaction rate (particles
  move faster)
• Surface - the smaller the particle size of a
  given mass the larger the surface area. ? surface
  area ? reaction rate. (greater exposure)
• Concentration number of particles in a given
  volume. ? the concentration ? the reaction rate.
  (increase particles increase collisions)
• Stirring ? reaction rate by ? particle
  collisions.
• Catalysts a substance that affects reaction
  rate without being used up in the reaction (ex.
  Phenol red)

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Equilibrium

• Physical equilibrium when a physical change
  does not go to completion a physical equilibrium
  is established between the forward and reverse
  changes.
• Chemical equilibrium Most reactions dont go
  only in one direction they are reversible.
  Reactants ? Products and Products ? Reactants
  happens simultaneously. When a chemical reaction
  does not go to completion a chemical equilibrium
  is established between the forward and reverse
  reactions.

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Look at the equation for hints

• ? this suggests the rxn proceeds in one direction
• ? this suggests an equilibrium

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• Factors affecting chemical equilibrium
• When a change is introduced to a system in
  equilibrium, the equilibrium shifts in the
  direction that relieves the change
• Temperature equilibrium shifts in the direction
  that removes heat from the system.
• Pressure increase pressure causes equilibrium
  to shift in direction that produces the least gas
  molecules.
• Concentration decrease concentration of a
  substance and system would shift in the direction
  that produces more concentration of substance
  removed