Honors ICPE

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Honors ICPE

• Chapter 5 Chemical Reactions

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• The nature of Chemical Reactions
• Chemical Reactions Change Substances
• Chemical reactions occur when substances undergo
  chemical changes to form new substances.
• Production of gas and change of color are signs
  of chemical reactions.

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• Chemical reactions rearrange atoms.
• Example C8H18 O2 ? CO2 H2O
• Isooctane oxygen yields carbon dioxide
  water
• Reactants yields products

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Energy and Reactions

• Energy must be added to break bonds.
• Many forms of energy are transferred as heat,
  like a spark that starts isooctane-oxygen
  reaction.
• Energy also can be transferred as electricity,
  sound or light.
• Forming bonds releases energy see Figure 5-4.

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• Energy is conserved in chemical reactions
• Chemical energy the energy stored within atoms
  and molecules that can be released when a
  substance reacts.
• Reactions that release energy are exothermic
  reactions.
• Reactions that absorb energy are endothermic
  reactions.

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• Sometimes reactions are described as exergonic or
  endergonic.
• These terms refer to the ease with which
  reactions occur.
• Ex. Bioluminescence and respiration are
  exergonic reactions.
• Photosynthesis is an endergonic reaction.

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Reaction Types

• Synthesis is reaction that combine substances
• Ex. 2H2 O2 ? 2H2O
• 2. Decomposition reactions break substances
  apart
• Ex. 2H2O ? 2H2 O2
• Breakdown of water through process of
  electrolysis.
• Electrolysis the decomposition of a compound by
  an electric current.

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• 3. Combustion reactions use oxygen as a reactant
• Ex. 2CH4 4O2 ? 2CO2 4H2O
• The amount of oxygen available will determine the
  products produced.
• 4. Single displacement reaction atoms of one
  element take the place of atoms of another
  element in a compound
• Ex. 3CuCl2 2Al ? 2AlCl3 3Cu

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• 5. Double displacement reaction ions appear to
  be exchanged between compounds.
• Ex. Pb(NO3)2 K2CrO4 ? PbCrO4 2KNO3

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Electrons and Chemical Reactions

• With the discovery of the electron and its role
  in chemical bonding, another way to classify
  reactions was developed.
• We understand many reactions as transfer of
  electrons.
• Electrons are transferred in redox reactions.
• Reduction/oxidation (redox) reaction is a
  reaction that occurs when electrons are
  transferred from one reactant to another.

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• Radicals have electrons available for bonding.
• Radicals are the fragments of molecules that have
  at least one electron available for bonding.

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Balancing Chemical Equations

• Describing Reactions
• Word equation
• Methane plus oxygen yields carbon dioxide and
  water
• Chemical equations summarize reactions.

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• A chemical equation is an equation that uses
  chemical formulas and symbols to show the
  reactants and products in a chemical reaction.
• The reactants, which are on the left-hand side of
  the arrow, form the products, which are on the
  right hand side.
• When chemical equations are written, ? means
  "gives" or "yields".

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• Balanced chemical equations account for the
  conservation of mass.
• Same number of atoms of each element on each side
  of the equation.
• This is a balanced equation.

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How to balance chemical equations

• Identify the reactants and products
• Write a word equation for the reaction
• Write the equation using formulas for the
  elements and compounds in the word equation
• Balance the equation one element at a time. Must
  use coefficients do not change subscripts.

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• Look at information from a balanced equation
• Determining Mole Ratios
• A coefficient of 1 is never written Ex 1O
• Balanced equations indicate particles and moles
• Balanced equations show the conservation of mass

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• The law of definite proportions states a compound
  always contains the same elements in the same
  proportions, regardless of how the compound is
  made, or how much of the compound is formed.

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• Mole ratios can be derived from balanced
  equations.
• Mole ratio is the smallest relative number of
  moles of the substance involved in a reaction.

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Rates of Change

• Factors affecting reaction rates
• Most reactions go faster at higher temperatures
• A large surface area speeds up reactions
• Concentrated solutions react faster
• Reactions are quicker at higher pressure
• Massive, bulky molecules react slower
• Catalyst changes the rates of chemical reactions.

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• A catalyst is a substance that changes the rate
  of chemical reactions without being consumed.
• Enzymes are biological catalysts.
• An enzyme is a protein that speeds up a specific
  biochemical reaction.
• See Table 5-1 Common Enzymes and their uses page
  171
• A substrate is the specific substance affected by
  an enzyme

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• Equilibrium Systems
• Some changes are reversible ?
• ?
• Equilibrium results when rates balance
• Systems in equilibrium respond to minimize change
• Le Chateliers' principle predicts changes in
  equilibrium if a change is made to a system in
  chemical equilibrium, the equilibrium shifts to
  oppose the change until a new equilibrium is
  reached