Chapter 2

  Chemical Reaction – process which the chemical & physical properties of original substance change as the new substance formed takes on new sets of properties  Ex. Burning of gasoline, rusting of iron Change in energy also occurs  Either absorbed or released  Ex. Gasoline burns; heat is released or heat energy is absorbed when sugar changes to caramel

  All chemical reactions share certain characteristics  New substance formed 2 kinds of substances involved in chemical reactions:  Reactant – beginning substance  Product – new substance formed after reaction

Chemical reaction is when reactants change into products

   Reactants must have the ability to combine w/other substances to form products Bonding capacity – ease that an atom will form chemical bonds Bonding capacity of an atom determines the ability of an atom to undergo a chemical reaction

  Atoms can form molecules which can break apart & form atoms or molecules can react w/other molecules Either way, new substances are formed when bonds are broken, atoms rearranged & new bonds formed

 Chemical equation –an expression in which symbols & formulas are used to represent a chemical reaction    Symbols & formulas ( +) represents “and” An “arrow” represents the reactants becoming the product  “arrow” is read “yield” and shows direction of change

  Atoms cannot be created or destroyed during a chemical reaction   # of atoms of each element are the same before and after a chemical reaction Only the arrangement of the atoms change Every atom has a particular mass  Mass never changes, not lost or gained  equal on both sides of equation  Law of Conservation of Mass – mass remains constant in a chemical reaction

 Balanced Chemical Equation- # of atoms of each element is the same on both sides of equation   Cannot change the subscripts of an element Coefficient - # placed in front of an element or formula  Used to balance an equation

   Elements combine to form compounds Compounds break down into elements 1 element can replace another element  1.

4 Types of Chemical reactions

Synthesis 2.

3.

4.

Decomposition Single replacement Double replacement

 2 or more simple substances combine to form a new complex substance  A + B  C    Corrosion of metals Substances burning in oxygen Ex. Combination of Iron with Oxygen to form Iron Oxide (rust)

  Complex substances break down into 2 or more simpler substances Reverse of Synthesis reactions  C  A + B  Ex. Decomposition of water in presence of electricity

 An uncombined element replaces an element that is part of a compound   A + BX  AX + B “X” switches partners in equation  Sodium and Water    Very active metal Sodium must be stored in oil not water; explodes when combined with water Sodium replaces Hydrogen in water & release energy Most don’t explode

 Different atoms of 2 different compounds replace each other   AX + BY  AY + BX “X” & “Y” switch partners  Ex. Upset Stomach & medicine  Medicine containing Magnesium Carbonate reacts with stomach acids (Hydrochloric Acid) and result in Magnesium Chloride (harmless) and Carbonic Acid (water and carbon dioxide)

  1.

Classifications Exothermic 2.

Released or Absorbed Endothermic   Energy is neither created nor destroyed, just changed position or form Ex. Heat or light

     Energy is released Burning or combustion  Ex. Combustion of methane gas in a stove released large amts. of heat Energy released originally stored in molecules of reactants Energy of products is less than that of reactants

Reactants have higher energy than products

   Energy is absorbed or taken in Usually in form of heat or light  Ex. Decomposition of Sodium Chloride Energy is stored in molecules of products 

Energy of products is higher than that of reactants

  Total energy released or absorbed not only energy change in a chemical reaction For reactants to form products, molecules of reactants combine to form short-lived, high energy, extremely unstable molecules.

  Atoms of these are rearranged to form products Process requires energy

  Reactants “climb” to top of “energy hill” b/4 products are formed This is called Activation Energy  after reactants absorb this energy they “slide down the hill” and form products 

All chemical reactions require Activation Energy!

   Kinetics – Study of the rate of reaction rates Reaction Rates – measure of how quickly reactants turn into products 1.

Collision Theory – the rate of a reaction is affected by 4 factors: Concentration 2.

3.

4.

Surface area Temperature catalysts

   Measure of the amount of that substance in a given unit of volume high concentration = more particles/unit volume = more collisions High concentration of reactants = higher rate of reaction

  How much of a material is exposed Increase in surface area = increase in collision btwn reactant particles

   Measure of energy of motion of particles Increase in temperature generally increases reaction rate Reaction rates double/triple with an increase of 10 degrees above room temperature

     Catalyst – a substance that increases the rate of a reaction but is not itself changed by the reaction Alters reactions & can be recovered at the end of reaction Lowers the “energy hill” / activation energy Involved on 1 or more early steps of reaction Reformed in later steps – ability to be recovered

 Used in many chemical processes  Ex. Catalytic converter in automobiles  Catalysts in human body a.k.a. enzymes (metabolism)