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Unit 7 Chemical Reactions

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In chemistry one mole is equal to 6.022 x 1023 particles (a.k.a. Avogadro's number) ... of the reactants (energy is absorbed into the reaction); test tube gets cooler ... – PowerPoint PPT presentation

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Title: Unit 7 Chemical Reactions


1
Unit 7 Chemical Reactions
  • Part 2 Formula Mass, the Mole, Energy of
    Reactions, Rates of Reactions, Limiting Reactants

2
Calculating Formula Mass
  • Formula mass can be calculated in amus or grams
    of a substance by multiplying the number of atoms
    of each element by the mass in amus or grams of
    the element. Then add the values together. (YES,
    sig figs COUNT!!)
  • Example CaSO4
  • ( atoms each element x mass total mass of
    element in compound)
  • 1 Ca x 40.1g 40.1 g
  • 1 S x 32.1g 32.1 g
  • 4 O x 16.0g 64.0g
  • Then add masses of all elements together



136.2g 136g
3
The Mole
  • In chemistry one mole is equal to 6.022 x 1023
    particles (a.k.a. Avogadros number).
  • The formula mass of any compound is the mass of 1
    mole of the compound in grams.
  • 1 mole 6.022 x 1023 is similar to
  • 12 eggs 1 dozen
  • 52 weeks 1 year
  • 144 items 1 gross

4
Percent Composition
  • The percent composition of a compound is the
    formula mass of each element in a compound
    relative to the total formula mass (FM) of the
    compound
  • Found by dividing the formula mass of the element
    by the formula mass of the compound multiplying
    the answer by 100 (to change it to a percent)
  • Example CaSO4
  • Ca40.1g (40.1g/136g) x 100 29.5
  • S 32.1g (32.1g/136g) x 100 23.6
  • O 64.0g (64.0g/136g) x 100 47.1
  • FM 136g

5
Limiting Reactants
  • The limiting reactant is the reactant that
    determines the maximum amount of product that is
    formed.
  • The limiting reactant will be completely used up
    in a reaction and then the reaction stops.
  • The other reactant will have some unchanged so it
    is said to be the excess reactant.
  • For example, you need to make 10 chicken
    sandwiches. You have 10 slices of bread and 10
    pieces of chicken. If each sandwich requires 2
    slices of bread and 1 piece of chicken, which is
    the limiting reactant? Excess reactant?

6
Rates of Reactions
  • The reaction rate is the change in concentration
    of reactants and products in a certain amount of
    time.
  • Rate at which the reactants disappear and the
    products appear.
  • Combining two substances (causing a reaction)
    means forcing their particles to hit, or collide
    with each another
  • Collision Theory states that molecules must
    collide in order to react

7
Activation Energy
  • The activation energy is the energy needed to
    start the reaction like the heat of the Bunsen
    burner
  • When particles collide with sufficient energy
    at least equal to the activation energy
    existing bonds may be disrupted and new bonds can
    form
  • Endothermic reaction the energy of the product
    is greater than that of the reactants (energy is
    absorbed into the reaction) test tube gets
    cooler
  • Exothermic reaction the energy of the products
    is lower than that of the reactants (energy is
    released from the reaction) test tube gets hotter

8
Factors Affecting Reaction Rates
  • 1. Nature of Reactants
  • Depends on the state of particular reactants and
    the complexity of the bonds that have to be
    broken and formed in order for the reaction to
    proceed
  • The more bonds to be broken, then the longer the
    reaction takes
  • A reaction between two gases will be quicker than
    a reaction between two liquids or two solids.

9
Factors Affecting Reaction Rates
  • 2. Temperature
  • The higher the temperature at which a reaction
    occurs, the faster the particles will move and
    the more frequent the collisions
  • For example, food spoils faster at room
    temperature than when it is refrigerated.

10
Factors Affecting Reaction Rates
  • 3. Concentration
  • Deals with how many particles are there
  • An increase in concentration means that there are
    more particles within a given volume
  • Thus, the higher the concentration of reactants,
    the greater the frequency of collisions among
    their particles.
  • For example, the more people there are in a room
    the more people you will bump into as you walk
    through the room.

11
Factors Affecting Reaction Rates
  • 4. Surface Area
  • Surface area deals with the number of particles
    that are exposed for reaction.
  • The larger the surface area the greater the
    number of particles that are exposed for
    reaction.
  • For example, many small pieces of coal will burn
    faster than a lump of coal (small pieces have
    more particles exposed to react with more oxygen
    particles)

12
Factors Affecting Reaction Rates
  • 5. Catalysts
  • A catalyst is a substance that increases the rate
    of the reaction without itself being used up in
    the reaction (it does not appear as a reactant or
    a product in the chemical equation)
  • Catalysts lower the activation energy required
    for a reaction to occur.
  • Thus a catalyst creates a different pathway from
    reactants to products one that requires less
    energy.
  • Catalysts in the body are enzymes they speed up
    reactions in the body that are essential to life.
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