Honors Chemistry

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Honors Chemistry Unit 5Chemical Equations

• Chapter 10

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What is a chemical rxn?

• A process where one or more substances changes to
  one or more other substances.
• Exchange of electrons!!!
• This is NOT a nuclear rxn! All atoms remain the
  same they just rearrange

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Parts of a chemical reaction

• Reactants the original substances
• Products the resulting substances

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ASK YOURSELF

• Which are the products and which are the
  reactants in these equations?
• Na Cl- ?NaCl
• 2H2O ? 2H2 O2

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Quick Demo

• Magnesium

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A word equation

• Magnesium metal reacts with oxygen gas to form
  white magnesium oxide solid.
• We must learn how to translate these!

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Ways to Represent Reactions

• Chemical equation symbols that describe a
  chemical reaction and show what atoms (and how
  many, relatively speaking) are involved.
• Word equation the names of each product/reactant
  are written out. Doesnt show how many of each

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Write out the word equation

• Magnesium metal reacts with oxygen gas to form
  magnesium oxide solid.
• Mg O ? MgO

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Remember BrINClOFH

• Br2 a liquid
• I2 - a solid
• N2 - a gas
• Cl2 - a gas
• O2 - a gas
• F2 - a gas
• H2 - a gas

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Check for BrINClOFH

• Any time you see 1 of the 7 diatomic elements
  alone, write them with a 2 subscript
• Mg O2 ? MgO

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Balance the charges of ionic comp

• In our reaction we have already done this, but
  make sure to use proper formulas (for molecular
  comp too!)
• MgO Mg2 O2-
• Mg O2 ? MgO

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Balance number of atoms/ions on each side

• In ordinary chemical changes, atoms just
  rearrange
• This is the LAW OF CONSERVATION OF MATTER
• We do this with coefficients
• 2Mg O2 ? 2MgO

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Example

• If you take Ca(NO3)2 and change it to 2Ca(NO3)2,
  how many of each element do you have?

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Put in physical states

• Symbols used in chemical equations
• (s) solid
• (l) liquid
• (g) gas
• (aq) aqueous solution (dissolved in H2O)

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Put in physical states

• I will give you physical states in word problems
• Magnesium metal reacts with oxygen gas to form
  magnesium oxide solid
• 2Mg(s) O2(g) ? 2MgO(s)

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Balancing equations Tips

• __Fe2O3 __H2SO4 ? __Fe2(SO4)3 __H2O
• If possible treat polyatomic ions as 1 unit
• Balance hydrogen 2nd last
• Balance oxygen last
• If the last element does not balance double
  everything

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Word Equation example

• Hydrogen gas and Oxygen gas combine to form
  liquid water.
• 2H2(g) O2(g) ? 2H2O(l)

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Example

• Sodium solid reacts with chlorine gas to form
  sodium chloride solid.
• 2Na(s) Cl2(g) ? 2NaCl(s)

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Try this one

• Dicarbon dihydride reacts with oxygen gas to
  produce carbon dioxide gas and water

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Evidence of rxns

• All rxns involve chemical change
• Look for evidence of those changes
• Light, heat, gas, color change, precipitate

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Other Symbols

• ? precipitate forms
• ? gas forms
• ? produces or yields
• ?? products can reform in to reactants end
  result is a mix of the two.
• ? means that heat was added

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Chemistry Chemical Equations

• Part II

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Experiment 17

• Please read over steps 1-8 carefully
• Get the proper safety gear, and the proper
  equipment
• Do steps 1-8, then set the solid aside to dry
  (step 9)
• Return to your seat while the solid dries, you
  can work on A7
• At the end of class record the mass of your
  beaker and solid.

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Balanced equations show RATIOS

• The coefficients of a balanced equation reveal
  the RATIO of each reactant to each product
• 2H2 O2 ? 2H2O means there is a 212 ratio of
  H2 to O2 to H2O.
• This is the same as a 424 ratio, etc. etc.

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The activity series

• Writing an equation doesnt mean it will actually
  happen
• How do we know it will happen?
• Single replacement reactions are predicted with
  the activity series
• This activity series has been created through
  experiment.

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The activity series

• Things on the list will displace anything below
  but not above other things on the list
• There are two lists one for elements that
  become positive ions, and one for elements that
  become negative ions.

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Element Reactivity Element Reactivity
LiRbKBaCaNa React with cold H2O and acids, replacing hydrogen F2Cl2Br2 I2 Listed from most reactive to least reactive
MgAlMnZnCrFe React with acids or steam but usually not liquid water, to replace hydrogen
NiSnPb All react with acids but not water, to replace hydrogen
H2CuHg All react with oxygen to form oxides
AgPtAu Mostly unreactive
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? When a rxn releases energy, where does it come
from?

• You have to put energy in to break bonds
• If bonding is stronger in the products they will
  be more stable
• More stable lower energy
• Lower energy means energy must be RELEASED

Is this an example of an ENDOthermic or
EXOthermic reaction?
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• If bonding is weaker in the products, they will
  be less stable, have more energy, and will
  therefore absorb it.

Is this an endo or exothermic rxn?