Chemical Reactions

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Unit 5

• Chemical Reactions

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Chemistry Change

• Chemistry is all about change
• Explain in a sentence what may have chemically
  changed in each of the following pictures

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• All CHEMICAL reactions start and end with
  electrons
• Metals
• Lose valence shell
• Become charged
• Types I and II
• Non-Metals
• Tend to gain electrons
• Become charged
• Dont always follow the rules (sometimes they
  share etc.)
• All chemical reactions are decided by ENERGY!
• 1st Law of Thermodynamics Energy cannot be
  created or destroyed
• 2nd Law of Thermodynamics Changes in nature tend
  to decrease available energy (or release energy)

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• We keep track of these changes by using chemical
  equations

Here we have the chemical equation for the
formation of water. Notice the position of the
reactants and the products as well as the
coeficients.
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Reactions and the 1st Law of Thermodynamics

• Notice on the previous picture that atoms are not
  created or destroyed
• This holds true with the 1st Law of TD
• This is the first rule in dealing with chemical
  equations
• RULE 1 all atoms present in the reactants must
  also be accounted for in the products

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Balancing Act

• When we follow Rule 1, we have balanced chemical
  equations.
• Consider the following reaction
• CH4 O2 ? CO2 H2O
• Is it balanced? How do we do it?

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Balance the following reactions

• 1. K(s) H2O(l) ? H2(g) KOH
• 2. HgO(s) ? Hg(l) O2(g)
• 3. C(s) O2(g) ? CO2(g)

Besides just coefficients and chemicals, notice
the small letters indicating the STATE of the
chemical. l liquid aq aqueous (dissolved in
water) s solid g gas
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Types of Reactions

• There are 5 types of reactions you need to know
• Synthesis
• Single replacement
• Double replacement
• Decomposition
• Combustion

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Synthesis 2 into 1

• Synthesis or combination reactions form one new
  compound from two reactants
• H2 O2 ? H2O
• Balance this reaction
• Notice how two reactants COMBINE to form a new
  product!

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Synthesis is
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Single Replacement

• In single replacement reactions, ONE element
  reacts with ONE compound.
• A new compound is formed
• One ion is released or left over without a partner

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• Example
• Zn HCl ? H2 ZnCl2
• Balance this equation
• Notice only one anion is exchanged, only one
  partner has switched

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Double Replacement

• In double replacement reactions, a compound
  reacts with another compound
• Two new compounds are formed
• Both ions switch partners

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• Example
• Pb(NO3)3 NaCl ? PbCl3(s) Na(NO3)(aq)
• Balance this reaction.
• What is PbCl3(s) called?
• Notice two new compounds are formed and both
  ions switch partners

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Decomposition 1 to 2

• In a decomposition reaction, 1 reactant breaks
  apart (decomposes) into two products.
• HgO(s) ? Hg (l) O2(g)
• Balance this reaction
• Notice there is 1 reactant! This is the key to
  identifying decomposition reactions.

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Combustion

• Hydrocarbon O2 ? CO2 H2O
• Hydro (H and possibly O) and Carbon (C)
• ALWAYS the same 2 products! All you have to do is
  balance them!

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Predict and Balance

• Composition/Synthesis Double Rep
• Na O2 ? AgNO3 K2SO4 ?
• Fe (iron 3) O2 ? Mg(OH)2 HCl ?
• Combustion
• CH4 O2 ?
• C6H12O6 O2 ?
• Single Rep
• KI Cl2 ?
• Cu (copper 2) AgNO3 ?

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• C3H8 O2 ? CO2 H2O
• Balance this reaction

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Acid Base

• Acid/base reactions follow the same rules as
  single and double replacement reactions.
• H acts as a metal cation, so it is replaced by a
  metal cation

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Oxidation/Reduction

• Know a couple examples
• Rust
• Corrosion
• Tarnish
• Electroplating
• Oxidation Number Charge on the ion