1. Chapter 11: Chemical Reactions

2. Chemical Reactions A rearrangement of atoms
Chemical Reaction example: Methane in natural gas burns in oxygen; carbon dioxide and water are formed.
Chemical Equation:
CH4 + O2  H2O + CO2
Reactants: present before the reaction (methane and oxygen)
Products: chemicals formed in a reaction (water and carbon dioxide)

3. Evidence of a Chemical Reaction
Color Change
Bubbles form
Precipitate forms (solid from the
combination of two liquids)
Heat is absorbed or released
A flame occurs (combustion)

4. Law of Conservation of Mass
In a chemical reaction, atoms are neither created nor destroyed.
All atoms in the reactants must be accounted for in the products
Chemical rxns list state of compounds
(s) = solid
(l) = liquid
(g) = gas
(aq) = in aqueous solution (dissolved in water)

5. Example
Potassium reacts with water to produce hydrogen gas and potassium hydroxide.
Write the balanced chemical equation with physical states listed.
K(s) + H2O(l)  H2(g) + KOH(aq)

6. Rules for Balancing Never change chemical formulas
Only balance using coefficients
The coefficient is distributed to all of the atoms in a compound
Example: 2 NaOH = 2 Na, 2 O, 2 H
Reduce coefficients if possible

7. Practice Problems
Solid Zinc reacts with hydrochloric acid and produces hydrogen gas and solid zinc chloride.
Zn(s) + HCl(aq)  H2(g) + ZnCl2(s)
Sulfuric acid plus solid sodium hydroxide yields water and solid sodium sulfate.
H2SO4(aq) + NaOH(s)  H2O(l) + Na2SO4(s)

8. Practice Problems
Solid Potassium chlorate is broken down into solid potassium chloride and oxygen gas.
KClO3(s)  KCl(s) + O2(g)
Lead(II)nitrate reacts with sodium iodide to produce Lead(II)iodide and sodium nitrate.
Pb(NO3)2 + NaI  PbI2 + NaNO3

9. Practice Problems
Solid Copper reacts with aqueous silver hypoiodite to make aqueous copper(I)hypoiodite and solid silver.
Cu(s) + AgIO(aq)  CuIO(aq) + Ag(s)
Aluminum carbonate reacts with barium chloride to produce aluminum chloride and barium carbonate.
Al2(CO3)3 + BaCl2  AlCl3 + BaCO3

10. Combination (synthesis) Reactions
Two or more substances combine to form a new compound
A + X  AX
2 Mg + O2  2 MgO
8 Ba + S8  8 BaS

11. Decomposition Reactions
A single compound undergoes a reaction that produces two or more similar substances
AX  A + X
2 HgO  2 Hg + O2
CaCO3  CaO + CO2

12. Single-Replacement Reaction
One element replaces a similar element in a compound
A + BX  AX + B
Mg + 2 HCl  H2 + MgCl2
2 Al + 3 Pb(NO3)2  3 Pb + 2 Al(NO3)3

13. Metal Reactivity
When finely divided copper metal is heated, it gradually reacts with oxygen in the air to produce a black copper(II) oxide:
2 Cu(s) O2(g)  2 CuO(s)
Magnesium metal burns in oxygen with a blinding light: 2 Mg(s) O2(g)  2 MgO(s)
Gold does not react with oxygen
What is the relative reactivity of Cu, Mg, Au?

15. Metal Activity Series Why did early Man discover gold, but not sodium?
Element
Metal ion(s) found in Minerals Na
Na+ Mg
Mg2+ Zn
Zn2+ Fe
Fe3+, Fe2+ Cu
Cu2+ Ag
Ag+ Au
Au3+
most active metals
(not found as free element)
least active metals
(can be found as free element)

16. Trends in Metal Activity
A more reactive metal (higher in the activity series) will cause ions of a less reactive metal (lower in the activity series) to change to their corresponding metal
Zn + CuCl2  ZnCl2 + Cu this reaction will take place;
Zn is high on the Activity Series
and tends to form it’s cation
Cu + ZnCl2  N. R this reaction does not occur;
Cu is low on the Activity Series
and does not easily form it’s
cation

17. Metal Reactivity
Activity Series – Elements ranked by relative order of chemical reactivities
Gold does not react with any components of air, including oxygen
Long-lasting decorative objects (jewelry)
Gold-plated electrical connectors for automobile air bags and audio cable connectors

18. Activity Series
How would you rank copper, gold, and magnesium based on what we observed?
Magnesium – reacts readily
Copper – reacts with much added heat
Gold – does not react

19. Double-Replacement Reactions
The ions of two compounds exchange places in an aqueous solution to form two new compounds
AX + BY  AY + BX
2 KI + Pb(NO3)2  PbI2 + 2 KNO3
HCl + NaOH  NaCl + HOH (H2O)

20. Combustion Reactions
A substance combines with oxygen, releasing a large amount of energy in the form of light and heat
Hydrocarbon + oxygen  carbon dioxide + water
CxHy + O2  CO2 + H2O
C3H8 + 5 O2  3 CO2 + 4 H2O

21. SR Types of Reactions Synthesis Decomposition (direct combination-DC)
Large molecule breaks down to smaller pieces
opposites
Small pieces are put together to form large molecules
AB  A + B
A + B  AB
Only one reactant
Only one product
Ex:  +
Ex: + 
Types of Reactions SR
Single Replacement C DR
Double Replacement
Combustion
One element replaces an element from a compound.
Combustible organic (Carbon-containing) compound that reacts with O2 to form CO2 + H2O
Two compounds exchange ions
Active metal can replace another less active metal (cationic SR)
AX + BY  AY + BX
A + BX  AX + B
cpd. + cpd.  cpd. + cpd. +  +
metal
Less active metal
Active non-metal can push out less active non-metal (anionic SR) +  +
The cpds. switch cations
A + BX  BA + X
CH4 + 2O2  CO2 + 2H2O +  +
Look for 2 cpds. for reactants, 2 cpds. for products
Non-metal
Weaker non-metal in compound
Non-metal
(always the products)