Chemical Reaction s
Jan 21, 2016
Chemical Reactions
Chemical Reactions
• Reactants:– A starting substance in a chemical reaction.
• Products:– A substance formed in a chemical reaction.
Reactants Products
• Reactants are always on the left of the arrow.
• Products are always on the right of the arrow.
• What does the arrow mean?– Yields or produces
For Review: Law of Conservation of Mass
• Are atoms ever created or destroyed in a chemical reaction?
NO!!!
Definitions
• Chemical Equation:– The arrow separates the formulas of the
reactants from the formulas of the products.
• Skeleton Equation:– A chemical equation that doesn’t indicate the
relative amounts of the reactants and products involved.
– Symbols you will see: (s), (l), (g), and/or (aq)
More Definitions:• Catalyst:
–Substance that speeds up the rate of a reaction but is not used up in the reaction.• If a catalyst is used, the symbol or formula
is written above the arrow.
Evidence of Chemical Reaction
• Energy change
– Temp increase/decrease
– Light emitted
• Precipitate produced
• Unexpected color change
• Gas released/bubbles
Types of Reactions
• There are 5 types of reactions.–1. Combination (a.k.a. synthesis)–2. Decomposition–3. Single replacement–4. Double replacement–5. Combustion
Combination (Synthesis)• Start with TWO elements/compounds and
merge to form ONE productTwo elements combine• A + B C• Mg(s) + O2(g) MgO(s)
• Element/cmpd + element/cmpd Compound
• Example: Complete the following combination reaction:
K(s) + Cl2(g)KCl
2 2
2 2
Combination, cont.
• Nonmetal oxides react with water to form acids–SO2(g) + H2O(l) H2SO3 (aq)
• Metallic oxides react with water to from bases–CaO(s) + H2O(l) Ca(OH)2
Synthesis ReactionsSynthesis Reactions
• Here is another example of a synthesis reaction
PracticePractice
• Predict the products. Write and balance the following synthesis reaction equations.
• Sodium metal reacts with chlorine gas
Na(s) + Cl2(g) • Solid Magnesium reacts with fluorine gas
Mg(s) + F2(g) • Aluminum metal reacts with fluorine gas
Al(s) + F2(g)
Decomposition:
• A compound breaks down into a two or more simpler substances.
• C A + B• CaCO3(s) CaO(s) + CO2(g)
• *Most decomposition reactions require energy in the form of heat, light or electricity.
• Cmpd element/cmpd + element/cmpd
Decomposition Rxn.• Start with ONE cmpd and it breaks into TWO
pieces (two elements or two smaller cmpds)
– Ionic Salt M + NM
NaCl
– Carbonate salt metal oxide + CO2
CaCO3
– Chlorate/perchlorate salt metal chloride + O2
KClO3
Decomposition ReactionsDecomposition Reactions
• Another view of a decomposition reaction:
PracticePractice
• Predict the products. Then, write and balance the following decomposition reaction equations:
• Solid Lead (IV) oxide decomposes PbO2(s)
• Aluminum nitride decomposes
AlN(s)
Single Replacement
• One element replaces a second element in a compound.
• A + BC AC + BElement + Compound Compound +
Element
Single Displacement Rxns.• Element reacts with a cmpd to produce a new element
and a new cmpd.– Metal + cmpd new metal + new cmpd
AgNO3 (aq) + Cu (s) – NM + cmpd new NM + new cmpd
Cl2 (g) + NaI (aq) – Metal + water H2 + metal hydroxide (base)
Li (s) + HOH (l)
• Use REACTIVITY SERIES to make sure these rxns will occur
• Element must be more reactive than the ion in the cmpd in order to replace it
Single Replacement, cont.• Whether or not a metal will displace
another metal depends on its reactivity to other metals. – A metal must be more reactive to “kick out”
another metal. If it is not, there is no reaction.
– Activity Series of Metals. Reactivity decreases as you go down, so metals will only replace another one below it.
• A nonmetal can also replace another nonmetal from a compound.– This is usually limited to halogen group.
– Activity of halogens decreases as you go down the column (F is more reactive than I).
Single Replacement ReactionsSingle Replacement Reactions
• Another view:
Single Replacement Practice
• Complete the following reactions:
• 1. Zn + H2SO4 • 2. Sn + NaNO3
• 3. Cl2 + NaBr
ZnSO4 + H2
No Reaction
NaCl + Br2
Single Replacement Practice
• Complete the following reactions:
• 1. Mg + KCl • 2. Ag + HCl• 3. l2 + KF• 4. Zn + LiCl• 5. Ca + PbO2
Double Replacement• Involves an exchange of positive ions
between two reacting compounds.
• AB + CD AD + CB
• Cmpd + Cmpd Cmpd + Cmpd
For a double replacement reaction to occur, one of the following is usually true:
1. A precipitate is formed.2. A gas is produced and bubbles out of solution.3. One product is a molecular compound such as water.
Double Replacement ReactionsDouble Replacement Reactions
• Think about it like “foil”ing in algebra, first and last ions go together + inside ions go together
• Example:
AgNO3(aq) + NaCl(s) AgCl(s) + NaNO3(aq)
• Another example:
K2SO4(aq) + Ba(NO3)2(aq) KNO3(aq) + BaSO4(s) 2
PracticePractice
Predict the products
1. HCl(aq) + AgNO3(aq)
2. CaCl2(aq) + Na3PO4(aq)
3. Pb(NO3)2(aq) + BaCl2(aq)
4. FeCl3(aq) + NaOH(aq)
5. H2SO4(aq) + NaOH(aq)
6. KOH(aq) + CuSO4(aq)
Combustion• An element or a compound reacts with oxygen (O2) often
producing energy as heat and light.
X + O2 XO
Often involves hydrocarbons: C and H• C#H#, C#H#O# or C#H#OH
Complete combustion of a hydrocarbon looks like this:
C3H8 + O2(g) CO2 + H2O
Combustion ReactionsCombustion Reactions• In general:
CxHy + O2 CO2 + H2O
• Products in combustion are ALWAYS carbon dioxide and water. (although incomplete burning does cause some by-products like carbon monoxide)
• Combustion is used to heat homes and run automobiles (octane, as in gasoline, is C8H18)
Identify the following reactions1. S + O2 → SO3
2.CaCl2 + Na3PO4 Ca3(PO4)2 + NaCl3. K + Cl2 → KCl4. H2O2 → H2O + O2
5. Na + H2O → NaOH + H2
6. KClO3 → KCl + O2
7. C2H6 + O2 → CO + H2O8.KI + Cl2 KCl + l2
9. CH4 + O2 → CO2 + H2O10. N2 + H2 → NH3
11. P + O2 → P4O10
12. Fe(OH)3 → Fe2O3 + H2O
PracticePractice
Predict the products. Identify the type of reaction.1. S + O2
2. CaCl2 + Na3PO4)
3. Pb(NO3)2 + BaCl2
4. KI + Cl2
5. CH4 + O2
6. C3H8 + O2
7. CaCO3
8. Na + H2O
Balancing Combustion Balancing Combustion ReactionsReactions
When balancing combustion reactions, start with Carbon and balance Oxygen last.
If you end up needing an odd number of diatomic oxygen, multiply the coefficient in front of the hydrocarbon by 2 and rebalance.
• Here is what I mean…
– Balance the following equation:
C3H8 + O2 CO2 + H2O
C4H10 + O2 CO2 + H2O
C4H10 + O2 CO2 + H2O
3 45
4 5
2 8 1013
6.5
Combustion ReactionsCombustion Reactions
Combustion Practice
• Combust the following compounds:
• C6H6
• CH3OH
• Be sure to balance these!
+ 15 O2
+ 3 O2
12 CO2 + 6 H2O
2 CO2 + 4 H2O
2
2
Balancing Equations
• Balanced Equation:
– Each side of the equation has the same number of atoms of each element.
• Why balance an equation?
– To obey the law of conservation of mass.
How do we balance?
• Use Coefficients:– “The big numbers in front.” Place the
coefficient in front of the symbols for the respective parts. If no number is written, it is understood to be one.
Rules for Balancing
• 1. Determine the correct formulas– If states are given, include these in
parenthesis.
• 2. Write the formula for reactants on left and products on the right.– Separate with a yield sign.– If there are two or more reactants or
products, separate with a plus sign.
Rules for Balancing, cont.
• 3. Count the number of atoms of each element.– A polyatomic ion appearing unchanged on
both sides of the equation is counted as a single unit.
– Example:
CaS(s) + NaOH(aq) Ca(OH)2(aq) + Na2S(s)
Versus: NaHCO3(s)+HCl(aq) NaCl(aq)+H2O(l)+ CO2(g)
Rules for Balancing, cont.
• 4. Balance the elements one at a time.• 5. Check to be sure each atom or
polyatomic ion is balanced.• 6. Make sure all coefficients are in the
lowest possible ratio.