unit 8 chemical reactions page 123. describing chemical reactions a chemical reaction is the process...
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Unit 8 Unit 8 Chemical ReactionsChemical Reactions
Unit 8 Unit 8 Chemical ReactionsChemical Reactions
PAGE 123PAGE 123
Describing Chemical Reactions
• A chemical reaction is the process by which one or more substances are changed into one or more different substances.
•They are described by chemical equations
Describing Chemical Reactions
• In a chemical reaction:– bonds are broken in the reactants
–Bonds are formed in the products
Parts of a Chemical Parts of a Chemical EquationEquation
Parts of a Chemical Parts of a Chemical EquationEquation
Reactants•original substances in a chemical reaction
•written on the left hand side of a chemical equation
Products•The resulting substances produced in a chemical reaction.
•written on the right hand side of the chemical equation
yields
Subscripts in (parenthesis)
•represents the physical states of the compounds (elements)
•Ex: (s)= solid, (l)=liquid, (g)= gas, (aq)= aqueous (dissolved in water)
•Words or symbols are placed over/under the arrow(s) to indicate certain conditions under which the reaction is carried out.
– : heat is applied– catalyst
• substance that speeds up a chemical reaction without becoming part of the reaction.
Law of conservation of mass
According to the law ofconservation of mass, •the total mass of reactants must equal the total mass of products for any given chemical reaction.
Law of conservation of mass
Mass must be equal50 g + 45 g
Balance atoms H2 (g) + O2 (g) H2O (g)22
95g
Translate Chemical Translate Chemical Equations into WordsEquations into WordsTranslate Chemical Translate Chemical
Equations into WordsEquations into Words
Chemical Equations
•1) include all of the symbols appearing in the equation (with the exception of the
coefficients)
Chemical Equations•3. Diatomic Elements: Elements that combine with each other when found alone in nature
Diatomic Elements•7 diatomic elements (know)
H2 O2 F2 Br2 I2 N2 Cl2
Example #2Mg(s) + 2HCl(aq) MgCl2(aq) +
H2(g)Solid magnesium
reacts with
aqueous hydrochloric acid
to produceaqueous magnesium
chlorideand hydrogen gas
Check for Understanding
CaCO3(s) CaO(s) + CO2(g)
Solid calcium carbonate is heated and produces solid calcium oxide and gaseous carbon dioxide
Check for Understanding #2
3KOH(aq) + Fe(NO3)3(aq) Fe(OH)3(s) + 3KNO3(aq)
Aqueous potassium hydroxide reacts with aqueous iron (III) nitrate to produce solid iron (III) hydroxide and aqueous potassium nitrate
Writing Chemical Equations Writing Chemical Equations from a Written from a Written
DescriptionDescription
Writing Chemical Equations Writing Chemical Equations from a Written from a Written
DescriptionDescription
Example #1
Aluminum metal reacts with oxygen in the air to form solid aluminum oxide.
4 Al (s) + O2 (g)3 Al2O3 (s)2
Example #2
When solid mercury(II) sulfide is heated with oxygen gas, liquid mercury metal and gaseous sulfur dioxide are produced.
+ SO2 (g)HgS (s) + O2
(g)
Hg (l)
Check for Understanding
Oxygen gas can be produced by heating solid potassium chlorate in the presence of the catalyst manganese dioxide. Potassium chloride is a solid residue.
MnO2 + O2 (g)2 2 3KClO3 (s) KCl (s)
Check for Understanding
Aqueous potassium nitrate and a precipitate of barium chromate are formed when aqueous solutions of barium nitrate and potassium chromate are mixed.
Check for Understanding
Aqueous potassium nitrate and a precipitate of barium chromate are formed when aqueous solutions of barium nitrate and potassium chromate are mixed.
Ba(NO3)2 (aq) + K2CrO4 (aq)
KNO3 (aq) + BaCrO4 (s)2
Types of Chemical Types of Chemical ReactionsReactions
Types of Chemical Types of Chemical ReactionsReactions
Synthesis (Combination) Reactions
Two or more substances combine to form a new compound.
.
Formation of only ONE product.
Synthesis (Combination) Reactions
Examples:
2H2 + O2 2H2O
CaO + H2O Ca(OH)2
Decomposition Reactions
• A single compound undergoes a reaction that produces two or more simpler substances.
Decomposition Reactions
Decomposition Reactions Decomposition reactions are the opposite of synthesis reactions.
There is only one reactant.
Examples
•CaCO3 CaO + CO2
•Na2CO3 Na2O + CO2
Single-Replacement Reactions
• one element replaces a similar element in a compound.
• a single element reacts with a compound.
Single-Replacement Reactions
Distinguishing Characteristics
• Examples:
Pb + 2HNO3 Pb(NO3)2 + H2
F2 + 2NaI 2NaF + I2
Predict the Products
Zn + CuCl2 ZnCl2 + Cu
Double Replacement Reactions
The ions of two compounds exchange places in an aqueous solution to form two new compounds.
Two ionic compounds “swap” ions.
Double Replacement Reactions
Usually forms a precipitate
• precipitate: when two aqueous solutions are mixed and a solid product that is formed
AgNO3(aq) + KCl(aq) AgCl(s) + KNO3(aq)
Precipitate
Examples
• PbSO4 + 2KCl K2SO4 + PbCl2
• AgNO3(aq) + KCl(aq) ‑‑‑‑> AgCl(s) + KNO3(aq)
Predict the Products
H2O 2KOH + H2SO4 K2SO4 +2HOH
Combustion Reactions
A hydrocarbon (CxHy) combines with oxygen, releasing a large amount of energy in the form of light and heat.
General Form
CxHy + O2 CO2 + H2O
Example
• C3H8 + 5O2 3 CO2 + 4 H2O
• CH4 + O2 CO2 + H2O
Predict the Products
H2O C2H6 + O2 CO2+2 4 12101 5 2 6
1) 2 Li + 2 H2O 2 LiOH + H2
single replacement / displacement
2) NH4Cl NH3 + HCl
decomposition
3) AgNO3 + NaCl AgCl + NaNO3
double replacement / displacement
4) 2 C6H14 + 19 O2 12 CO2 + 14 H2O
combustion
5) N2 + 3 H2 2 NH3
combination / synthesis
Oxidation – Oxidation – ReductionReductionProcessesProcesses
Oxidation – Oxidation – ReductionReductionProcessesProcesses
Redox ReactionAny chemical reaction that involves the transfer of one or more electrons between atoms. One reactant gains electrons and the other reactant loses electrons.
Examples of Redox Reactions
•Combustion of gasoline •Burning of wood•Energy from food•Bleaching stains•Iron rusting
Oxidation and Reduction
• Opposing reactions
•Must occur together–Redox (Reduction – Oxidation)
Oxidation•Electrons are fully or partially
lost
Mg0 (s) Mg 2+ + 2e-
Electrons lost: written as a product
Reduction• Electrons are fully or
partially gained
S0(s) + 2 e- S2-
electrons gained: written as a reactant
Mnemonic Device
• LEO the lion goes GER
LEO: Loss of Electrons is Oxidation
GER: Gain of Electrons is Reduction
Mnemonic Device
•OIL RIG
–Oxidation Is Loss of electrons
–Reduction Is Gain of electrons
Complete Redox Reaction
• Add together the reduction half-reaction with the oxidation half-reaction to get the complete redox reaction.
Complete Redox Reaction
Mg0 (s) Mg 2+ + 2e-
S0(s) + 2 e- S2-
+
Mg0 (s) + S0
(s) Mg 2+ + S2-
Mg + S Mg+2+ S -2
Assigning Oxidation Assigning Oxidation NumbersNumbers
Assigning Oxidation Assigning Oxidation NumbersNumbers
Oxidation Numbers•Numbers assigned to all of
the elements involved in the reaction to determine if electrons have been transferred between atoms in a reaction.
Oxidation Numbers
• The oxidation number is usually equal to the charge on the ion if it was formed.
Rule #1•Free elements are assigned
an oxidation state of 0.
–Al = 0–Na = 0
–H2 = 0
Rule #2• The oxidation state for any
simple one-atom ion is equal to its charge.
–Na+ = +1–Be2+= +2–F- = -1
Rule #3•The alkali metals (Li, Na, K, Rb,
Cs and Fr) in compounds are always assigned an oxidation state of +1.
–LiOH: Li = +1–Na2SO4: Na= +1
Rule #4
•Fluorine in compounds is always assigned an oxidation state of -1.
–HF: F= -1
–MgF2: F= -1
Rule #5• The alkaline earth metals (Be,
Mg, Ca, Sr, Ba, and Ra) and also Zn and Cd in compounds are always assigned an oxidation state of +2. Similarly, Al & Ga are always +3.
•MgF2: Mg = +2
•CaO: Ca= +2
•Al2O3: Al=+3
Rule #6• Hydrogen in compounds is assigned an
oxidation state of +1.
Exception - Hydrides, ex. LiH (H=-1).
•H2SO4: H=+1
•HCl: H=+1
Rule #7
• Oxygen in compounds is assigned an oxidation state of -2.
Exception - Peroxide, ex. H2O2 (O = -1).
– H3PO4: O=-2
– H2O: O=-2
Rule #8
•The sum of the oxidation numbers of all atoms in a neutral compound is 0.
–H2SO4: add up to 0–CO2: add up to 0
Rule #9
•The sum of the oxidation numbers of all atoms in a polyatomic ion equals the charge on the ion.
–SO42-: add up to -2
–NH4+: add up to +1
Examples•Al(s):
–Al = ____
Rule #1
0
Examples•CaCl2:
–Ca = ____–Cl=____
Rule #5
Ca+2 Rule #8
1 (+2) + 2 (Cl) = 0
Cl = -1
-1
Examples•HNO3:
–H= ____–N= ____–O= ____
Rule # 6
Rule # 7
Rule # 8+1
-2 1(1) + 1(N) + 3(-2) = 0
N = +5
+5
Examples• SO4
2-:
– S= ____– O= ____
Rule #7
-2
Rule #9
1(S) + 4(-2) = -2+6
S= +6
Examples• H2O:
– H= ____– O= ____
Rule #6
+1Rule #7
-2
Examples•(NH4)2CO3
NH4+ CO3
2-
N=____ C=___ H=____ O=___
Rule 6
+1
Rule 7
-2
Rule 9
1(N)+4(+1)=+1N= -3
-3
1(C)+3(-2)=-2C=+4
+4
Check for Understanding
• S8:
– S= ____
• AsO43-:
– As= ____ – O= ____
0
-2+5
Check for Understanding
• Cr(OH)3:
– Cr= ____ – O= ____ – H= ____
-2
+1
+3
OH-
Check for Understanding
• (NH4)3PO4:
– N= ____ – H=_____ – P= ____ – O= ____
NH4+ PO4
-3
+1-3
-2
+5
Oxidized Element
• Atoms that lose electrons in a chemical reaction.
– Elements that lose electrons are undergoing oxidation and are said to be oxidized.
Oxidized Element
• The substance that loses electrons is the oxidized element.
• Atoms that are oxidized will have an increase in their oxidation number
Oxidized Element
Mg0 (s) + S0
(s) Mg 2+ + S2-
Increase in oxidation number
Mg is oxidized
Reduced Element
• Atoms that gain electrons in a chemical reaction.
– Elements that gain electrons are undergoing reduction and are said to be reduced.
Reduced Element
• The substance that gains electrons is the reduced element.
• Atoms that are reduced will have an decrease in their oxidation number
Reduced Element
Mg0 (s) + S0
(s) Mg 2+ + S2-
Decrease in oxidation number
S is reduced
Oxidizing Agent•The element or compound that
is reduced.
•It is called the oxidizing agent because it oxidizes the other element or compound.
Oxidizing Agent
Mg0 (s) + S0
(s) Mg 2+ + S2-
Decrease in oxidation number
S is reduced
S is the oxidizing agent(it is oxidizing Mg)
Reducing Agent•The element or compound that
is oxidized.
• It is called the reducing agent because it reduces the other element or compound.
Reducing Agent
Mg0 (s) + S0
(s) Mg 2+ + S2-
Increase in oxidation number
Mg is oxidizedMg is the reducing agent
(it is reducing S)
Examples
• Are the following redox reactions?
• If yes:– What element is oxidized?– What element is reduced?– What is the oxidizing agent?– What is the reducing agent?
Examples
Mg(s) + S(s) MgS
Step #1: Assign Oxidation Numbers
0 0 +2-2
Step # 2: Do the oxidation numbers change?
- if no it is not a redox reaction
- if yes it is a redox reaction
Yes – It is a redoxWhat element is oxidized?What element is reduced?What is the oxidizing agent?What is the reducing agent?
MgS
SMg
Examples
Step #1: Assign Oxidation Numbers
2AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2Ag(s)
0 0
NO3- NO3
-
+1 +2
Yes – It is a redox
Step # 2: Do the oxidation numbers change?
- if no it is not a redox reaction
- if yes it is a redox reaction
What element is oxidized?What element is reduced?What is the oxidizing agent?What is the reducing agent?
CuAg
AgNO3(aq)
Cu(s)
Check for Understanding
• Are the following redox reactions?• If yes:
– What element is oxidized?– What element is reduced?– What is the oxidizing agent?– What is the reducing agent?
Check for Understanding
4Fe(s) + 3O2(g) 2Fe2O3(s)
0 0 -2+3
YesWhat element is oxidized?What element is reduced?What is the oxidizing agent?What is the reducing agent?
FeOO2
Fe
Check for Understanding
Ca(s) + H2O(l) Ca(OH)2(aq) + H2(g)
0 0+1 -2
OH-
+2 +1-2
YesWhat element is oxidized?What element is reduced?What is the oxidizing agent?What is the reducing agent?
CaH
H2OCa
Check for Understanding
HCl + NaOH NaCl + H2O +1-1 +1-2+1 +1-1 +1 -2
No – it is not a redox
Check for Understanding
Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)
-2 +2-2 0+3 +4-2
YesWhat element is oxidized?What element is reduced?What is the oxidizing agent?What is the reducing agent?
CFe
Fe2O3
CO
Check for Understanding
2AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2Ag (s)
NO3- NO3
-
+1 0 +2 +0
YesWhat element is oxidized?What element is reduced?What is the oxidizing agent?What is the reducing agent?
CuAg
AgNO3Cu
Check for Understanding
NaCl + AgNO3 NaNO3 + AgCl+1-1 +1
NO3- NO3
-
+1 +1-1
No – it is not a redox
Check for Understanding
2 H2(g) + O2(g) 2 H2O(l)
0 0 +1-1
YesWhat element is oxidized?What element is reduced?What is the oxidizing agent?What is the reducing agent?
HO
O2H2
Check for Understanding
CH4 + 2 O2 CO2 + 2 H2O -4+1 0 +4-2 +1-2
YesWhat element is oxidized?What element is reduced?What is the oxidizing agent?What is the reducing agent?
CO
O2
CH4