chapter 8 chemical reactions. physical change in a physical change, the identity and composition of...
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Chapter 8
Chemical Reactions
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Physical Change
In a physical change,
• The identity and composition of the substance do not change
• The state can change or the material can be torn into smaller pieces
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Chemical Change
In a chemical change,
• Reacting substances form new substances with different compositions and properties
• A chemical reaction takes place
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Chemical Reaction
In a chemical reaction,
• Old bonds are broken and new bonds are formed
• Atoms in the reactants are rearranged to form one or more different substances
• Fe and O2 form rust (Fe2O3)
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Chemical Reaction
In a chemical reaction,
• A chemical change produces one or more new substances
• There is a change in the composition of one or more substances
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Evidence of a Chemical Reaction
• Changes that can be seen are evidence of a chemical reaction.
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Writing a Chemical Reaction
• Chemists use a shorthand approach when writing the specifics of a chemical reaction. This approach is called the chemical equation.
Reactants -----> Products
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Chemical Equations
A chemical equation,
• Gives the chemical formulas of the reactants on the left of an arrow and the products on the right
Reactants Product
C(s)
O2 (g)CO2 (g)
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Symbols Used in Equations
Symbols used in chemical
equations show:
• The states of the reactants
• The states of the products
• The reaction conditions
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Chemical Equations Are Balanced
In a balanced chemical reaction,
• Atoms are not gained or lost
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Chemical Equations Are Balanced
In a balanced chemical reaction,
• The number of reactant atoms are equal to the number of product atoms
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Chemical Equations•Chemical equations: symbolic descriptions of chemical reactions.•Two parts to an equation:
•reactants and products
H2 + O2 H2OA Chemical Equation must also be balanced.
2H2 + O2 --> 2H2O
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Balanced Chemical Equations
• Chemical Equations must be balanced– There must be equal numbers of atoms of
each element on both sides of the equation (both sides of the arrow)
1. Write the correct symbols and formulas for all of the reactants and products.
2. Count the number of each type of atom on BOTH sides of the equation.
3. Insert coefficients until there are the equal numbers of each kind of atom on both sides of the equation.
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A Balanced Chemical Equation
Al + S Al2S3 Not Balanced
How many atoms of Al and S are on each side of the equation?
coefficients
2Al + 3S Al2S3 Balanced
2 Al = 2 Al
3 S = 3 S
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Learning Check
State the number of atoms of each element on thereactant and on the product sides of the equations:
P4(s) + 6 Br2(l) → 4 PBr3(g)
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Learning Check
State the number of atoms of each element on thereactant and on the product sides of the equations:
2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s)
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Check the balance of atoms in the following:
Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)
1. Number of _____atoms in reactants.
2. Number of ____ atoms in reactants.
3. Number of ____ atoms in reactants.
Learning Check
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Learning CheckDetermine if each equation is balanced or not.
Na(s) + N2(g) → NaN3(s)
C2H4(g) + H2O(l) → C2H5OH(l)
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Balancing Equations
• Methane (CH4) reacts with oxygen (combustion reaction) to form carbon dioxide and water.
Write a properly balanced chemical equation
1. Write out chemical formulas
CH4 + O2 CO2 + H2O
2. Use coefficients to balance the equation
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The Numbers in Chemical Equations
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More Practice: Balancing Reactions
C2H6 + O2 CO2 + H2O
C3H6 + O2 CO2 + H2O
NH3 + O2 NO + H2O
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And more practice….
B2H6 + H2O H3BO3 + H2
C3H8O + O2 CO2 + H2O
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Balancing with Polyatomic Ions
MgCl2(aq) + Na3(PO4)(aq) NaCl(aq) + Mg3(PO4)2(s)
• HINT: Balance PO43- as a unit.
Set of # coef?
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Type of Reactions
Chemical reactions can be classified as
• Combination reactions.
• Decomposition reactions.
• Single Replacement reactions.
• Double Replacement reactions.
• Combustion reactions.
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Combination In a combination reaction,
• Two or more elements form one product.
• Or simple compounds combine to form one product.
+
2Mg(s) + O2(g) 2MgO(s)
2Na(s) + Cl2(g) 2NaCl(s)
SO3(g) + H2O(l) H2SO4(aq)
A B A B
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DecompositionIn a decomposition reaction,
• One substance splits into two or more simpler substances.
2HgO(s) 2Hg(l) + O2(g)
2KClO3(s) 2KCl(s) + 3O2(g)
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Learning Check
Classify the following reactions as
A) combination or B) decomposition:
1.
2.
3.
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Single ReplacementIn a single replacement reaction,
• One element takes the place of a different element in a reacting compound.
Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)
Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s)
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Double ReplacementIn a double replacement reaction,• Two elements in the reactants exchange places.
AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)
ZnS(s) + 2HCl(aq) ZnCl2(aq) + H2S(g)
Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings
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Learning Check Classify the following reactions as
A) single replacement B) double replacement
1.
2.
3.
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In a combustion reaction,
• A compound such as carbon reacts with oxygen, O2.
C(s) + O2(g) CO2(g)
CH4(g) + 2O2(g) CO2 (g) + 2H2O(g)
C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)
• Fuels burned in oxygen produce CO2, H2O, and energy.
Combustion
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Learning Check
Balance the combustion equation
___ + ___ ___ + ___
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Summary of Reaction Types
Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings
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Heat of Reaction
The heat of reaction,• Is the amount of heat absorbed or released
during a reaction at constant pressure.• Is the difference in the energy of the
reactants and the products.• Is shown as the symbol ΔH. ΔH = Hproducts − Hreactants
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Endothermic Reactions
In an endothermic reaction,
• Heat is absorbed.
• The sign of ΔH is +.
• The energy of the products is greater than the energy of the reactants.
• Heat is a reactant.
N2(g) + O2 (g) + 181 kJ 2NO(g)
ΔH = +181 kJ (heat added)
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Exothermic Reactions
In an exothermic reaction,• Heat is released.• The sign of ΔH is -.• The energy of the products is less
than the energy of the reactants.
• Heat is a product.
C(s) + 2H2(g) CH4(g) + 75 kJ
ΔH = - 75 kJ (heat given off) Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Learning Check
Identify each reaction as
1) exothermic or 2) endothermic.
A. N2 + 3H2 2NH3 + 92 kJ
B. CaCO3 + 556 kJ CaO + CO2
C. 2SO2 + O2 2SO3 + heat