Theme 7:
Chemical Reactions
1
Outline
• Balancing Reactions
• Solubility
• Electrolytes and non electrolytes
• Types of Reactions
o Precipitation Reactions
o Acid-Base Reactions
o Gas forming Reactions
o Oxidation Reduction Reactions
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Chemical Reactions: Definition
• A chemical reaction: a process in which reactants are transformed into products.
• Represented by a chemical equation:
◦ Reactants:
◦ Products:
substances combined in the reaction.
Stoichiometric coefficients:
number of molecules
(reactants & products)
involved in a reaction.
1 A2
(g) +
2 B4 (l)
Substances produced
3 C (s) + 4D2 (aq)
Physical states of reactants & products
3
Subscripts and Coefficients
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Balancing Chemical Equations
• A balanced equation
◦ shows the same number of atoms of each element on each side of the equation.
◦ shows relationship between the quantities of all reactants and products
• Tips for balancing chemical equations:
i.
Formulas for reactants & products must be correct, e.g. formula for the formation
of water
ii. Subscripts in the formulas cannot be changed to balance the equation e.g. CO2 to
CO
iii. Use stoichiometric coefficients to balance chemical equations
iv. The entire chemical formula is multiplied by the stoichiometric coefficient
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Balancing Chemical Equations
Practice Problem 1:
Balance the following equations by providing the missing coefficients:
2
a) __Na
(s) +
2 (s)
b) __Al
__H
2 2O(l)
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+ __HCl
(aq)
__NaOH
2
1 2 (g)
(aq) + __H
__AlCl
2
3(aq)
+ __H
2 2(g)
Practice Problem 2: Lecture Quiz
The following diagram represents a chemical reaction in which the
red spheres are oxygen atoms and the blue spheres are nitrogen
atoms. a) Write the chemical formulas for the reactants and products.
b) Write a balanced equation for the reaction. c) Is the diagram consistent
with the law of conservation of mass? Explain.
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Solutions
•
What does the term “aqueous medium” mean?
A solution in which water is the dissolving medium
• What is a solution?
Homogeneous mixtures of two or more pure substances.
• What are the components of a solution?
1. Solvent, a medium in which another substance dissolve
2. Substances that dissolve solutes.
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Solubility and Miscibility
— Solubility: ability of solute
to dissolve in a liquid
Solid solute
— Miscibility: liquids to mix &
form homogenous solution
Liquid
solute
— Immiscible: liquids cannot mix & form
homogenous solution e.g. oil & water
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Dissociation
Breaking/separating of ionic
compounds into separate ions
Solubility
Why are some substances soluble and others not?
• Water is a polar molecule
Net dipole is not zero
• Example 1: NaCl in water (ionic compounds in water)
◦ When placed in water, NaCl dissociates into its ions.
◦ Ions formed are surrounded by water
Attraction between
the solute & solvent
particles must must
be greater than the
intermolecular forces
of those particles
Picture from: https://www.grandinetti.org/solution-chemistry
9
Like Dissolves Like
Polar dissolves polar e.g. ethanol in H2O &
•
glucose in H2O
• Polar also dissolves ionic e.g. salt (NaCl) in
water
• Nonpolar dissolves nonpolar
• Polar will not dissolve nonpolar e.g. water & oil
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Dissociation, Solvation, Hydration
• Dissociation – takes place in ionic compounds where water
breaks down the compound into separate ions
• Solvation – the process of attraction and association of
molecules of a solvent with molecules or ions of a solute.
• Hydration – specific case of hydration when the solvent is water
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Solubility of Ionic Compounds
Refer to Chapter 3, page 108,
figure 3.10, in 9th edition.
Chapter 3, figure 3.10, pg. 134
10th ed
Solubility
Insoluble
Partially
soluble
Soluble
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Practice Problem
Predict whether the following ionic compounds are likely to be soluble in water.
For soluble compounds, list the ions present in solution.
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Solubility and Electrolytic Properties
Solubility
Insoluble
Partially
soluble
Soluble
Strong
electrolytes
Weak
electrolytes
Nonelectrolytes
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Electrolytes and Non-Electrolytes
• An electrolyte is a substances that dissociates
into ions when dissolved in water.
• Strong electrolytes e.g. Ionic substances and
strong acids & bases
• Weak electrolytes e.g. weak acids and bases
• A nonelectrolyte may dissolve in water, but
it does not dissociate into ions.
• For example molecular compounds
e.g. sucrose(table sugar)
Important: Molecular compounds tend to be
nonelectrolytes, except for acids and bases.
Weak electrolytes
dissociate partially when
dissolved in water
Strong electrolytes
dissociate completely
when dissolved in water
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Strong and Weak Electrolytes
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Summary
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Practice Problem: Give it some thought
Which of the following solutes would you use if you want a light bulb to glow brightly?
Explain your choice: CH3OH & MgBr2
18
Reflection Time
https://sites.google.com/site/eportfolioteam1sharr/home/reflection-time
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Types of reactions
• Precipitation reactions
• Acid-base reactions
• Gas-forming reactions
• Oxidation-reduction reactions
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Precipitation Reactions
• Referred to as Exchange Reactions
◦ Ions of reactants exchange partners
Precipitate
For example:
2 KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2 KNO3 (aq)
Which of these products
is insoluble?
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Complete Ionic & Net Ionic Equations
2 KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2 KNO3 (aq)
2 K+ (aq) + 2 I- (aq) + Pb2+
(aq)
Molecular
Equation
+ 2 NO-3 (aq) ⎯→ PbI2 (s) + 2 K+ (aq) + 2 NO-3 (aq)
Complete ionic equation: it shows that soluble ionic compounds are
present in solution as dissociated ions.
2 K+ (aq) + 2 I- (aq) + Pb2+
(aq)
+ 2 NO-3 (aq) ⎯→ PbI2 (s) + 2 K+ (aq) + 2 NO-3 (aq)
Spectator ions: they do not participate in the chemical reaction and they are found
in solution both before and after the reaction, they do not change during the reaction
2I─(aq) + Pb2+(aq) ⎯→ PbI2(s)
Net ionic equation: only shows the ions that are involved
in the chemical reaction. Spectator ions are removed
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Practice Problem
1. Molecular equation:
Remember to balance the Equations
AgNO3 (aq) + KCl(aq) → AgCl (s) + KNO3 (aq)
AgNO3 (aq) + KCl(aq) → AgCl (s) + KNO3 (aq)
2. Total Ionic Equation/Complete Ionic Equation
+ NO+
Ag(aq)
3 (aq) + K Cl (aq) →
AgCl(s) + K+ NO-3 (aq)
3. Net Ionic Equation
+
Ag(aq)
+ Cl-(aq)
AgCl(s)
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Summary: Writing Net Ionic Equations
1. Write a balanced molecular equation.
2. Dissociate all strong electrolytes.
3. Cross out anything that remains unchanged from the left side to the
right side of the equation.
4. Write the net ionic equation with the species that remain.
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Acids and Bases
Definitions
Acid
Base
Arrhenius
Increase the concentration of H+ when
dissolved in water.
e.g. HCl(aq)
H+(aq) + Cl- (aq)
Brønsted and Lowry
Proton donors
NH4 (aq) + OH- (aq)
Increase the concentration of OH−
Proton acceptors
when dissolved in water.
NH3 (aq) + H2O(l)
+
e.g. NaOH(aq)
OH (aq) + Na (aq)
NH3 (aq) + H2O(l)
NH4 (aq) + OH- (aq)
The Arrhenius definition is limited as it assumes that ALL bases are substances increase
that increase OH- ions when dissolved in water. This is not always true as substances such
as NH3 are bases yet they do not increase OH- ions when dissolved in water. Therefore the
Brønsted–Lowry definition is mainly preferred and used by chemists
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Acids and Bases
• Acids are either:
o Strong acids: ionize completely in solution and are strong
electrolytes
o Weak acids: ionize partially in solution and are weak electrolytes
strong acid: HCl + H2O → H3O+ + Cl−
weak acid: HC2H3O2 + H2O
H3O+ + C2H3O2−
• Bases are either:
o Strong bases: ionize completely in solution and are strong
electrolytes
o Weak bases: : ionize partially in solution and are weak electrolytes
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Acids and Bases
Table 3.1, Chapter 3,
page 115, 9th edition
Table 3.1, Chapter 3,
page 140, 10th edition
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◦ e.g.
Acid Base Reactions
Neutralization Reactions and Salts
• React to produce water and salt
• Reactions between strong acids & strong bases are referred to as
neutralizations reactions
◦ Solutions are neither acidic nor basic
HCl (aq) + NaOH(aq)
Acid
Base
H2O(l) + NaCl(aq)
water
salt
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Acid Base Reactions
1. Molecular Equation
HCl (aq) + NaOH(aq)
H2O(l) + NaCl(aq)
2. Total/Complete ionic equation
H+(aq) + Cl- (aq) + Na+ (aq) + OH-(aq)
3. Net ionic equation
H+ (aq) + OH-(aq)
H2O(l) + Na+ (aq) + Cl-
(aq)
Spectator ions
H2O(l)
29
Conjugate Acid-Base Pairs
HF(aq) +
acid1
H2O(ℓ)
base2
→
F−(aq)
base1
+ H3O+(aq)
acid2
Pair 1: F− is the conjugate base of HF
Pair 2: H3O+ is the conjugate acid of H2O
A conjugate acid-base pair always differ by one H
30
Acid-Base Reactions with Gas Formation
• Addition of an acid to a metal carbonate, metal hydrogen carbonate, metal
sulfite or metal sulfide leads to the formation of gas(CO2, SO2 or H2S).
CaCO3 (s) + 2HCl (aq)
⎯→ CaCl2 (aq) + CO2 (g) + H2O (l)
NaHCO3 (s) + HCl (aq) ⎯→ NaCl (aq) + CO2 (g) + H2O (l)
NaHCO3 (aq) + HBr (aq) ⎯→ NaBr (aq) + CO2 (g) + H2O (l)
SrSO3 (s) + 2 HI (aq) ⎯→
SrI2 (aq) + SO2 (g) + H2O (l)
Na2S (aq) + H2SO4 (aq) ⎯→ Na2SO4(aq) + H2S(g)
no water
31
Reflection Time
https://sites.google.com/site/eportfolioteam1sharr/home/reflection-time
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Lecture Quiz
Question 1
Refer to slide 6 for the lecture quiz question.
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The End!!!
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