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Types of Chemical Reactions
and
Solution Stoichiometry
Aqueous Solutions
Water is the dissolving
medium, or solvent.
Some Properties of Water
Water is “bent” or V-shaped.
The O-H bonds are covalent.
Water is a polar molecule.
Hydration occurs when salts dissolve in
water.
Figure 4.1: (Left) The water molecule is polar. (Right) A space-filling model of the water molecule.
Figure 4.3: (a) The ethanol molecule contains a polar O—H bond similar to those in the water molecule. (b) The polar water molecule interacts strongly with the polar O—H bond in ethanol. This is a case of "like dissolving like."
A Solute
dissolves in water (or other “solvent”)
changes phase (if different from the solvent)
is present in lesser amount (if the same
phase as the solvent)
A Solvent
retains its phase (if different from the
solute)
is present in greater amount (if the same
phase as the solute)
Ions in Aqueous Solution
- Some molecular compounds dissolve but do
not dissociate into ions.
C H O (s)(glucose) C 6 H 12 O 6 (aq ) HO 6 12 6 →^2
Ionic Theory of Solutions
- These compounds are referred to as nonelectrolytes. They dissolve in water to give a nonconducting solution.
Ions in Aqueous Solution
- Many ionic compounds dissociate into
independent ions when dissolved in water
Ionic Theory of Solutions
- These compounds that “freely” dissociate into independent ions in aqueous solution are called electrolytes.
- Their aqueous solutions are capable of conducting an electric current.
NaCl( s) Na (aq) Cl (aq )
H 2 O
Figure 4.2: Polar water molecules interact with the positive and negative ions of a salt assisting in the dissolving process.
Figure 4.5: When solid NaCl dissolves, the Na+^ and Cl-^ ions are randomly dispersed in the water.
Ions in solution conduct electricity. Strong electrolytes in solution conduct electricity!
Electrolytes
Strong - conduct current efficiently
NaCl, HNO 3
Weak - conduct only a small current
vinegar, tap water
Non - no current flows
pure water, sugar solution
Figure 4.7:
An aqueous
solution of
sodium
hydroxide.
Ions in Aqueous Solution
- Strong and weak electrolytes.
- A weak electrolyte is an electrolyte that dissolves in water to give a relatively small percentage of ions.
- NH 3 (g) + H 2 O (l) º NH 4 OH (aq) NH 4 OH(aq) ← NH 4 (aq) OH (aq)
→
- – Most soluble molecular compounds are either nonelectrolytes or weak electrolytes.
Ionic Theory of Solutions
Figure
4.9: The
reaction
of NH 3 in
water.
Ions in Aqueous Solution
- In summary, substances that dissolve in water
are either electrolytes or nonelectrolytes.
Ionic Theory of Solutions: Summary
- Nonelectrolytes form nonconducting solutions because they dissolve as molecules.
- Electrolyte s form conducting solutions because they dissolve as ions.
Ions in Aqueous Solution
- Electrolytes can be strong or weak.
- Almost all ionic substances that dissolve are strong electrolytes.
- Molecular substances that dissolve are either nonelectrolytes or weak electrolytes.
Ionic Theory of Solutions: Summary
Working with Solutions
- Molar concentration , or molarity (M) , is
defined as the moles of solute dissolved in
one liter (cubic decimeter) of solution.
Molar Concentration
litersof solution
molesofsolute Molarity (M) =
Molarity
Molarity ( M ) = moles of solute per volume of
solution in liters:
M
M
molarity
moles of solute
liters of solution
HCl
moles of HCl
liters of solution
Common Terms of Solution
Concentration
Stock - routinely used solutions prepared in
concentrated form.
Concentrated - relatively large ratio of solute
to solvent. (5.0 M NaCl)
Dilute - relatively small ratio of solute to
solvent. (0.01 M NaCl)
Working with Solutions
- The majority of chemical reactions
discussed here occur in aqueous solution.
- When you run reactions in liquid solutions, it is convenient to dispense the amounts of reactants by measuring out volumes of reactant solutions.
Figure 4.11: (a) A measuring pipet is graduated and can be used to measure various volumes of liquid accurately. (b) a volumetric (transfer) pipet is designed to measure one volume accurately.
Figure 4.10: Steps involved in
the preparation of a standard
aqueous solution.
Working with Solutions
- When we dissolve a substance in a liquid,
we call the substance the solute and the
liquid the solvent.
Molar Concentration
- The general term concentration refers to the quantity of solute in a standard quantity of solution.
Types of Solution Reactions
Precipitation reactions
AgNO 3 ( aq ) + NaCl( aq ) → AgCl( s ) + NaNO 3 ( aq )
Acid-base reactions
NaOH( aq ) + HCl( aq ) → NaCl( aq ) + H 2 O( l )
Oxidation-reduction reactions
Fe 2 O 3 ( s ) + 2 Al( s ) → 2 Fe( s ) + Al 2 O 3 ( s )
Ions in Aqueous Solution
- A molecular equation is one in which the
reactants and products are written as if they
were molecules, even though they may
actually exist in solution as ions.
Molecular and Ionic Equations
- Note that Ca(OH) 2 , Na 2 CO 3 , and NaOH are all soluble compounds but CaCO 3 is not.
Ca( OH) 2 ( aq) + Na 2 CO 3 (aq) → CaCO 3 (s) + 2 NaOH(aq)
Simple Rules for Solubility
- Most nitrate (NO 3 −) salts are soluble.
- Most alkali (group 1A) salts and NH 4 +^ are soluble.
- Most Cl−, Br−, and I−^ salts are soluble (NOT Ag +, Pb 2+, Hg 2 2+)
- Most sulfate salts are soluble (NOT BaSO 4 , PbSO 4 , HgSO 4 , CaSO 4 )
- Most OH−^ salts are only slightly soluble (NaOH, KOH are soluble, Ba(OH) 2 , Ca(OH) 2 are marginally soluble)
- Most S^2 −, CO 32 −, CrO 42 −, PO 43 −^ salts are only slightly soluble.
Ions in Aqueous Solution
- An ionic equation , however, represents
strong electrolytes as separate independent
ions. This is a more accurate representation
of the way electrolytes behave in solution.
Molecular and Ionic Equations
- − + − Ca (aq) 2 OH(aq) 2 Na(aq) CO (aq) 2 3
2
CaCO 3 (s) ↓+ 2 Na +^ (aq) + 2 OH^ − (aq)
- A complete ionic equation is a chemical equation in which strong electrolytes (such as soluble ionic compounds) are written as separate ions in solution.
Ions in Aqueous Solution
- Complete and net ionic equations
Molecular and Ionic Equations
- − + − Ca (aq) 2 NO (aq) 2 K(aq) CO (aq) 2 3 3
2
Ca (NO 3 ) 2 (aq) + K 2 CO 3 (aq) → (strong) (strong)^ ( insoluble ) (strong)
CaCO 3 (s) + 2 KNO 3 (aq )
CaCO 3 (s) + 2 K +^ (aq) + 2 NO 3^ − (aq )
Ions in Aqueous Solution
- Complete and net ionic equations.
Molecular and Ionic Equations
- A net ionic equation is a chemical equation from which the spectator ions have been removed.
- A spectator ion is an ion in an ionic equation that does not take part in the reaction. Ca 2 +^ (aq) + 2 NO 3 − (aq) + 2 K + (aq) + CO 32 − (aq) → CaCO 3 (s) 2 K(aq) 2 NO 3 (aq)
Ions in Aqueous Solution
- Complete and net ionic equations
- Separating the strong electrolytes into separate ions, we obtain the complete ionic equation.
Molecular and Ionic Equations
- − 2 H (aq) 2 NO 3 (aq) Mg(OH) 2 (s)
2 H 2 O(l) Mg^2 (aq) 2 NO 3 (aq)
- Note that the nitrate ions did not participate in the reaction. These are spectator ions.
Ions in Aqueous Solution
- Complete and net ionic equations
- Eliminating the spectator ions results in the net ionic equation.
Molecular and Ionic Equations
- − 2 H (aq) 2 NO 3 (aq) Mg(OH) 2 (s)
2 H 2 O(l) Mg^2 (aq) 2 NO 3 (aq)
2 H +^ (aq) + Mg(OH) 2 (s) → 2 H 2 O(l) + Mg^2^ + (aq ) This equation represents the “essential” reaction.
Figure 4.16: Precipitation of silver chloride by mixing solutions of silver nitrate and potassium chloride. The K+ and NO 3 - ions remain in solution.
Figure 4.17: The reaction of
KCl( aq ) with AgNO 3 to form
AgCl( s ).
Describing Reactions in Solution
1. Molecular equation (reactants and
products as compounds)
AgNO 3 ( aq ) + KCl( aq ) → AgCl( s ) + KNO 3 ( aq )
2. Complete ionic equation (all strong
electrolytes shown as ions)
Ag +( aq ) + NO 3 −( aq ) + K +( aq ) + Cl−( aq ) → AgCl( s ) + K +( aq ) + NO 3 −( aq )
Types of Chemical Reactions Acid-Base Reactions
the H 2 O molecule is the acid because it donates a proton. The NH 3 molecule is a base, because it accepts a proton.
NH 3 (aq) H 2 O(l) → NH 4 (aq) OH^ − (aq)
← +
H+
In the reaction of ammonia with water,
- The Brønsted-Lowry Concept
Types of Chemical Reactions Acid-Base Reactions
This “mode of transportation” for the H+^ ion is called the hydronium ion.
H (aq) H 2 O(l) H 3 O (aq )
The H+(aq) ion associates itself with water to form H 3 O +(aq).
- The Brønsted-Lowry Concept
Types of Chemical Reactions Acid-Base Reactions
HNO 3 (aq) H 2 O(l) NO 3 (aq) H 3 O^ + (aq )
−
The dissolution of nitric acid, HNO 3 , in water is therefore a proton-transfer reaction
- The Brønsted-Lowry Concept
where HNO 3 is an acid (proton donor) and H 2 O is a base (proton acceptor).
H+
Types of Chemical Reactions Acid-Base Reactions
- The Arrhenius concept acid: proton (H+^ ) donor
base: hydroxide ion (OH -^ ) donor
- In summary, the Arrhenius concept and the
Brønsted-Lowry concept are essentially the
same in aqueous solution.
Types of Chemical Reactions Acid-Base Reactions
- The Brønsted-Lowry concept acid: proton (H+^ ) donor
base: proton (H +^ ) acceptor
- In summary, the Arrhenius concept and the
Brønsted-Lowry concept are essentially the
same in aqueous solution.
Types of Chemical Reactions Acid-Base Reactions
- Strong and Weak Acids and Bases
- A strong acid is an acid that ionizes completely in water; it is a strong electrolyte.
HNO 3 (aq) H 2 O(l) NO 3 (aq) H 3 O (aq )
− (^) +
HCl( aq) H 2 O(l) Cl (aq) H 3 O(aq )
− +
- – Table 4.3 lists the common strong acids.
Types of Chemical Reactions Acid-Base Reactions
- Strong and Weak Acids and Bases
- A strong base is a base that is present entirely as ions, one of which is OH-^ ; it is a strong electrolyte.
- – The hydroxides of Group IA and IIA elements, except for beryllium hydroxide, are strong bases. (see Table 4.3)
NaOH( s) Na(aq) OH(aq )
H 2 O
Return to Slide 43
Types of Chemical Reactions Acid-Base Reactions
- Strong and Weak Acids and Bases
- You will find it important to be able to identify an acid or base as strong or weak.
- When you write an ionic equation, strong acids and bases are represented as separate ions.
- Weak acids and bases are represented as undissociated “molecules” in ionic equations.
Types of Chemical Reactions Acid-Base Reactions
- Strong and Weak Acids and Bases
- A weak acid is an acid that only partially ionizes in water; it is a weak electrolyte.
- The hydrogen cyanide molecule, HCN, reacts with water to produce a small percentage of ions in solution.
HCN( aq) H 2 O(l) → CN(aq) H 3 O(aq) − +
←
Types of Chemical Reactions Acid-Base Reactions
- Strong and Weak Acids and Bases
- A weak base is a base that is only partially ionized in water; it is a weak electrolyte.
- Ammonia, NH 3 , is an example.
NH 3 (aq) H 2 O(l) → NH 4 (aq) OH(aq)
← + −
Types of Chemical Reactions Acid-Base Reactions
- Neutralization Reactions
- One of the chemical properties of acids and bases is that they neutralize one another.
- A neutralization reaction is a reaction of an acid and a base that results in an ionic compound and water.
- The ionic compound that is the product of a neutralization reaction is called a salt.
HCN( aq) + KOH(aq) → KCN(aq) + H 2 O(l )
acid base salt
Performing Calculations for
Acid-Base Reactions
1. List initial species and predict reaction.
2. Write balanced net ionic reaction.
3. Calculate moles of reactants.
4. Determine limiting reactant.
5. Calculate moles of required reactant/product.
6. Convert to grams or volume, as required.
Types of Chemical Reactions Acid-Base Reactions
- Neutralization Reactions
- The net ionic equation for each acid-base neutralization reaction involves a transfer of a proton.
- Consider the reaction of the strong acid , HCl( aq ) and a strong base, LiOH( aq ).
HCl( aq) + KOH(aq) → KCl(aq) + H 2 O(l )
Types of Chemical Reactions Acid-Base Reactions
- Neutralization Reactions
- Writing the strong electrolytes in the form of ions gives the complete ionic equation.
H (aq) Cl (aq) K (aq) OH(aq)
K (aq) + Cl(aq) + H 2 O(l)
Types of Chemical Reactions Acid-Base Reactions
- Neutralization Reactions
- Canceling the spectator ions results in the net ionic equation. Note the proton transfer.
H (aq) Cl (aq) K (aq) OH (aq)
K + (aq) + Cl − (aq) + H 2 O(l)
H+
H (aq) + OH(aq) → H 2 O(l)
Types of Chemical Reactions Acid-Base Reactions
- Neutralization Reactions
- In a reaction involving HCN( aq ), a weak acid, and KOH( aq ), a strong base, the product is KCN, a strong electrolyte.
- The net ionic equation for this reaction is
HCN( aq) + OH(aq) → CN(aq) + H 2 O(l )
− −
H+^
Note the proton transfer.
Types of Chemical Reactions Acid-Base Reactions
- Acid-Base Reactions with Gas Formation
- Sulfides react with acids to form H 2 S, hydrogen sulfide gas.
Na (^) 2 S + 2 HCl → 2 NaCl + H 2 S ↑
- These reactions are summarized in Table 4.4.
Types of Chemical Reactions Acid-Base Reactions
- Acid-Base Reactions with Gas Formation
- Carbonates react with acids to form CO 2 , carbon dioxide gas.
Na 2 CO 3 + 2 HCl → 2 NaCl + H 2 O + CO 2 ↑
- Sulfites react with acids to form SO 2 , sulfur dioxide gas. Na 2 SO 3 + 2 HCl → 2 NaCl + H 2 O + SO 2 ↑
Acid/Base Reactions
Return to Slide 52
Types of Chemical Reactions
- Oxidation-Reduction Reactions
- Oxidation-reduction reactions involve the transfer of electrons from one species to another.
- Oxidation is defined as the loss of electrons.
- Reduction is defined as the gain of electrons.
- Oxidation and reduction always occur simultaneously.
Figure 4.19: The reaction of solid sodium and gaseous chlorine to form solid sodium chloride.
Figure 4.13: Combination reaction.
Return to Slide 63
Figure 4.20: A
summary of an
oxidation-
reduction
process, in
which M is
oxidized and X
is reduced.
Types of Chemical Reactions
- Describing Oxidation-Reduction Reactions
Oxidation-Reduction Reactions
- Look again at the reaction of iron with copper(II) sulfate.
Fe( s) Cu (aq) Fe (aq) Cu(s ) 2 2
- We can write this reaction in terms of two half- reactions.
Types of Chemical Reactions
- Describing Oxidation-Reduction Reactions
Oxidation-Reduction Reactions
- A half-reaction is one of the two parts of an oxidation-reduction reaction. One involves the loss of electrons (oxidation) and the other involves the gain of electrons (reduction).
2
Cu (aq) 2 e Cu(s) 2
oxidation half-reaction
reduction half-reaction
Types of Chemical Reactions
- Describing Oxidation-Reduction Reactions
Oxidation-Reduction Reactions
- An oxidizing agent is a species that oxidizes another species; it is itself reduced.
- A reducing agent is a species that reduces another species; it is itself oxidized.
Fe( s) Cu (aq) Fe (aq) Cu(s ) 2 2
oxidizing agent
reducing agent
Loss of 2 e -^ oxidation
Gain of 2 e-^ reduction
Types of Chemical Reactions
- Balancing Simple Oxidation-Reduction Reactions
Oxidation-Reduction Reactions
- At first glance, the equation representing the reaction of zinc metal with silver(I) ions might appear to be balanced.
Zn( s) Ag (aq) Zn (aq) Ag(s ) 2
- However, a balanced equation must have a charge balance as well as a mass balance.
Types of Chemical Reactions
- Balancing Simple Oxidation-Reduction Reactions
Oxidation-Reduction Reactions
- Since the number of electrons lost in the oxidation half-reaction must equal the number gained in the reduction half-reaction,
we must double the reaction involving the reduction of the silver.
Zn( s) → Zn^2 +^ (aq) + 2 e^ − Ag (aq) + e → Ag(s)
oxidation half-reaction
2 2 2 reduction half-reaction
Types of Chemical Reactions
- Balancing Simple Oxidation-Reduction Reactions
Oxidation-Reduction Reactions
- Adding the two half-reactions together, the electrons cancel, + −
Zn( s) → Zn (aq) + 2 e
2
2 Ag + (aq) + 2 e −→ 2 Ag(s)
oxidation half-reaction
_____________________________________ reduction half-reaction Zn (s) + 2 Ag 2+^ (aq) º Zn2+^ (aq) + 2 Ag (s)
Figure 4.12: The burning of calcium metal in chlorine. Photo courtesy of James Scherer.
Return to Slide 62
Ca (s) + Cl 2 (g) º CaCl 2 (s) Ca º Ca 2+^ + 2 e -
Cl 2 + 2 e -^ º 2 Cl-
Ca (s) + Cl 2 (g) º CaCl 2 (s)
Rules for Assigning Oxidation States
1. Oxidation state of an atom in an element = 0
2. Oxidation state of monatomic element = charge
3. Oxygen = −2 in covalent compounds (except in
peroxides where it = −1)
4. H = +1 in covalent compounds
5. Fluorine = −1 in compounds
6. Sum of oxidation states = 0 in compounds
Sum of oxidation states = charge of the ion
Balancing by Half-Reaction
Method
1. Write separate reduction, oxidation
reactions.
2. For each half-reaction:
Balance elements (except H, O)
Balance O using H 2 O
Balance H using H+
Balance charge using electrons
Half-Reaction Method -
Balancing in Base
1. Balance as in acid.
2. Add OH−^ that equals H+^ ions (both
sides!)
3. Form water by combining H+^ , OH−.
4. Check elements and charges for balance.
Balancing by Half-Reaction
Method (continued)
3. If necessary, multiply by integer to
equalize electron count.
4. Add half-reactions.
5. Check that elements and charges are
balanced.
Stoichiometry Steps for reactions
in solution.
