SCH 4U - Acids & Bases

Dissociation of Water

H₂O(l)  ↔  H⁺(aq)  +  OH^-(aq)     very small K

 

This process is called the autoionization of water (1:10⁸ ionized: unionized)

K_W = [H⁺][OH^-] = 1.00 x 10^-14 at 25°C

 

The equilibrium constant, K_W, is called the ion product constant.  This equation is valid for solutions as well as, pure water.

• If [H⁺] > [OH^-], solution is acidic
• if [H⁺] < [OH^-], solution is basic
• if [H⁺] = [OH^-], solution is neutral

The Proton in Water

Water can dissociate to form the hydrogen ion (which is equivalent to a single proton*).  But, when H⁺ is in solution, its positive charge is attracted to the negative end of the polar water molecule to form a hydrated hydrogen ion – the hydronium ion (H₃O⁺).  

 

 

Since a hydronium ion is simply a hydrated hydrogen ion, we can use the hydronium ion and the hydrogen ion interchangeably in reaction equations.

* A hydrogen atom, H, has 1 proton, 1 electron and 0 neutrons.  Thus, the hydrogen ion, H⁺, has 1 proton, 0 electrons and 0 neutrons.

Arrhenius Acids and Bases

One the first acid-base definitions (Svante Arrhenius 1859-1927)

“Acids are substances that, when dissolved in water, increase the hydrogen ion concentration.  Likewise, bases are substances, that when dissolved in water, increase the hydroxide ion concentration.”

Acid Properties

• electrolyte (a solute that produces ions in solution – conducts electricity)
• reacts with most metals to form hydrogen gas
• indicators: litmus turns red; bromthymol blue turns yellow; phenolphthalein stays colourless
• tastes sour

Base Properties

• electrolyte (a solute that produces ions in solution – conducts electricity)
• indicators: litmus turns blue; bromthymol blue stays blue; phenolphthalein turns pink
• tastes bitter and feels soapy

Brønsted-Lowry Acids and Bases

"An acid is a proton donor and a base is a proton acceptor."

Compared to the Arrhenius definition, the Brønsted-Lowry definition allows for a broader range of substances which can be acidic or basic. 

HCl(g)  +  H₂O(l)  ↔  H₃O⁺(aq)  +  Cl^-(aq)

   A              B

HNO₃(aq)  +  NH₃(g)  ↔  NH₄⁺(aq)  +  NO₃^-(aq)

   A                   B

Conjugate Acid-Base Pairs

In any acid-base equilibrium, both the forward and reverse reactions involve proton transfer.

HNO₃(aq)  +  H₂O(l)  ↔  NO₃^-(aq)  +  H₃O⁺(aq)

     A                  B               CB                CA

      ↑_________________↑

                          ↑_________________↑

NH₃(aq)   +   H₂O(l)   ↔   NH₄⁺(aq)  +  OH^-(aq)

     B                  A                CA                 CB

      ↑_________________↑

                        ↑_________________↑

 

Notice in these two examples that the acid (A) and its conjugate base (CB) differ by a single proton.  The same can be said for the base (B) and its conjugate acid (CA).

TryIt!:

Finish the reaction below.  Label the acid, base, conjugate acid and conjugate base.  Link the related substances across the reaction, like in the example above.

 

HClO₄(aq)  +  NH₃(aq)  ↔  ________  +  ________

Conjugate Acid-Base Strengths

The stronger an acid, the weaker its conjugate base; the weaker the acid, the stronger its conjugate base.  This will be important to remember in the coming lessons.

Strong Electrolytes

A strong electrolyte is a substance which dissolves to produce exclusively ions.  

• Some common strong acids: hydrochloric acid, hydrobromic acid, nitric acid, perchloric acid.
• Some common strong bases: all group 1 & 2 metal hydroxides, like sodium hydroxide, barium hydroxide.
  

HCl(aq)  +  H₂O(l)  →  H₃O⁺(aq)  +  Cl^-(aq) 

The above equation can be written in a simplified form as well:

HCl(aq)  →  H⁺(aq)  +  Cl^-(aq)

Regardless of whether we write the simplified version of the equation or not, notice that the arrow we use only points in one direction.  This is because the acid dissociates 100% in solution.  Since the acid dissociates completely, for every 1 acid molecule that dissociates, 1 hydrogen (or hydronium) ion ends up in solution.

Weak Electrolytes

A weak electrolyte is a substance that dissolves to produce ions only to a limited extent.

• Some common weak acids: acetic acid, lactic acid, benzoic acid, hydrocyanic acid. 
• Some common weak bases: all non-group 1 & 2 metal hydroxides, like ammonia.

CH₃COOH(aq)  +  H₂O(l)  ↔  H₃O⁺(aq)  +  CH₃COO^-(aq)

The above equation can be written in a simplified form as well:

CH₃COOH(aq)  ↔  H⁺(aq)  +  CH₃COO^-(aq)

Regardless of whether we write the simplified version of the equation or not, notice that the arrow we use points in two directions.  This is because the acid dissociates less than 100% in solution (in the case of the acetic acid, the amount of dissociation is about 1.6%). 

 

TryIt! Answer:

 

Homework #1, 4, 5 
Answer Keys can be found here.

SCH 4U - Acids & Bases Answer Keys

Question #1-5

Question #6-9

Question #10-14

Question #15, 16

Question #17-20

Question #21-23

Question #24-26

Question #27-30

Question #31-37

SCH 4C - Alkenes & Alkynes

Alkenes (unsaturated hydrocarbons, C_nH_2n)

Alkenes are similar to the alkanes, but they contain at least one C-C double bond.

When naming an alkene, the prefix stays the same, but the ending changes to ‘-ene.’

Alkenes can be found as different isomers, depending on where the double bond is situated.  This means we typically need to indicate the location of the double bond within the chain.  To do this, we begin numbering from the end which provides the lowest number for the double bond.

Physical Properties of Alkenes

Like the alkanes, alkenes molecules are essentially non-polar.  Therefore, the physical properties of the alkenes are very similar to those of the alkanes.

Lighter alkenes are gases at RT and as chain length increases, they become liquids and solids.

Alkenes are insoluble in water, but dissolve in other liquid hydrocarbons.

They have relatively low melting points and boiling points.

 

Chemical Properties of the Alkenes

Combustion Reactions

Like the alkanes, the alkenes undergo combustion in which they burn in oxygen to produce carbon dioxide and water.

 

Addition Reactions

The double bond gives the alkenes a greater reactivity than the alkanes.  An addition reaction can occur in which a molecule adds on to the alkene carbons, leaving a single bond in its wake.

Check out these videos for an explanation of how addition reactions work:

Addition of Hydrogen, Addition of Halogen, Addition of Hydrogen Chloride, Addition of Water

   

Alkynes (unsaturated hydrocarbons, C_nH_2n+2)

The alkynes contain at least one triple bond.

Again the prefix stays the same, but the ending changes to ‘-yne.’ 

Alkynes can be found as different isomers, depending on where the triple bond is situated.  Like with the alkenes, we begin numbering the main chain from the end which provides the lowest number for the triple bond.

3-heptyne

 

Physical Properties of the Alkynes

The physical properties of the alkanes are similar to those of the alkanes and alkenes.

Alkynes are gases, liquids or low melting solids at room temp.

They do not dissolve in water.

They have relatively low melting points and boiling points.

 

Chemical Properties of the Alkynes

The chemical properties of the alkynes are similar to those of the alkenes, due to multiple bond.

They undergo combustion reactions and addition reactions with hydrogen, the halogens and hydrogen halides.

 

Homework:  

Practice, p. 186 # 3, 4

Practice, p. 188 # 5cd

Practice, p. 190 # 6, 7

Section Questions, p. 190 # 3-5, 7, 10

Read "Functional Groups" p. 199-200 then do p. 201 # 1-6

 

Answers:

SCH 4C - Alkane Properties & Reactions

Physical Properties of the Alkanes

Since the bonds in an alkane are either C-C (ΔEneg = 0) or C-H (ΔEneg = 2.5 – 2.1 = 0.4), they are all essentially non-polar.  Thus, there are no δ⁺ and δ^- that would be attracted to each other, so alkanes have very weak forces of attraction between molecules. 

As the number of C atoms in a molecule increases, so do the attractive intermolecular forces between molecules so the bp increases with chain length (gas → liquid → solid).

Due to their non-polarity, they are water-insoluble.

 

Chemical Properties of the Alkanes

Combustion Reaction

• Alkanes are fairly unreactive compounds.
• They are used for the manufacture of other organic compounds and fuels.
• They do undergo a complete combustion reaction with oxygen, always producing carbon dioxide and water:

 C₃H₈(g)  +  5O₂(g)  →  3CO₂(g)  +  4H₂O(g)

• This reaction produces much of the energy used by society for heating and cooking.
• If a limited supply of oxygen is available, incomplete combustion occurs, which can be dangerous since carbon monoxide is one of the products.

 

Substitution Reactions

• Alkanes can react with the halogens during a substitution reaction.
• In the presence of heat or UV radiation, alkanes react with chlorine and bromine.
• One or more of the alkane’s hydrogens are replaced with a halogen atom.
• The reaction can stop here to produce CH₃Cl, or continue to produce CH₂Cl₂, CHCl₃ or CCl₄.

 See this video for a description of how this reaction proceeds.
 

Homework: 

Practice, p. 188 # 5ab

Show the substitution reaction between (a) methane and bromine, (b) ethane and chlorine.

Read "Fractional Distillation" p. 193-195 then do p. 196 # 1-6, 10.

 

Answers: