Module 6 need to know

Key terms

• Solution – homogeneous mixture

• Solvent – substance present in largest amount

• Solutes – other substances in the solution

When ionic compounds (solutes) dissolve in water:

1. The strong ionic forces that hold the ionic compound crystal together are overcome by the strong attractions between the ions and the polar water molecules

2. Each polar water molecule orient itself in a way to maximize its attraction with an ion

3. The strong ionic forces holding the positive and negative ions in the solid are replaced by strong water–ion interactions, and the solid dissolves (the ions disperse)

Looking at what happens to non-ionic compounds (solutes) when they dissolve in water:

• Contains polar O-H groups that can form hydrogen bonding with water

• The hydrogen bonding interaction with water is very strong and solubilizes the compound

Substances insoluble in water

• Nonpolar oil does not interact with polar water

• Water-water hydrogen bonds keep the water from mixing with the nonpolar molecules

How Substances Dissolve

• “like dissolves like”

• Water (polar) dissolves polar substances

• Nonpolar solvents dissolve nonpolar solutes

The solubility of a solute is limited

• Saturated solution – contains as much solute as will dissolve at that temperature

• Unsaturated solution – has not reached the limit of solute that will dissolve. When more solute is added to an unsaturated solution, it dissolves

variation in substances (solutes) amounts present in different solutions

• A relatively large amount of solute is dissolved in a concentrated solution

• A relatively small amount of solute is dissolved in a dilute solution

Mass percent

Describing the composition of a solution means specifying the amount of solute present in a given quantity of the solution

Concentration

- defined as the amount of solute in a given volume of solution

• expressed as molarity (M)

Molarity

Describes the amount of solute in moles and the volume of the solution in litres. (Molarity is the number of moles of solute per volume of solution in litres.)

To find the solution that contains the greatest number of moles of ions/solutes

1. Think about relative numbers of ions/solutes

2. How many moles of solute or each ion are in each solution?

The solution with the greatest number of ions is not necessarily the 1 in which:

• the volume of the solution is the largest

• the formula unit has the greatest number of ions

Dilution

- The process of adding water to a concentrated or stock solution to achieve the molarity desired for a particular solution

A dilute solution contains the same amount of solute in the concentrated solution it is made from. A dilute solution contains more water than the concentrated solution

Dilution calculation

A typical dilution calculation involves determining how much water must be added to an amount of stock solution to achieve a solution of the desired concentration

Steps for Solving Stoichiometric Problems Involving Solutions

1. Write the balanced equation for the reaction. For reactions involving ions, it is best to write the net ionic equation

2. Calculate the moles of reactants

3. Determine which reactant is limiting

4. Calculate the moles of other reactants or products, as required

5. Convert to grams or other units, if required

Neutralization reaction

-  An acid-base reaction

A strong acid, such as hydrochloric acid, HCl, dissociates (ionizes) completely in water

HCl(aq) → H⁺ (aq) + Cl⁻(aq)

Strong bases are water-soluble metal hydroxides, which are completely dissociated in water

NaOH(aq) → Na⁺(aq) + OH⁻(aq)

When a strong acid and a strong base react, the net ionic equation is

𝐇⁺ (𝐚𝐪) + 𝐎𝐇⁻ (𝐚𝐪) → 𝐇_𝟐𝐎 (𝐥)

Characterization of acids and bases

Acids are characterized by their sour taste. E.g. acetic acid in vinegar, and citric acid in lemons

Bases are characterized by their bitter taste and slippery feel. E.g. hand soaps

Arrhenius:

•      Acids produce H+ ions in solution, bases produce OH^– ions

 

 

This concept is limited because it allows for only 1 kind of base—the hydroxide ion

Brønsted–Lowry

•      Acids are proton (H⁺) donors, bases are proton acceptors

– an acid (HA) donates a proton to a water molecule to form a new acid (the conjugate acid) and a new base (the conjugate base)

A conjugate acid-base pair consists of 2 substances related to each other by the donating and accepting of a single proton (H⁺)

A Brønsted–Lowry acid is a proton donor while a Brønsted–Lowry base is a proton acceptor

 

 

 

 

 

The difference between a conjugate base and its acid, and a conjugate acid and its base is a single proton (H⁺)

Strong acid

completely ionized or completely dissociated in an aqueous solution

HA(aq) + H₂O(l) → H₃O⁺ (aq) + A⁻ (aq)

•      is one in which the forward reaction predominates

•      HA (aq) completely ionizes (dissociates) into its constituent ions (H⁺ or H₃O⁺ plus an anion).

•      Examples: HCl, HNO₃ and H₂SO₄

•      A strong acid contains a relatively weak conjugate base, A⁻

•      Excellent conductor of electricity

Weak acid

dissociates (ionizes) only to a very small extent in an aqueous solution

HA(aq) + H₂O(l) → H₃O⁺(aq) + A⁻ (aq)

The reverse reaction predominates:

H3O⁺ (aq) + A⁻ (aq) → HA(aq) + H₂O(l)

•      When placed in water, almost all the acid molecules remain intact (undissociated)

·       Example: acetic acid (CH₃COOH)

•      A weak acid contains a relatively strong conjugate base, A⁻

·       Conducts electric current weakly

The relationship of acid strength and conjugate base strength for the dissociation reaction:

Ways to decribe acid strength

Water as an acid and base

Water is an amphoteric substance which means it can behave either as an acid or a base

(water behaving as an acid)

 

Ionization equilibrium of water shows that the forward reaction does not occur to a great extent:

 

 

Therefore, there are only small amounts of ions at 25 °C:

The ion-product constant, K_w at 25 °C can be obtained by multiplying the concentrations of H₃O⁺ and OH^–

 

 

Or,

 

 

No matter what the solution contains, the product of [H⁺] and [OH^–] must always equal 1.0 × 10^–14 at 25°C

 

Since K_w is a constant, when [H⁺] increases, [OH^–] decreases, and vice versa

 

 

3 Possible Situations:

•      In a neutral solution, [H⁺] = [OH^–]

•      In a acidic solution, [H⁺] > [OH^–]

•      In a basic solution, [H⁺] < [OH^–]

In each case, however,

K_w = [H⁺][OH^–] = 1.0 × 10^–14

pH scale

Because the [H⁺] in an aqueous solution is typically quite small, the pH scale provides a convenient way to represent solution acidity

 pH = -log[H+]

 

 

To represent pH of the appropriate number of significant numbers, you need to know the following rule for logarithms: the number of decimal places for a log must be equal to the number of significant figures in the original number

 

pH decreases as [H⁺] increases

A convenient relationship between pH and pOH can be derived from K_w

pH + pOH = 14.00

Calculating the pH of strong acid solutions

A strong acid is completely ionized or completely dissociated in an aqueous solution

 

 

Even though the label on the bottle says 1.0 M HCl, the solution contains virtually no HCl molecules!

A 1.0 M HCl solution contains 1.0 M H⁺ and 1.0 M Cl⁻ ions rather than HCl molecules

We can do stoichiometric calculations for strong acids

A buffer solution is one which resists pH change when either an acidic or a basic solution is added to it

A buffer solution is made up of a weak acid (e.g acetic acid, CH3COOH) and the conjugate base (e.g acetate, CH3COO–) of the weak acid

How does a buffered solution work?

Any added strong acid, H+ reacts with the base A–.

H⁺(aq) + A^–(aq) → HA(aq)

 

 

 

 

 

Any added strong base, OH– reacts with the weak acid HA.

OH^–(aq) + HA(aq) → H₂O(l) + A^–(aq)

The Characteristics of a Buffer

1. The solution contains a weak acid HA and its conjugate base A^–

2. The buffer resists changes in pH by reacting with any added H+ or OH^– so that these ions do not accumulate

3. Any added H⁺ reacts with the base A^– H⁺(aq) + A^–(aq) → HA(aq)

4. Any added OH– reacts with the weak acid HA OH^–(aq) + HA(aq) → H₂O(l) + A^–(aq)