ACIDS & BASES GR11&12 by Lizelle Swanepoel

ACIDS BASES

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Make sure to stick to your syllabus for definitions and content

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INTRO: Acids and bases

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Acids and bases are two types of corrosive substances.

Properties:
Acids:

taste sour
are corrosive to metals
change litmus (a dye extracted from lichens) red
become less acidic when mixed with bases
Examples of strong acids - HI, HClO₄, HCl, HBr, H₂SO_4,HNO₃("Imagine a Perfectly Clear Breezy Summer Night")
Examples of weak acids - HF – hydrofluoric acid, H₃PO₄ – phosphoric acid, H₂SO₃ – sulfurous acid, (COOH)₂ – oxalic acid, CH₃COOH – ethanoic acid, carbonic acid (H₂CO₃)

Bases:

feel slippery (soapy)
are corrosive
change litmus blue
become less basic when mixed with acids.
Examples of strong bases - NaOH, KOH, LiOH (only the Group 1 metal hydroxides are strong bases).
Examples of weak bases - NH₃, NH₄OH, Al(OH)₃, Ca(OH)₂, Mg(OH)₂
The base NaOH reacts with fats (or oils) to form soap, through a process called saponification.

SAPONIFICATION - THE PROCESS OF SOAP MAKING

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Lowry-Brønsted Model (1923)

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Definitions:
Acid = A proton donor
Base = A proton acceptor

In acid-base reaction the proton is transferred from acid to base.

HCl + H₂O —> H₃O⁺+ Cl^-
HCl donates H⁺and acts as the acid
H₂O accepts H⁺ and acts as base

Watch this short video:

Conjugate acid-base pairs

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Identify conjugate acid-base pairs for given compounds or reactions
A strong acid has a very weak conjugate base.
A strong base has a very weak conjugate acid.

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Ampholytes = Amphiprotic substances

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Amphoteric (or amphiprotic) substance = A substance that can act as either an acid or a base.

Identify a substance as amphoteric (or amphiprotic), e.g. H₂O, HCO₃^-, HSO₄^-
Write equations to show how an amphoteric (or amphiprotic) substance can act as an acid or a base

Ampholyte

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Example 2

Strong acid vs. Weak acid

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Strong acids ionize completely in aqueous solution.
Weak acids ionize only partially in aqueous solution.
pH_{strong acid}< pH_{weak acid}(@ equal concentrations)

Concentrated vs. Dilute acid or base

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State what is meant by a concentrated acid and a concentrated base.
State what is meant by a dilute acid and a dilute base.

POLYPROTIC ACIDS

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A polyprotic acid is an acid which is capable of donating more than one proton (H⁺).
Common polyprotic acids are:

Phosphoric acid - H₃PO₄
Sulphuric acid - H₂SO₄
Oxalic acid - H₂C₂O₄
Carbonic acid - H₂CO₃

In all polyprotic acids, the loss of each successive proton becomes more difficult.
Sulphuric acid is able to donate two protons (diprotic acid) as shown in the following two equations:

Watch this video:

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The pH scale

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The pH scale is a measure of the hydronium ion (H₃O⁺) concentration in water, at 25 °C
Explain qualitatively the pH range of 0 to 14

pH of different substances

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What exactly is pH?

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pH = - log [H⁺]
The term "pH" was first described by Danish biochemist Søren Sørensen in 1909.
pH is an abbreviation for "potency of hydrogen"
"p" is short for the German word for potency or potential, potenz
H is the element symbol for hydrogen
pH = -log₁₀[H⁺]
[H⁺] = 10^-pH
The higher the concentration of hydronium ions in solution, the lower the pH-value.
Example 1:
Calculate the pH for a specific [H⁺]. Calculate pH given [H⁺]=1.4 x 10^-5 M
pH = -log₁₀[H⁺]
pH = -log₁₀(1.4 x 10^-5)
pH = 4.85

Example 2:
Calculate [H⁺] from a known pH. Find [H⁺] if pH = 8.5

[H⁺] = 10^-pH
[H⁺] = 10^-8.5
[H⁺] = 3.2 x 10^-9 M

pH _{strong acid} < pH _{weak acid}(at equal concentrations)
Because the strong acid donates more H⁺ ions to the solution.
Comment on the validity of this statement:

"Strong acids have lower pH than weak acids." ???
A. Always True
B. Always False
C. Sometimes True

ANSWER: C
REASON:

If the concentration of a weak acid is very high, it produces more H⁺ ions than the strong acid (even though it dissociates partially)
This leads to a lower pH for weak acid.

Importance of pH in our oceans

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Formation of carbonic acid

Importance of pH in our muscles

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Optimal functioning of myosin at pH 6

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An alternative test for acid strength is CONDUCTIVITY

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Factors affecting electrical conductivity of a acid or alkali

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How well a substance forms ions in solution, determines its conductivity.

Strong acids and alkalis ionize (almost) completely, therefore they have a large concentration of free ions in solution which makes them highly conductive.
Weak acids and alkalis ionize only partially, therefore have a low concentration of free ions in solution which gives them poor conductivity.

Explain the given data

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Data explained

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At the same concentrations, the stronger acid (HCl) will ionize more readily than the weaker acid (ethanoic acid).
A strong acid ionizes completely which produces a larger number of free ions in solution.
Consequently, the [H₃O⁺] is high which results in lower pH compared to weak acids, by the formula
pH= - log[H₃O⁺]

Acids ionize

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Ionisation = The reaction of a molecular substance with water to produce ions.

Acids ionize
Weak bases ionize
Because they form completely NEW IONS.
Write reaction equations for the ionisation of any known or given acids dissolving in water, using single arrows for strong acids and double arrows for weak acids, for example:
HCl(aq)+H₂O(l) -> H₃O⁺(aq)+ Cl^-(aq)
H₂SO₄(aq) + 2H₂O(l) -> 2H₃O⁺(aq) + SO₄^2-(aq)
CH₃COOH(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CH₃COO^-(aq)
H₃PO₄(aq) + 3H₂O(l) ⇌ 3H₃O⁺(aq) + PO₄^3-(aq)

Bases dissociate

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Strong bases dissociate, while weak bases ionize.

Dissociation = The splitting of an ionic compound into its ions.

A strong base = A base that dissociates completely in an aqueous solution, e.g. NaOH.
A weak base = A base that ionizes only partially, e.g. NH_3.
Write the reaction equations (using single arrows) for the dissociation of any known or given metal hydroxides dissolving in water.

The K-value

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ANSWERS - Exercise 1-5

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Watch this video

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LINK BETWEEN pH, pOH, concentration hydronium and concentration hydroxide ions

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Indicators

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They change colour to indicate whether a substance is acidic or basic.

Indicators are weak acids, which undergo ionisation in a REVERSIBLE REACTION, as follows:
HIn <—> H⁺+ In^-Color1 Color2

This is how the indicator changes colour in acid or base as explained by LE CHATELIER'S PRINCIPLE:

1) When an acid is added to an indicator, the equilibrium system experiences a stress in the form of an increase in [H⁺].

The system will respond to decrease the added H⁺, by consuming it.
The equilibrium point changes to favor the reverse reaction (equilibrium shifts to the left).
Concentration of the product decrease and conc. of reactant increases.
Colour changes from color2 to color1.

2) When a base is added to the indicator, the base reacts with and removes H⁺.

The system responds to replace the removed H⁺ions, hence the equilibrium position changes to favor the forward reaction which produces H⁺ ions.
The equilibrium shifts to the left and product increase and reactant concentration decreases.
Indicator changes color from color1 to color2.

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Household indicators

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Neutralization

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Neutralization = The reaction of an acid with a base to produce a salt and water.

Acids release H⁺ into solution.
Bases release OH^-into solution.
If we were to mix an acid and base together, the H⁺ ion would combine with the OH^- ion to produce the molecule H₂O.

Examples:

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Reactions of acids

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Learn this off by heart ❤️

Titration

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Acid-base titration = A volumetric method of adding a base to an acid in a very controlled way to neutralize it by forming a salt and water.
A salt = A substance in which the hydrogen of an acid has been replaced by a cation, e.g. NaCl
Use stoichiometry to determine the concentration of an unknown acid or base solution
CV_acid = CV_base(depending on the acid-to-base mole ratio)

Titration terms and definitions

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Standard solution = A solution of known concentration
Analyte (titrand) = the substance of unknown identity or concentration in the Erlen Meyer flask below the burette.
Titrant = the substance of known concentration that is added from the burette.
Equivalence point = The point where an acid and base have reacted so neither is in excess.
End point = The point where the indicator changes colour.
Indicator = A weak acid that changes colour to indicate the acidity or basicity of a compound.
For accurate titration, the end point needs to coincide with the equivalence point.

TITRATION

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HOW TO DO A TITRATION IN THE LAB:

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SALT HYDROLYSIS

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DEF:
Salt hydrolysis is a reaction of a salt with water, where water itself is decomposed.
Either an acidic or basic solution forms.

The reaction between an acid and base is called a NEUTRALIZATION reaction, even though the salt that forms is not necessarily neutral.
The produced salt can be ACIDIC, NEUTRAL or BASIC, which we have to determine.
The pH of the produced salt depends on the EXTENT TO WHICH THE SALT REACTS WITH WATER to produce H₃O⁺ and OH^- ions.

EXAMPLE:
1)
Na₂CO₃, dissolved in water, is BASIC.
EXPLANATION:

The salt dissociates in water by the reaction:
Na₂CO₃->2Na⁺(aq) + CO₃^2-(aq)
Then the ion of the weak acid or base reacts with water.
Hence, CO₃^2-+ H₂O <-> HCO₃^-+ OH^-
OH^-ions are in excess in solution.
[OH^-] > [H₃O⁺]
Therefore Na₂CO₃is a BASIC salt.

2)
NH₄Cl is an acidic salt when dissolved in water.
THE EXPLANATION:

NH₄Cl dissociates in water as follows:
NH₄Cl -> NH₄⁺(aq) + Cl^- (aq)
The ion from the weak parent base is hydrolysed as follows:
NH₄⁺+ H₂O <-> NH₃+ H₃O⁺
H₃O⁺ions are in excess in solution.
[H₃O⁺] > [OH^-]
Therefore NH₄Cl is an ACIDIC salt.

Hydrolysis

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You should be able to do the following:

Determine which cation or anion from a given salt will undergo hydrolysis.
Write an equation for the hydrolysis reaction.
Predict the approximate pH range of the salt solution.

Indicators

lizellexs — 2020-12-24 18:30:11 UTC

Indicators are WEAK ACIDS which indicate the acidity/alkalinity of a substance by changing colour.

Examples:

Litmus
Universal indicator
Phenolphthalein

Further Answers

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Exercise 6

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ANSWERS - EXERCISE 7-11

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MEMO TO EXAM QUESTION

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FURTHER EXAM QUESTION

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MEMO

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TITRATION EXERCISE

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TITRATION EXERCISE

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GENERAL QUESTIONS

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NOTES 2022

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NOTES - BISHOPS

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ANSWERS TO NOTES

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Negative log relationship : pH = -log[H3O+]

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pH = -log[H₃O⁺]

Logarithm power rule

The logarithm of x raised to the power of y is y times the logarithm of x.

log_b(x ^y) = y ∙ log_b(x)
For example:
log₁₀(2⁸) = 8∙ log₁₀(2)
-log 10^-2= - (-2) log 10 = 2 (1) = 2

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AUTO-PROTOLYSIS

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