Topic 7 Acids and Bases

Topic 7

Asids and Bases

Learning Outcomes

By the end of this topic, you should be able to:

1. Define acid and base using the Arrhenius and the Bronsted-Lowry theories

2. Explain what an alkali is

3. Differentiate between acid strength and base strength

4. Discuss the autoionisation of water

5. Interpret the pH scale for acidic, neutral and alkaline solutions

Learning Outcomes

By the end of this topic, you should be able to:

6. Identify the properties of acids, bases and alkalis

7. Test a solution to see whether it is an acid or a base using indicators

8. Determine the relationship between the number of moles and molarity

9. Prepare a standard solution whose molarity is known

Asids and Bases

Citric acid is found in lemons and other citrus food.

The acid in vinegar is acetic acid.

The acid used in car batteries is sulphuric acid.

The lining of our stomachs produce gastric juice which is essential for digestion. The acid in gastric juice is hydrochloric acid.

Ammonia are used in household cleaning products.

Sodium hydroxide is used to make soap.

7.1 ACID, BASE AND ALKALI

Substances that ionise in water to produce hydrogen ions, H+, are acids.

Substances that ionise in water to yield hydroxide ions, OH-, are bases.

Arrhenius Theory

Limitations

It can only be used to explain acids and bases that are dissolved in water.

It does not explain why substances such as ammonia, NH3, which does not contain an OH group like most bases, show basic properties in water.

Arrhenius Theory

An acid is a substance that donates a proton (H+) to a base.

A base is a substance that accepts a proton (H+) from an acid.

Brønsted-Lowry Acid-Base

An acid is a proton donor. A base is a proton accepter.


In addition, according to Bronsted and Lowry, acid-base reactions are reversible reactions.

The acid has a conjugate base and the base has its conjugate acid.


The products of a Brønsted-Lowry acid-base reaction are also acids and bases.



Example 1

CH3COOH + H2O H3O+ + CH3COO-

H+ donor H+ acceptor H+ donor H+ acceptor

Acid Base Conjugate acid Conjugate base

Example 2


When a Brønsted-Lowry base such as ammonia is placed in water, it accepts a proton from the solvent (water). Hence, water acts as a Brønsted-Lowry acid.

NH3 + H2O OH- + NH4+

H+ acceptor H+ donor H+ acceptor H+ donor Base Acid Conjugate base Conjugate acid

NH4+ is the conjugate acid of the base NH3

OH- is the conjugate base of the acid H2O

Tip… Conjugate acid-base pairs are chemical spesies whose

formulae differ only by one proton, H+

Alkali

Alkali is originally from the Arabic word, “Al-Qaly”.

They are metal oxides or basic oxides of alkali metals that dissolve in water to produce the corresponding metal hydroxides.

Hydrated Protons and Hydronium Ions

Proton is fundamental to both the Arrhenius and Bronsted-Lowry definitions of an acid.

Although the symbol H+(aq) is convenient to use in equations, it does not really represent the structure of the ion present in an aqueous solution.

Hydrated Protons and Hydronium Ions

This is because as a bare proton with no electron nearby, H+, is much too reactive to exist by itself.

The H+ attaches to a water molecule, giving the more stable hydronium ion, H3O+.

SELF-CHECK 7.1

Account for the acidic properties of nitrous acid (HNO2) in terms of the Arrhenius theory and Bronsted-Lowry theory. Then, state the conjugate base of nitrous acid.

SELF-CHECK 7.1

Acid Strength and Base Strength

Dissociation

When an ionic compound dissolved in water or in aqueous solutions, it separates into its ions.


Ionisation

Unlike ionic compounds, covalent compounds are not made up of ions.

However, many such compounds, when dissolved in water, form ions in solution.

This process is called ionisation.

HCl


Different acids and bases ionise to different extents in aqueous solutions.

Those acids or bases that ionise completely in aqueous solution form respectively strong acids and strong bases.

Those acids and bases that ionise only to a small extent are weak acids and weak bases respectively.

Autoionisation of Water

Water ionises slightly in solution to produce equal numbers of hydrated hydrogen ions and hydroxide ions.


According to the Bronsted-Lowry theory, this autoionisation or self-ionisation of water is an acid-base reaction.

One H2O molecule gives a proton to another H2O molecule as follows:


Do you know that water is said to be amphiprotic?

It means that H2O molecules can donate protons (acting as an acid) as well as accept protons (acting as a base).

7.2 pH Scale

pH Scale

The pH scale is a scale of numbers from 0 to 14 to show how acidic or basic/alkaline a substance is.

pH Scale

7.3 CHARACTERISTIC OF ACIDS,

BASES AND ALKALIS

Properties of Acids

a) Acids have a sour taste.

b) Acids turn litmus paper from blue to red and cause bromothymol blue to change from blue to yellow.

c) Acids react with metals to form salt and hydrogen gas, H2.

Properties of Acids

d) Acids react with bases or alkali to form salt and water. The reaction is a neutralisation reaction.

e) Acids react with salts of weaker acids to form the weaker acid and the salts of the stronger acids.

Properties of Acids

f) Acids conduct electricity in aqueous solutions. Acids ionise into ions in solutions that are free to

move in an electric field, thus conducting electricity.

g) Carbonate salts react with acids, forming salt, carbon dioxide gas and water.

h) Concentrated acids are corrosive and can cause injury to the skin and corrode metals.

Properties of Bases and Alkalis

a) Bases have a bitter taste.

b) Bases feel soapy to touch.

c) Bases turn litmus paper from red to blue and bromthymol blue from yellow to blue.

d) Bases neutralise acids, forming salt and water.

e) Bases conduct electric current in aqueous solutions because bases are ionised to some extent.

f) Concentrated bases are corrosive and can burn the skin.

7.4TESTS FOR ACIDS AND ALKALIS

Test for Acids and Alkalis

We can do test for acids and alkalis by using litmus paper, or indicators such as universal indicator.

Acidicity and alkalinity are measured by acid-base indicators that can change colour in acid or base solutions.

Or, we can use the pH meter for a more accurate reading of pH value.


Universal indicator


Activity 7.3: Red cabbage indicator


pH meter

7.5CONCENTRATION OF ACID AND

BASE

Concentration of Acid and Base

The concentration of an acid or base is the quantity of solute in a given volume of solution which is usually 1 dm-3.

Thus, the concentration of a solution determines the amount of solute in a given volume of solution.

Thus, the amount of solute in a given volume of solution determines the concentration of a solution .

The greater the amount of dissolved solute, the higher is the concentration of the solution.

Relationship between the Number of Moles and Molarity

Unit for concentration: g dm-3 or mol dm-3.

The unit of concentration that is widely used is molarity (mol / dm3) or molar concentration (M).


Problem 1

The concentration of nitric acid, HNO3, is 126 g dm-3. Find its molarity.

(Relative atomic mass: H, 1; N, 14; O, 16)

Answer: 2.0 mol dm-3


Problem 2

25 cm3 of sodium hydroxide solution was poured into a beaker. The concentration of the alkali was 1.5 mol dm-3. Calculate the number of moles of sodium hydroxide in the beaker.

Answer: 0.0375 mol


SELF-CHECK 7.2

The concentration of cholesterol (C27H46O) in normal blood is approximately 0.005 M. How many grams of cholesterol are in 750 cm3 of blood?

Answer: 1.45 g

Preparation of a Standard Solution

Standard solutions are solutions of accurately known concentrations.


How would you prepare a standard solution of 1.0 dm3 of 0.50 mol dm-3 sodium chloride?


SELF-CHECK 7.3 A science investigation requires

1.50 L of 0.250 M glucose, C6H12O6.

Explain how you would prepare the above solution.

Answer:

Weigh 67.5 g of glucose. Put into 1.5 L volumetric flask. Add water until 1.5 L.

Solution Liquefaction

Dilution Method In the dilution method, the amount of dissolved

solute used is fixed but the amount of solvent used is increased, thus liquefying the solution.

DDCC VM VM C = concentrateD = diluteDilution Equation:


How do we prepare 200 cm3 of 0.50 M H2SO4 solution from a 2.00 M solution?

Answer: Calculate the volume of the 2.00 M solution required.

(50 cm3) By using a pipette, 50 cm3 of the 2.00 M solution is

transferred into a 200 cm3 volumetric flask. Distilled water is added until the total volume of the

solution is exactly 200 cm3. The solution is shaked well to ensure thorough mixing.


SELF-CHECK 7.4

You are given a 10.0 M solution. 10.0 cm3 of this solution is withdrawn and then diluted to 250 cm3. Determine the concentration of the final solution.

Answer: 0.4 M


ACTIVITY 7.4

You are given a 2.0 M concentrated acid. To dilute the acid, you pour some water into a beaker and then add the acid to the water. More water is then added to further dilute the acid. Discuss why in diluting the concentrated acid, you did not add water directly to the acid.

Answer:

The reaction between water and acid is exothermic, generating intense heat, causing spattering.

Topic 7 Acids and Bases

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