Acids and Bases

Acids are substances that turn blue litmus red. Examples of Acids found in household items: oranges, lemons, vinegar!

Strong acids..

Bases are substances that turn red litmus indicator blue. Examples of bases In the household: soap, toothpaste

Strong bases…

When an acid is mixed with a base they react with each other form a salt and water. This is called a neutralisation reaction. Acid + Base Salt + Water

Uses of neutralisation reactions: 1. Treating wasp stings A wasp sting is basic. Vinegar, an acid, can be used to treat a wasp sting because when they react with each other a neutralisation reaction happens

Uses of neutralisation reactions: 2. Neutralising soil Many plants do not grow well in acidic soil. Farmers often add lime (a base) to acidic soil to make their plants grow better.

Arrhenius – a new way to think about acids and bases

Arrhenius acids and bases An Arrhenius acid is a substance that dissociates in water, forming hydrogen ions HA H+ + A-

Arrhenius said… HA H+ + A- A strong acid is one which fully dissociates in water A weak acid is one which is only slightly dissociates in water HA H+ + A-

Arrhenius acids and bases An Arrhenius base is a substance that dissociates in water, forming hydroxide ions MOH OH- + M+

Arrhenius said… MOH OH- + M+ A strong base is one which fully dissociates in water A weak base is one which is only slightly dissociates in water MOH OH- + M+

Bronsted Lowry Theory of acids and bases An acid is a proton donor A base is a proton acceptor V. Important - Proton is the same as a (Hydrogen ion H+)

Identify the Bronsted acids/ bases in each case An acid is a proton donor A base is a proton acceptor

A conjugate acid – base pair Reversible reactions Some reactions can go in both directions, forward and backward NH4+ H20 NH3 +H30+ Conjugate base Bronsted acid Bronsted base Conjugate acid Conjugate means having something in common but also opposites in some ways A conjugate acid – base pair an acid and a base by differ in structure by a proton

Self ionisation of water Water can dissociate itself the following way: H20 H+ + OH- In a sample of water at room temperature this will only happen to a very small number of molecules

The ionic product of water Kw = [OH-][H+] (Note - Square Brackets mean concentration in moles per litre) At 25OC Kw = 1 x 10-14 As temperature increases Kw will increase as more water molecules ionise. Can you guess why reaction rates increase with increasing temperature?

Regardless of how many there are!! H20 H+ + OH- So in water, [H+] = [OH-] Regardless of how many there are!!

Remember that Kw = [OH-][H+] 1 x 10-14 = [OH-] [H+] at 25OC We can also say Kw = [H+][H+] Or Kw = [H+]2 From this 1 x 10-14 =[H+]2 √ 1 x 10-14 = [H+] 1 x 10-7 = [H+] at 25OC Also 1 x 10-7 = [OH-] at 25OC Once number of h+ and oh are the same the solution is balanced and is nuetral. Idea with the pH scale. Balanced is nuetral.

In any solution If [H+] = [OH-] The solution is neutral If [H+] > [OH-] The solution is acidic If [H+] < [OH-] The solution is basic

pH

the square brackets mean concentration in moles per litre pH Sorensen – a Danish chemist Expressed the acidity of a solution by measuring the amount of H+ ions present in a solution and using the formula: To find out what Log base 5 (25) is, we'd ask ourselves "what power do you raise 5 to to get 25?" Since 5^2 = 25, the answer to this one is 2. So the Logarithm to the base 5 of 25 is 2. pH= - log10 [H+], the square brackets mean concentration in moles per litre

Example 1: A solution has a concentration of 10-4 moles of H+ per litre. What is the pH? pH= - log10 [H+], pH= - log10 [10-4], pH= 4

Example 2: A solution has a concentration of 7x10-7 moles of H+ per litre. What is the pH? pH= - log10 [H+], pH= - log10 [7 x10-7], pH= 6.15

[H+] = 1.995 X 10-4 moles per litre Example 3: A solution has a pH of 3.7. What is the concentration of hydrogen ions in moles per litre? pH= - log10 [H+], 3.7 = - log10 [H+], Antilog INV AND THEN LOG OG 10 to the x [H+] = antilog - 3.7 [H+] = 1.995 X 10-4 moles per litre

The PH scale The pH of an acidic solution is less than 7. The pH scale is a scale from 0 – 14 which tells how acidic or basic a solution is. The pH of an acidic solution is less than 7. The pH of an neutral solution is 7. The pH of an basic solution is more than 7.

Measuring the pH of a solution: Universal indicator Dip the universal indicator paper into the solution. Compare the colour of the paper to the scale supplied with the indicator to find the pH.

Measuring the pH of a solution: pH meter Dip the pH meter into the solution. Read the pH off the meter

pH scale

Limitations of the pH scale You can only find the pH of a substance if it is in a dilute aqueous solution. It only really works at 25oC

Strong acids Strong acids fully dissociate in water to produce H+ ions. So for a strong acid, like HCl, in water the following happens: HCl H+ + Cl-

Calculating the pH of a strong acid Example: Calculate the pH of a 0.01 M HCl solution: HCl H+ + Cl- 0.01 M 0.01 M pH= - log10 [H+], pH= - log10 [0.01], pH= 2

HNO3 H+ + NO3 0.03 M 0.03 M pH= - log10 [H+], pH= - log10 [0.03], pH= Example: Calculate the pH of a 0.03 M HNO3 solution: HNO3 H+ + NO3 0.03 M 0.03 M pH= - log10 [H+], pH= - log10 [0.03], pH=

Example: Calculate the pH of a 0.5 M HF solution: HF H+ + F- 0.5 M 0.5 M pH= - log10 [H+], pH= - log10 [0.5], pH=

Example 3 The concentration of a solution of HCl is 7.3g per litre. (i) What is the concentration of the solution in moles per litre? 7.3g of HCl in one litre of solution. Number of moles of HCl = 7.3 36.5(mass of one mole) Number of moles of HCl in one litre = 0.2

Example 3 HCl H+ + Cl- 0.2 M 0.2 M pH= - log10 [H+], (ii) What is the pH of the solution? Number of moles of HCl = 0.2 HCl H+ + Cl- 0.2 M 0.2 M pH= - log10 [H+], pH= - log10 [0.2], pH= 0.69

Example 4 The concentration of a solution of HNO3 is 6.3g per litre. (i) What is the pH? 6.3g of HNO3 in one litre of solution. Number of moles of HNO3 = 6.3 63(mass of one mole) Number of moles of HNO3 in one litre = 0.1

Example 4 HNO3 H+ + NO3- 0.1 M 0.1 M pH= - log10 [H+], (ii) What is the pH of the solution? Number of moles of HCl = 0.1 HNO3 H+ + NO3- 0.1 M 0.1 M pH= - log10 [H+], pH= - log10 [0.1], pH= 1

Find the pH of a 0.4M solution of sulphuric acid. An unusual case H2SO4 2H+ + SO4-2 Find the pH of a 0.4M solution of sulphuric acid.

Strong bases NaOH Na+ + OH- Strong bases fully dissociate in water to produce OH- ions. So for a strong base, like NaOH, in water the following happens: NaOH Na+ + OH-

To find the pH of a strong base There are two steps 1. Use this formula pOH= - log10 [OH-] 2. pH = 14 - pOH

To find the pH of a strong base Example 1: Calculate the pH of a 0.5 M NaOH solution: To find the pH of a strong base Two steps 1. Use this formula: pOH= - log10 [OH-] pOH= - log10 [0.5] pOH= 2. pH = 14 – pOH pH = 14 – pH =

To find the pH of a strong base Example 2: Calculate the pH of a 0.2 M KOH solution: To find the pH of a strong base Two steps 1. Use this formula: pOH= - log10 [OH-] pOH= - log10 [0.2] pOH= 2. pH = 14 – pOH pH = 14 – pH =

Example 3 The concentration of a solution of KOH is 5.6g per litre. (i) What is the concentration of the solution in moles per litre? 5.6g of KOH in one litre of solution. Number of moles of KOH = 5.6 56(mass of one mole) Number of moles of KOH in one litre = 0.1

Example (ii) What is the pH of the solution? Number of moles of KOH= 0.1 Two steps pOH= - log10 [OH-] pOH= - log10 [0.1] pOH= 2. pH = 14 – pOH pH = 14 – pH =

Example 4 The concentration of a solution of NaOH is 8g per litre. (i) What is the concentration of the solution in moles per litre? 8g of NaOH in one litre of solution. Number of moles of NaOH = 8 40(mass of one mole) Number of moles of NaOH in one litre = 0.2

Example (ii) What is the pH of the solution? Number of moles of KOH= 0.1 Two steps pOH= - log10 [OH-] pOH= - log10 [0.1] pOH= 2. pH = 14 – pOH pH = 14 – pH =

Example 4 The concentration of a solution of NaOH is 8g per litre. (i) What is the concentration of the solution in moles per litre? 8g of NaOH in one litre of solution. Number of moles of NaOH = 8 40(mass of one mole) Number of moles of NaOH in one litre = 0.2

Example (ii) What is the pH of the solution? Number of moles of NaOH= 0.2 Two steps pOH= - log10 [OH-] pOH= - log10 [0.2] pOH= 2. pH = 14 – pOH pH = 14 – pH =

Calculating pHs of weak acids and bases

Weak acids A weak acid only slightly dissociates in water to produce H+ ions. HA H+ + A- Ka is the acid dissociation constant. It will tell you to what extent a weak acid will actually dissociate in water.

Useful formulas for doing calculations on weak acids pH= - log10 [H+] [H+] = √Ka x [Acid] ****Square brackets mean concentration in moles per litre!

But [H+] is not given for a weak acid [H+] = √Ka x [Acid] Calculate the pH of a 0.1 solution of methanoic acid given that the value of the acid dissociation constant is 1.8 X 10-4 pH= - log10 [H+] But [H+] is not given for a weak acid [H+] = √Ka x [Acid] [H+] = √1.8 x 10-4 x [0.1] [H+] = √.00018 [H+] = 0.0042 pH= - log10 [0.0042] = 2.37

Weak bases A weak base only slightly dissociates in water to produce OH- ions. NH3 +H20 NH4+ + OH- KB is the BASE dissociation constant. It will tell you to what extent a weak base will actually cause the production of OH- in water.

Useful formulas for doing calculations on weak bases pH = 14 - pOH pOH= - log10 [OH-] [OH-] = √Kb x [Base] ****Square brackets mean concentration in moles per litre!

Calculate the pH of a 0.01 solution of ammonia given that the value of the base dissociation constant is 1.8 X 10-5 pH = 14 - pOH pOH= - log10 [OH-] But [OH-] is not given for a weak base [OH-] = √Kb x [Base] [OH-] = √1.8 x 10-5 x [0.01] [OH- ] = √.00000018 [OH- ] = 0.00042 pOH= - log10 [0.00042] = 3.37 pH = 14 – pOH Ph = 14 – 3.37 = 10.6

Acid – Base Indicators

Acid-base indicators Indicators are substances that change colour depending on the pH of the solution in which they are placed

Lab indicators

How indicators work They are weak acids mostly so will act as follows.. HIn H+ + In- They have a different colour depending on which form they are in – dissociated or undissociated

Causing a colour change to blue Example : Litmus HIn H+ + In- BLUE RED What happens when a base is added to red Litmus indicator? The base will accept H+ ions from the reaction and make it go to the left hand side, Causing a colour change to blue What happens when an acid is added to blue Litmus indicator? The acid will donate H+ ions and these will react with the indicator, making it go red.

Methyl orange At pH 4 and 5 methyl orange looks an indistinct colour 1 2 3 4 5 6 7 8 9 10 11 Methyl orange P Y At pH 4 and 5 methyl orange looks an indistinct colour

Litmus At pH 5,6,7 litmus looks an indistinct colour PH 1 2 3 4 5 6 7 8 9 10 11 Litmus R B At pH 5,6,7 litmus looks an indistinct colour

Phenolphthalein At pH 8,9 phenolphtalein has an indistinct colour PH 1 2 3 4 5 6 7 8 9 10 11 Phenolphthalein C B At pH 8,9 phenolphtalein has an indistinct colour

Titration curves Measuring the pH changes in a reaction between an acid and a base Often the pH will change quite suddenly, and an indicator can be used to detect this end point

Picking the correct indicator Titrating a strong acid against a strong base At the end point the pH jumps from 3 to 10 suddenly. Which indicator would tell when this endpoint has been reached? IT MUST SHOW A CLEAR COLOUR CHANGE FROM 3 – 10!! PH 1 2 3 4 5 6 7 8 9 10 11 Phenolphthalein C B PH 1 2 3 4 5 6 7 8 9 10 11 Litmus R B PH 1 2 3 4 5 6 7 8 9 10 11 Methyl orange P Y

Picking the correct indicator Titrating a strong acid against a weak base At the end point the pH jumps from 3 to 7 suddenly. Which indicator would tell when this endpoint has been reached? IT MUST SHOW A CLEAR COLOUR CHANGE FROM 3 – 7!! PH 1 2 3 4 5 6 7 8 9 10 11 Phenolphthalein C B PH 1 2 3 4 5 6 7 8 9 10 11 Litmus R B PH 1 2 3 4 5 6 7 8 9 10 11 Methyl orange P Y

Picking the correct indicator Titrating a weak acid against a strong base At the end point the pH jumps from 7 to 10 suddenly. Which indicator would tell when this endpoint has been reached? IT MUST SHOW A CLEAR COLOUR CHANGE FROM 7 – 10!! PH 1 2 3 4 5 6 7 8 9 10 11 Phenolphthalein C B PH 1 2 3 4 5 6 7 8 9 10 11 Litmus R B PH 1 2 3 4 5 6 7 8 9 10 11 Methyl orange P Y

Picking the correct indicator Titrating a weak acid against a weak base There is no sharp change in pH at the end point. It is difficult to find the exact point of neutralisation relying on indicator colour. A pH meter could be used PH 1 2 3 4 5 6 7 8 9 10 11 Phenolphthalein C B PH 1 2 3 4 5 6 7 8 9 10 11 Litmus R B PH 1 2 3 4 5 6 7 8 9 10 11 Methyl orange P Y