Chapter 29 – Acids, Bases and pH Section A – introduction to acids and bases Properties of ACIDS Properties of BASES Have a pH of _______________ Have a pH of _______________ Can have a ____________ taste Can have a ____________ feel Turn blue litmus _____________ Turn red litmus _____________ Examples of household bases: Examples of household acids: Neutralisation reactions Acid + Base Examples from everyday life: A basic wasp sting is neutralised by Acidic soil is neutralised by Section B – The Arrhenius theory of acids and bases Arrhenius acid A strong acid St. Dominic’s College A weak acid Chemistry notes Page 1 Chapter 29 – Acids, Bases and pH Arrhenius base A weak base A strong base Section C – Bronsted-Lowry theory of acids and bases An acid is a ___________ donor, and a base is a __________ acceptor. Section D – Reversible reactions and Conjugate base pairs A reversible reaction is one which happens in ______________ directions, this is indicated by the arrow shown below: NH3 +H30+ NH4+ H20 In the forward direction ____________ is the acid and it turns into ____________________ after it donates the proton. The base is _________________ and it turns into ______________ after it accepts the proton. In the reverse reaction____________ is the acid and it turns into ____________________ after it donates the proton. The base is _________________ and it turns into ______________ after it accepts the proton __________________. Conjugate acid- base pair *This means the Bronsted acid turns into the conjugate base, and the Bronsted base turns into the conjugate base. This example contains two conjugate base pairs: St. Dominic’s College Chemistry notes Page 2 Chapter 29 – Acids, Bases and pH Section E – Self ionisation of water Water can dissociate itself (_____________) in the following way: H+ + OH- H20 Kw = [OH-][H+] The ionic product of water : (Square Brackets mean concentration in moles per litre) *A note about Kw and temperature - At room temperature 25OC Kw = 1 x 10-14 but this will increase as temperature increases because as temperature increases more ______________ happen between water molecules causing more ions to be generated. in water: Kw = [OH-][H+] = [OH-] [H+] at 25OC [H+] = [OH-], because for every one water molecule that dissociates _______H+ ion is made and ______ OH- ion is made every one water molecule that dissociates _______H+ ion is made and ______ OH- ion is made Kw = [H+][H+] Kw = [H+]2 1 x 10-14 =[H+]2 = [H+] = [H+] at 25OC ******Also 1 x 10-7 = [OH-] at 25OC Any neutral solution – like pure water will have [H+] = [OH-], If extra H+ ions are added into a neutral solution then [H+] > [OH-], and the solution will be _______ If extra OH- ions are added or generated in a neutral solution then [H+] < [OH-] and the solution will be ______________. Section F – The pH scale pH= the square brackets mean concentration in moles per litre **Using this formula provides a way to discuss the acidity of a solution without having to express the concentration of [H+] in mol/L which can often involve very small numbers St. Dominic’s College Chemistry notes Page 3 Chapter 29 – Acids, Bases and pH Example 1 - A solution has a concentration of 7x10-7 moles of H+ per litre. What is the pH? pH= - log10 [H+], Answer: The pH scale This is a scale from _______________which tell you how acidic or basic a solution is. The pH of a solution can be measured by using: 1)___________________________________ 2) ________________________________________ The pH scale can be very useful but also has its limitations: it can only be used at a temperature of __________ and the substance you are measuring must be in a __________ ______________ solution. *An interesting example is the case of water at temperature over 25OC – we already mentioned that the Kw (__________________________) of water increases with increasing temperatures. At 100 OC is Kw = 52.3 x 10-14 [H+] = √52.3 x 10-14 [H+] = pH = So you might assume that the water is now _______________, but it is not because even though the concentration of [H+] is higher than normal we still know that in pure water [H+] = [OH-] and this means the solution is still neutral! St. Dominic’s College Chemistry notes Page 4 Chapter 29 – Acids, Bases and pH Section G – Calculating pH values Strong acids Strong acids _______________dissociate in water to produce H+ ions. Example 1 Calculate the pH of a 0.5 M HF solution Answer H+ + F- HF Example 2 Find the pH of a 0.4M solution of sulphuric acid Answer Strong bases Strong bases _______________dissociate in water to produce ____ ions. Calculating the pH of a strong base: pOH= - log10 [OH-] pH = Example 1 - Calculate the pH of 0.2 M KOH solution Answer : For a strong base like KOH To find pOH Use the formula pOH= - log10 [OH- pH = 14 - Poh St. Dominic’s College KOH Chemistry notes K + OH- Page 5 Chapter 29 – Acids, Bases and pH Weak acids A weak acid only ________________dissociates in water to produce H+ ions. For this weak acid which we will call HA: H+ + A- HA Ka is the ______________________________ It will tell you to what extent a weak acid will actually dissociate in water. For a weak acid : [H+] = √Ka x [Acid] pH = ****Square brackets mean concentration in moles per litre! Example 1 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 Answer [H+] = √Ka x [Acid] [H+] = [H+] = [H+] = pH= Weak bases A weak acid only partially _________________ in water to produce OH- ions. For this weak base like NH3 NH3+ H2O NH4+ OH- Kb is the ______________________________ It will tell you to what extent a weak acid will actually dissociate in water. pH = 14 - pOH pOH= - log10 [OH-] [OH-] = √Kb x [Base] ****Square brackets mean concentration in moles per litre! St. Dominic’s College Chemistry notes Page 6 Chapter 29 – Acids, Bases and pH Example 1 Calculate the pH of a 0.01 solution of ammonia NH3 given that the value of the base dissociation constant is 1.8 X 10-5 Answer [OH-] = √Kb x [Base] [OH-] = [OH- ] = [OH- ] = pOH= - log10 [OH-] pOH= pH = 14 – pOH pH = Section H – Acid Base Indicators Acid/ Base Indicators Since acid/ base indicators are themselves either acids or bases you should not add too much of them to an acid/base titration as they will themselves react in the reaction and cause an inaccurate titre result! Litmus as an example of an indicator In its undissociated form (HIn) Litmus is ___________ in colour. HIn H+ + In- In its dissociated form (In-) Litmus is ___________ in colour. If blue Litmus is mixed with an acid then then the acid will donate ____________ ions to Litmus IN – and will turn it into HIn which is ____________ in colour If red Litmus is mixed with a base then the Indicator HIn will donate ______________ ions to the base and it will change to In- which is _______________ in colour. *We will look at the dynamics of these reactions in more detail in ch. Chemical Equilibrium St. Dominic’s College Chemistry notes Page 7 Chapter 29 – Acids, Bases and pH Section G - Titration curves and acid- base indicators During an acid- base titration the pH will change gradually at first but will make a s___________ change close at the ____________ of the reaction. Using an indicator can tell us when the end point has been reached by changing colour at this exact moment. Strong acid strong base titration Example: At the end point the pH jumps suddenly from ____ to ____________. A suitable indicator must have one colour at pH 3 and a different colour at pH10 Suitable indicators: Strong acid weak base titrations Example: At the end point the pH jumps suddenly from ____ to ____________ so the indicator must have a distinct colour change in this range Suitable indicator: St. Dominic’s College Chemistry notes Page 8 Chapter 29 – Acids, Bases and pH Weak acid strong base titration Example At the end point the pH jumps suddenly from ____ to ____________ so the indicator must have a distinct colour change in this range Suitable indicator: Weak acid weak base titration Example There is NO sharp or sudden pH change during this reaction so there is NOT a suitable indicator available to detect the end point by means of a colour change. St. Dominic’s College Chemistry notes Page 9 Chapter 29 – Acids, Bases and pH Self-assessment of Green = I already know this Orange = I am not sure – have to check it Red = I don’t know this yet – have to start learning Green Orange Red Household acids and bases (two examples of each). Everyday examples of neutralisation, e.g. use of lime in agriculture, use of stomach powders for acid indigestion. Neutralisation – formation of a salt from an acid and a base. Arrhenius theories of acids and bases Brønsted-Lowry theories of acids and bases Conjugate acid-base pairs. ( Definition and you must be able to identify these pairs in an equation) HL ONLY Self-ionisation of water; K w; ( Definition and you must be able to show why [H+] and [OH-] are 1 x 10 -7 pH scale – explain what it is Use of universal indicator paper or pH meter to measure pH of a solution Limitations of the pH scale – usefulness confined to dilute aqueous solutions. Calculation of pH of strong acids and bases HL ONLY Calculation of pH of weak acids and bases. HL ONLY Theory of acid-base indicators.( what an indicator is, how they work) HL ONLY Titration curves.( for each type of acid- base titration you must be able to draw the curve, and know the end points) HL ONLY Choice of indicator( for each type of indicator you must know its colours at various pHs) St. Dominic’s College Chemistry notes Page 10 Chapter 29 – Acids, Bases and pH St. Dominic’s College Chemistry notes Page 11