Equilibria involving acids and bases Chapter 17 Summary Equilibria involving acids and bases

What we should already know… Acids are proton donors Bases are proton acceptors In an acid-base reaction, there is a transfer of one or more protons from an acid to a base An example of an amphiprotic substance is ________ The pH scale is designed to measure acidity

Ionisation of water Self ionisation is the term used to describe water reacting with itself H2O(l) + H2O(l) H3O+(aq) + OH-(aq) At equilibrium K = [H3O+][OH-] [H2O]2 In an aqueous solution, water molecules are always more abundant than other substances present. We can say it is virtually always at a constant concentration of 56M. Because the concentration of [H2O]2 is a constant, we can eliminate it when writing the ionisation constant expression (Kw).

Ionisation of water K x [H2O]2 = [H3O+][OH-] Where [H2O]= 56M And [H3O+] = 10-7 [OH-] = 10-7 LOOK AT THE CONCENTRATION OF WATER MOLEULES COMPARED TO HYDRONIUM AND HYDROXIDE IONS! This can be re-written as Kw = [H3O+][OH-] In a neutral solution or pure water at 25°C Kw = 10-7 x 10-7 Or Kw = 1.0 x 10-14 M2 This expression applies to pure water as well as all aqueous solutions.

Affect of adding acid/ base In an acidic solution, H3O+ ions are formed by the dissociation of an acid, as well as from the self ionisation of water. Therefore: [H3O+] = >10-7M at 25°C. Because the total [H3O+][OH-] = 10-14, [OH-] = <10-7 M at 25°C In a basic solution [H3O+] = <10-7M [OH-] = >10-7 M The higher the concentration of [H3O+] in a solution, the more acidic the solution is and the lower the pH. For example, in a 1M HCl sol [H3O+] = 1 M BUT in a 1M NaOH solution, [H3O+] = 10-14 M

Ph scale Most acidic solutions are slightly less than zero The most basic solutions have a pH value of ~14 Formulas we need in this topic: pH = -log10[H3O+] [H3O+] = 10-pH [H3O+][OH-] = 1.0x10-14 M2

It is important to note: Strong acids and base reactions are examples Please turn to p.287 of Heniemann 2 Examine calculations in neutral, acidic and basic solutions. It is important to note: Strong acids and base reactions are virtually complete – almost all of the reactant becomes products.

What happens if the temp is not 25°C? The self ionisation of water is endothermic. ΔH = +57 kJ mol-1 If we increase the temperature of an endothermic reaction: We get more ______________ This means the equilibrium positions shifts to the ___________ Consequently, Kw would __________. As Kw increases, pH decreases (more H3O+).

Kw and ph As we increase temperature of the self ionisation of water reaction, the Kw also increases. The pH decreases, and the solution becomes more acidic. What would happen if we decreased the temperature?

Kw is called the ________________. Ka is the acidity constant! Ka is used to measure acid strength. A strong acid is almost always fully converted into products. With weak acids, a small amount is produced as the equilibrium constants favor the back reaction. When writing the acidity constant for a certain reaction at a particular temperature, we can eliminate water from the constant. We do this because the concentration of H2O is virtually constant for every acid/ base reaction!

Please flip to p.289 and let’s work through some questions on using the acidity constant, Ka for both a strong acid (HCl) and weak acid (CH3COOH). Step 1. Write a balanced acid/ base equation. Step 2. Write the Ka expression (no need to include H2O). Acid = [HA], Conjugate Base = [A-]. For weak acids ONLY [H3O+] = [A-], thus we can square [H3O+]. Remember that there is very little (almost no) ionisation with a weak acid. Concentration of acid product pretty much remains the same concentration! What would a Ka value of 105 tell us? A Ka value of 1.4x10-5?

In summary: A strong acid almost completely reacts. The % ionisation simply refers to the amount the acid product (conjugate base) that has been formed. It can be worked out by dividing the concentration of the products (which are the same) divided by the concentration of the reactants (for a weak acid this is the same as the initial concentration) x 100. E.g. [CH3COO-] x 100 [CH3COOH] In summary: A strong acid almost completely reacts. A weak acid does not ionizes to a great extent.

Questions to complete as a class: 3 4a 5a, b