Chapter 20 Acids
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What does the Arrhenius model of acids and bases state?
Acids disassociate and release H+ ions when dissolved in solution Alkalis disassociate and release OH- ions when dissolved in solution
What is a Brønsted-Lowry acid?
A proton donor
What is a Brønsted-Lowry base?
A proton acceptor
What happens when H+ ions dissolve in water really?
Proton transferred from the acid to a water molecule This forms the hydroxonium ion, which is what we really mean by protons The charge density is too high to allow lone protons
How do you name acid-base pairs?
Use the same molecule for the numbers E.g Acid 1 becomes B1 Base 2 becomes Acid 2 The previous acid, then becomes a base
What do mono, di, and tribasic mean?
They refer to the number of hydrogen ions that can be replaced when making a salt 1, 2 or 3 e.g HCl, H2SO4, H3BO4 Monobasic acid- release 1 mol of protons when 1 mol of acid is dissolved in water, each molecule with release 1 proton when the acid is dissolved
How do you calculate pH?
pH= log10([H+]) [H+]= 10^-pH
How do you calculate pH for strong acids? What pH are acids/alkalis?
As fully disassociate, -log10(concentration) x number of protons that disassociate Acids<7 Alkalis>7 neutral=7
What is Ka, what is special about it?
The acid-disassociation constant Special equilibrium for weak acids As an equilibrium constant, changes with temperature
What is pKa? How does acid strength vary with pKa?
Easier to compare Ka pKa= -log10(Ka) The lower the pKa, the stronger the acid (like pH)
What are the assumptions used for calculating the pH of weak acids?
The concentration of H+ = A- at equilibrium The concentration of HA at the start is the same at equilibrium
When do the pH assumptions break down?
For H+ = A-, ignores disassociation of water as normally small so negligible. But if the acid is weak enough, the H+ from water can become important So when pH>6 For HA start = equilibrium If a stronger weak acid, significant amount disassociate to reduce concentration, when Ka> 10^ -2
What is the equation for calculating pH of weak acids using assumptions?
Ka = ([H+]^2)/[HA]
How do you right acid base equilibrium between acids?
Normally an equilibrium sign Proton from the stronger acid donated to the weaker acid Charges
What is the ionisation of water?
Proton transferred from one water molecule to another Simplified, splits into H+ and OH-
What is Kw?
Ionic product of water Equilibrium constant for disassociation of water A constant= concentration of water x Ka
What is the concentration of water? What is the numerical value of Kw?
55.6 mol/dm3, think about moles in 1dm3 density 1 x 10^-14
Why is Kw important with pH?
The concentration of H+ multiplied by OH- is always Kw When they are equal, gives the pH for neutral, 7 at 25 degrees celcius
At higher temperatures, is pH 7 still neutral?
No, as the forward reaction is endothermic, Kw increases More disassociation So the pH is higher, as the pH scale runs longer But water is always neutral, as there is the same concentration of protons as hydroxide ions
How can you calculate the pH of strong bases using pOH?
pOH of strong base = -log10(concentration strong base ) pH= 14- p(OH) 14 only works at 25 degrees celcius, as pH scale runs different
How does equivalence point relate to Ka?
Look at equivalence point volume (half way of vertical) Half of this volume, look the pH pH=Pka 10^-pH= Ka