Acids and bases Flashcards
What is the Bronsted-Lowry definition of an acid?
a proton donor
What is the Bronsted-Lowry definition of an base?
a proton acceptor
Why are H3O+ (oxonium/hydroxonium) ions present in the solution instead of H+ only?
Because the H+ ion is attracted to the lone pair of electrons on a water molecule so a co-ordinate bond is formed.
What are the assumptions made for K- acid disassociation constant?
- only a small amount of acid disassociates so HA at equilibrium is the same as the start
- The dissociation of acid is greater than the dissociation of water present in the solution so we assume all the H+ ions come from the acid
What does it mean when acid is mono/diprotic?
They only donate one/two moles of protons per mole of acid
What is the definition of pH?
the concentration of H+ ions in a solution -log10
Define the term Kw constant and its value?
1 x 10-14
Kw = [H+][OH–]
What is the point of pka? What happens what the value of pKa becomes lower?
its used to determine whether something is a weak or strong acid
the lower the value of pka, the larger the value of ka, the greater the ionisation of acid in the water so the stronger the acid
What is a buffer solution?
pH changes by a small amount when acid/ alkali is added
Why don’t strong acids have buffer solutions?
because they fully ionise
What is a buffer? What is an acidic buffer and basic buffer?
A chemical that resists (not stop) the change in pH when small amounts of acid or base are added
Acidic = keep solution below ph7 and made from a weak acid and its salt
Basic - keep solution above ph&, made from weak base and its salt
What is the difference in the position of equilibrium for weak acids vs salt of weak acid
Weak acids dissociate weakly = left
salt dissociate strongly = right
What happens when acids (H+) and bases (OH-) are added to acidic buffers?
acid = the salt decreases and the acid increases (because H+ reacts with the salt to produce more acid)
Base = salt increases and acid decreases (cause OH reacts with H+ ions (for example CH3 COOH → CH3COO- + H+) so the equilibrium shifts to the right to oppose the decrease in H+, aka CH3COOH dissociate even more
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What happens when acids (H+) and bases (OH-) are added to basic buffers?
Example = NH3 + H2O→ NH4 +OH-
Acid = reaction becomes more acidic and some of the H+ ions react with OH - to make H2o, causing equilibrium position to move to the right to oppose the change to make more OH- ions
Base = reaction more basic. some OH- react with NH4+ ions to from NH3 and H2o , so equilibrium with shift to the left to oppose the change
Why does dilution have little to no effect on the pH value of a buffer solution
because the ration of the concentrations of the acid and its salt remain the same
When a small quantity of hydrochloric acid is added to a buffer, the pH remains ALMOST constant. Why?
adding HCL, the H+ will react with the salt of the weak acid. So the concentration of salt will decrease and concentration of acid will increase slightly
State and then explain what would happen to the pH of water, if the temperature were increased.
pH would decreae
increase in temp = equilibrium shifts to the right = endothermic reaction = causes an increase in H+ concentration
Explain why chloroethanoic acid is a stronger acid than ethanoic acid.[2 marks]
M1 (Electronegative) chlorine withdraws electrons
M2 Stabilises/reduces the charge on COO
weakens O-H bond
makes O-H more polar
Suggest a value for the pH of a 1.26 M solution of sodium chloride and explain your answer.
7 because concentration of H+ = concentration of OH-
Explain why the expression for Kw does not include the concentration of water
- the concentration of H2O is very high and only dissociates very slightly.
- The equilibrium lies far to the left
- H2o is effectively constant
Explain why the value of Kw increases as the temperature increases
Bond breaking is endothermic, so the equilibrium moves to the right-hand side to oppose increase in temperature
(h2o → H+ + OH-)
State why calibrating pH meter just before it is used improves the accuracy of the pH measurement
Over time / after storage meter does not give accurate readings
Describe how you would obtain the pH curve for titration? (5 marks)
- Measure pH (of the acid)
- Add alkali in known small portions (Allow 1 - 2cm3)
- Stir mixture
- Measure pH (after each addition)
- Repeat until alkali in excess (Allow 27 - 50cm3)
- Add in smaller increments near endpoint (Allow 0.1 - 0.5cm3)
