Chapter 20 Flashcards
According to the Arrhenius model, what are acids and bases?
- Acids: any substance that adds H+ ions to water
- Bases: any substance that adds OH- ions to water
What do H+ ions often exist as, and why? Which form tends to be used in equations, and why?
- Hydronium ions
- H3O+
- Protons can’t exist on their own
- H+, as it is simpler to use
What is inaccurate about Arrhenius’ model?
- It only describes acids and bases that are in solution
- Acids and bases were only described in terms of H+ and OH- ions, which doesn’t include all of them (e.g. ammonia)
What replaced the Arrhenius model?
- The Brønsted-Lowry model
How are acids and bases defined in the Brønsted-Lowry model?
- In terms of their reactions with each other; a proton is transferred from the acid to the base
- Acids: proton donors
- Bases: proton acceptors
How is the dissociation of weak and strong acids shown?
- Weak acid <-> H+ + anion
- Strong acid -> H+ + anion (as equilibrium is so far to the right)
What type of acid is HF?
- A weak acid
Give the 2 versions of the equations for the dissociation of water.
- H2O <-> H+ + OH-
- H2O + H2O <-> H3O+ + OH-
In the reaction of a strong acid and a weak acid, which acts as a Brønsted-Lowry acid and which acts as a Brønsted-Lowry base?
- The strong acid is the Brønsted-Lowry acid
- The weak acid is the Brønsted-Lowry base
What happens to a Brønsted-Lowry base in a reaction?
- It accepts a proton to form its conjugate acid
What happens to a Brønsted-Lowry acid in a reaction?
- It donates a proton to form its conjugate base
What are an acid and its conjugate base (and alternatively a base and its conjugate acid) called together?
- A conjugate acid-base pair (or just a conjugate pair)
What role do conjugate pairs have in acid-base equilibria?
- According to the Brønsted-Lowry model, acid-base equilibria always contain 2 conjugate pairs
What are monobasic, dibasic and tribasic acids?
- Monobasic acid: an acid where 1 hydrogen ion can be replaced per molecule in an acid-base reaction
- Dibasic acid: an acid where 2 hydrogen ions can be replaced per molecule in an acid-base reaction
- Tribasic acid: an acid where 3 hydrogen ions can be replaced per molecule in an acid-base reaction
Which terms can be used instead of monobasic, dibasic and tribasic?
- Monoprotic
- Diprotic
- Triprotic
What needs to be remembered when writing ionic equations?
- Only aqueous compounds (not even solid ionic compounds) can be split into their ions (since ions can only be aqueous)
How does the pH scale work, and why?
- The pH scale is actually a logarithmic scale used to quantify [H+] more easily
- Each number (x) on the pH scale actually represents [H+] of 1x10^-x moldm^-3
- Log base 10 was used to remove the powers of 10, and the negatives were then removed (which is why a low value actually means high acidity)
- Although the pH scale goes from 0 to 14, it is possible to have values outside the range- however, they are so dangerous that it isn’t common to use them
How do you calculate the concentration of hydrogen ions from pH values? How is this rearranged to calculate pH from the concentration of hydrogen ions?
- [H+] = 10^-pH
- pH = - log [H+]
What is the difference in concentration between solutions with a difference in pH of 1?
- The one with the lower pH has a concentration 10 times larger
How can you calculate the pH of a strong, monobasic acid if given its concentration?
- Since the ratio of the acid and the hydrogen ions is 1:1 in the equation of their dissociation, they have the same concentration
- You can put this value into the equation to find the pH of a solution
How can you calculate the pH of a strong, monobasic acid if it is diluted?
- You can use the concentration of the acid and the starting volume to find the number of moles
- Since the number of moles stayed the same but the volume changed, you find the new value for [H+], and use this to find the pH
How can you calculate the pH of a solution after a strong, monobasic acid is reacted with a base (solid and aqueous)?
- If the base is solid:
- you can assume the volume stays the same, since the volume of water produced would be negligible
- you use stoichiometry to find the number of excess moles of acid (some are used up forming salt and water)
- using the new number of moles, you can calculate [H+], and therefore the pH
- If the base is aqueous:
- the volume changes (it is the volume of the acid and base added together)
- you calculate the number of moles in the same way as when the base was solid
- you find the concentration from the new values for number of moles and volume (the volume of water is still ignored), and use this to find the pH
What is Ka?
- The acid dissociation constant
How is Ka calculated? What does this mean for it?
- It is Kc for the dissociation of a weak, monobasic acid
- Its units are always moldm^-3
What do values of Ka tell us about weak acids?
- The higher the Ka value, the stronger the acid (since equilibrium is further to the right)
What issue do Ka values have, and what has been done about this?
- Just like values of [H+], Ka values can be very small, and therefore hard to compare
- Just as [H+] has pH, Ka has pKa
How do you convert between Ka and pKa?
- pKa= -logKa
- Ka= 10^-pKa
What do values of pKa tell us?
- The larger the value of pKa, the weaker an acid is
What 2 approximations can be made when finding Ka for weak acids, and why can each be made?
- The concentration of the acid stays mostly constant, as the dissociation of weak acids is small
- There is an equal concentration of protons and anions (the production of H+ ions from the dissoication of water is negligible)
How can Ka be rewritten?
- Ka= [H+]^2/ [HA] (start)
What limitation is there to using approximations to find Ka for weak acids?
- In ‘stronger’ weak acids, [H+] becomes significant, so you can no longer assume that the concentration of the acid at the start and at equilibrium is equivalent
What is Kw?
- The ionic product of water
Where does Kw come from?
- Water can be treated as a weak acid as it dissociates slightly
- Ka= [H+][OH-] / [H2O]
- As water is mainly undissociated, [H2O] is a constant
- Kw= Ka × [H2O]
What is the formula for Kw, and therefore its units?
- Kw = [H+][OH-]
- mol^2 dm^-6
Even if water doesn’t have a pH of 7, how do we know that it’s always neutral?
- [H+] = [OH-]
What is the other form of the formula for Kw when dealing with water, and why does it work?
- Kw = [H+]^2
- [OH-] = [H+]
- Water is neutral
What can change the value of Kw?
- Temperature
How is the pH of strong bases calculated?
- In monoacidic bases like NaOH, their concentration is equal to the concentration of hydroxide ions (in the equation of their dissociation, they’re in a 1:1 ratio)
- You then use the equation for Kw to calculate [H+], and therefore the pH
