Acids And Bases Flashcards
Def of acid
A proton donor
Def of base
A proton acceptor
Def of salt
When a metal replaces the hydrogen ions on an acid in a neutralisation reaction
pH formula
pH = -log[H+]
Def of pH
The concentration of hydrogen ions in solution
How does the pH scale work?
- what is the logarithmic scale?
- pH 1 has 10x more hydrogen ions than in pH 2
- Therefore the scale goes up in multiples of 10
What type of reaction is dissociation?
- A reversible equation
- Therefore the reaction is in equilibrium
Different indicators
- Universal Indicator
- Phenolphthalein
- Methyl Orange
- Bromophenol Blue
Universal Indicator Colour Changes
- Acid: green to red
- Alkali: green to purple
Phenolphthalein colour changes
- Acid: colourless
- Alkali: pink/dark pink
Methyl Orange Colour changes
- Acid: 1-4 - red
- Alkali: above 4 is yellow
Bromophenol blue colour changes
- Acid: 1-3 - yellow
- Alkali: above 3 - blue
Conjugate acid-base pairs def
Conjugate acid-base pairs differ by the presence or absence of a transferable proton
What word describes water as an acid or base
Amphoteric
- It can act as both an acid and a base
- It can donate and accept protons
What are mono, di and tribasic acids
Monobasic - can donate 1 protons to a base/1 hydrogen ions
Dibasic - can donate 2 protons/hydrogen ions to a base
Tribasic - can donate 3 protons/hydrogen ions to a base
How to work out the pH of an acid when given its concentration in moles
- Write out equation for dissociation of the acid
- Work out if acid is monobasic, dibasic or tribasic
- Work out ratio of moles of H+ ions to base ions
- Work out moles of the number of protons/H+
- N = c x v
- Put concentration of H+ into: pH = -log[H+]
Why can’t pH = - log[H+] be used to work out pH of weak acids
They do not fully dissociate
What is the name given to Kw?
Ionic product
What is the expression for Kw?
Kw = [H+(aq)] x [OH-(aq)]
HA + H2O =(reversible) H3O+ + A-
What is Kc?
why is Kc always going to be very small?
- Kc is always going to be very small, as the no. of moles is always going to be close to 55 in 1dm3 of solution/water
- Therefore the value of Kc will always be very low
What substance is not included when calculating Ka
- Why?
- Water
- The concentration of water in solution is always going to be very high
- Therefore the concentration of water is effectively CONSTANT
What is Ka
Acid dissociation constant
Equation for Ka
Ka = [H+][A-] / [HA]
Times when approximations for Ka equation could be wrong
- If Ka is particularly high
- For very weak acids, pH > 6, the dissociation is no longer negligible
- Therefore [H+]equilibrium = [A-]equilibrium does not apply
- [HA]equilibrium = [HA]start only holds true for acids with a vet small Ka
- does not hold true for stringer weak acids or very dilute solutions