pH and Indicators Flashcards
equation for ionic product of water
H2O ⇌ H+ + OH-
or
H20 + H2O ⇌ H3O+ + OH-
definition of Kw
the product of the concentrations of hydrogen ions and hydroxide ions in water in moles per litre
Kw =
[H+] [OH-]
[ ] means
moles per litre
if the temperatures changes will Kw change and why
yes as it is temperature dependent
pure water
number of OH- equals H+ when water disassociates
[H+] =
√Kw
if acid is added to pure water
H+ ions will increase, system alters to oppose this and OH- in turn is also decreased
if base is added to pure water
OH- ions will increase, system alters to oppose this and H+ in turn is also decreased
if acids are bases are added, does Kw change
no, the ionic product is always 1 x 10^14
does the acidity or alkalinity of pure water change with temperature
no, as [H+] = [OH-] at all temperatures, so pure water is neutral at all temperatures
pH definition
minus the log to the base 10 of the hydrogen ion concentration (mol/l)
pH =
-log10 [H+]
pOH =
-log10 [OH-]
is neutral at 7 for all solutions?
only when at 25ºC
3 limitations of the pH scale
does not work at extremely low concentrations
at 25ºC only
goes for 0-14
what do we use to calculate with strong acids
disassociation equation and its molar ratio
4 points for strong acids
mol/l
disassociation equation
molar ratio
pH = -log10 [H+]
5 points for strong base
mol/l disassociation equation molar ratio pOH = -log10 [OH-] pH = 14 - pOH
Ka or Kb
disassociation constant for acid or base
weak acid equation
[H+] = √Ka[HA]
weak base equation
[OH-] = √Kb[BOH]
all weak acids on the course are
monobasic
3 points for weak acids
mol/l
[H+] = √Ka[HA]
pH = -log[H+]
4 points for weak bases
mol/l
[OH-] = √Kb[BOH]
pH = -log[H+]
pH = 14 - pOH
indicators definition
an indicator is a substance that changes colour depending on the pH of the solution in which it is placed
most indicators are
weak acids
why do indicators indicate
the undisassociated form or the indicator is different in colour to the disassociated form
for an indicator to work visibly there has to be
a change of at least 2 pH units to show an obvious colour change
the range or an indicator
the pH range over which there is a noticeable colour change
strong acid strong base
any indicator
strong acid weak base
change colour in acid conditions eg methyl orange (3-5 red to yellow)
weak acid and strong base
color change in alkaline conditions eg phenolphthalein (8-10 colourless to pink)
weak acid and weak base
there is not a change of 2 pH units - never a visible colour change, we do not carry out these titrations
give an example of an indicator that changes colour in acid conditions
methyl orange
give an example of an indicator that changes colour in alkaline conditions
phenolphthalein
