Flashcards in Chapter 5.1- Rates, equilibrium and pH Deck (27):
What is the equivalence point?
point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution. This matches the stoichometry of the reaction taking place.
What is a titration curve?
-pH meters or data loggers can be used to measure the pH of the reaction mixture as solution from the burette is added over time
-the varying pH values can then be plotted as a graph-known as an acid-base pH titration curve.
- y-axis the pH is plotted
-x-axis the volume of the base/acid is plotted
What is the basic setup of a titration experiment?
-burette with the known concentration and volume of an acid or base is added to a conical flask containing a known volume of acid or base but an unknown concentration of an acid or base
-a pH meter is put into the solution of the unknown concentration of acid or base. The meter is connected to a data logger which tells you the pH of the solution
In an acid-base titration, where a base is added to an unknown concentration of an acid, what are the 3 distinct areas on the titration curve?
1. Slight increase in pH- we add the base but the acid is in excess so theres little change
2. Sharp increase in pH- we have used up all the acid and small amounts of the base increases the pH a lot
3. Slight increase in pH- the base is now in excess so adding more base makes little difference
What does an indicator do?
they tell us where the equivalence point is
What is the end-point of a reaction?
point in a titration at which there are equal concentrations of the weak acid and conjugate base forms of the indicator. The colour of the end-point is midway between the colours of the acid and conjugate base forms
What is an acid-base indicator?
it is a weak acid with the formula Hln
What is the equilibrium reaction of a acid-base indicator?
Hln=H^+ + ln^-
How do the indicators work?
The indicator (Hln) and its conjugate base (ln^-) have different colours in solution
What range of units do most indicators show a colour change?
When choosing a suitable indicator for a experiment, what are the main points that we should think about?
- the colour change must be sharp- if it changes gradually then we will not be able to find the equivalence point
- the end point of the indicator must be near the equivalence point or we won't find the right answer
- the indicator is most useful when we have a clear colour change
What indicator is used in an acid-base titration where the indicator changes colour in acidic conditions (pH 2-4) and what is the colour change?
- Hln(weak acid)= red
- ln^-(conjugate base form)= yellow
What indicator is used in an acid-base titration where it changes colour in the region of pH 6-8 (neutral conditions) and what is the colour change?
-Hln (weak acid)= yellow
-ln^- (conjugate base form)= blue
-end point= light blue
What indicator is used in an acid-base titration where the indicator changes colour in alkaline conditions (pH 8-10) and what is the colour change?
- Hln (weak acid)= colourless
-ln^- (conjugate base form)= pink
-end point = light pink
In a strong acid-strong base titration, what are the features of the titration curve?
-the vertical section of the graph covers a large change in the pH, starting around pH 3 and ending around pH 11, with an equivalence point at pH 7
- both of the indicators, methyl orange and phenolphthalein, have end points that fall within this pH range so both can be used.
In a reaction between a strong acid and strong base what two indicators can be used?
In a strong acid- weak base titration, what are the features of the titration curve?
-the vertical section of the graph covers a smaller change in pH, starting around pH 3 and ending at around pH7.5, with the equivalence point occurring at a more acidic value, i.e. at a pH lower than 7
- methyl orange has an end-point that falls within this pH range
In a strong acid-weak base titration, what indicator should be used?
In a weak acid-strong base titration, what are the features of the titration curve?
-The vertical section of the graph covers a smaller change in pH, and occurs further up towards the higher pH values, starting around pH 6.5 and ending at around pH 11.5
-the equivalence point occurs at a more basic pH, i.e. a pH value above 7
-phenolphthalein has an end point that falls within this pH range
In a weak acid-strong base titration, what indicator should be used?
In a weak acid-weak base titration, what are the features of the titration curve?
-there is no real vertical section
- neither indicator has an end point near the equivalence point
-neither indicator is suitable
-an indicator would change colour gradually over a few cm^3 of base added
What is the rate of reaction?
change in concentration of a reactant or a product per unit time
What is the order of reaction?
The order with respect to a reactant is the power to which the concentration of the reactant is raised in the rate equation
What is the overall order of a reaction?
the sum of the individual orders
What is the rate constant?
-The rate constant is k
- constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation
What is meant if the order is zero with respect to a reactant?
-the rate is unaffected by changing the concentration of A
- any number to the power of 0 is equal to 1, so any reactants with order zero don't appear in the rate equation