5.1 rates, equilibrium and pH Flashcards
what is the formula for the rate of reaction
change in concentration / time
define rate of reaction
the change in concentration of a reactant or a product per unit time
what is the order with respect to the reactant
the power to which the concentration of a reaction is raised in the rate equation
define the rate constant, k
the constant that links the rate of reaction with the concentration of the reactants raised to the powers of their orders in the reaction
what is overall order
the sum of the individual orders
what is the equation for the rate of a reaction
change in concentration / tie
Define zero order
the rate is unaffected by the changing concentration of the reactant
define first order
the rate is directly proportional to the concentration
So, if the concentration of the reactant increases by 2 times so will the concentration
Define second order
The change in rate will be equal to the change in concentration squared
If the reactant increases by 2 times the rate increases by 2^2 = 4
What is the rate equation
Rate = k[A]x[B]y[C]z
define half life
the time taken for half the concentration of a reactant to reduce by half
What kind of concentration-time graph does a zero order rection produce
a straight-line graph with the concentration decreasing at a constant rate and the half-life decreases with time
what kind of concentration time graph does a first order reaction produce in respect to half-life
A curved graph with a constant half-life and halving of the reactant concentration occurring at equal time intervals
How can you use half-life to find the rate constant
Because first order reactions have constant half-lives the value of the half-life can be used to determine rate constant
give the equation of determining the rate constant from half-life
K = ln2/t1/2
K = natural log 2 / half life
What is a clock reaction
reactions that produce visible changes
And in these instances, the time taken for the visible event to occur is inversely proportional to the initial rate
give an example of a clock reaction
the reaction between sodium thiosulfate and hydrochloric acid
A cross is drawn and placed underneath the beaker and time taken for it to become not visible is measured
What kind of graph would you plot to see the effect of concentration on rate of reaction
1/T against concentrtion
What would a rate-concentration graph look for a zero order reaction
it would be a straight line as the changes in the concentration have no effect on the rate
What would a rate-concentration graph look like for a first order reaction
It would be a straight-line with a positive gradient
How would you determine the rate constant from first order rate-concentration graphs
k = rate / concentration
k = gradient (change in y/ change in x)
How would the rate-concentration graph look like for a second order reaction
it would be a slightly curved line with a positive gradient
What is the effect of temperature on the rate constant
the rate constant increases with temperature because the concentrations remain the same in the rate equation
What graphs are associated with Arrhenius plots
ln k -1/t graphs
What is the intercept equal to in the Arrhenius plots
it is equal to natural log A
What is the gradient equal to in the Arrhenius plots
It is equal to -Ea/ R with R as the gas constant
This can easily be rearranged to find the activation energy
Ea=-R/gradient
what is a colourimeter
monitors the concentration of a reactant or a product by measuring the amount of light passing through a substance
Define partial pressure
the pressure an individual gas in a substance would exert if it occupied a whole reaction vessel on its own
describe how you would figure out the partial pressure exerted by a gas
mole fraction * total pressure
what is Kp
The equilibrium constant for reactions involving gasses
What is the equation for Kp
(Pc)(Pd)/(Pa)(Pb) the multiplication sum partial pressure of the products divided by the multiplied partial pressures of the reactants
How do changes in pressure affect K
They have no effect
What is a Bronsted- Lowry acid
A proton/H+ donor
What is a Bronsted-Lowry base
A proton acceptor
What are conjugate acid-base pairs
A set of two species that transform into each other through the gain or loss of a proton
What is the definition of an alkali
A base that dissolves in water forming OH- ions
Define neutralisation
A chemical reaction in which an acid and base react together to form a salt and water
Define strong acid
An acid that completely dissociates in solution
Define weak acid
an acid that partially dissociates in water
Define Ka
The acid dissociation constant
What happens when a metal and acid react
salt and hydrogen are formed
What happens when a base and acid react
A salt and water formed
hat happens when a carbonate and acid react
salt water and carbon dioxide formed
What is the ionic equation for the reaction between an acid and carbonate
2H+ +CO3 2- –> CO2 +H20
What is the ionic equation for the reaction of alkali and acid
H+ +OH- –> H20
what is the ionic equation for the reaction of an acid and base (MgO)
2H+ +MgO –> Mg2+ H20
What is the equation for Ka
[H+][OH-] / [HOH]
What are the units of Ka
moldm-3
Describe the relation between pKa and pH
The smaller the pKa value the stronger the acid
How do you turn pKa into Ka
10**-pKa
How do you find the H+ ion concentration from pH
10**-pH
How do you find the pH from [H+]
-log([H+])
What do you assume when calculating the pH of a strong acid
That the monobasic acid has virtually complete dissociation in water so the [H+] ion concentration is equal to the concentration of the acid
How do you calculate the pH of a strong acid
-log(H+)
H+ = HOH
What do we assume when calculating the pH of a weak acid
that the concentration of the acid at equilibrium is the same as the concentration of it at the start
That the concentration of H+ and OH- is equal
How do you calculate the pH of a weak acid
Ka = [H+]**2 / [HOH]
[H+] = square root of Ka * HOH
pH = -log ([H+]
What is Kw
The ionic product of water
What is the value of Kw
at room temperature and pressure it is 1*10**-14
What is the equation of Kw
Kw =[H+][OH-]
Explain how water acts as an acid and base
This is because it can donate a proton like an acid (H20 ->H+ +OH-)
It can also accept a proton like a base (H20 + H+ -> H3O+)
How do you calculate the pH of strong bases
[H+] = Kw / [OH-]
pH = -log [H+]
What do we assume when calculating the pH of strong bases
That there is complete dissociation
Define a buffer solution
A mixture that minimises pH changes on addition of small amounts of acid or base
What is a buffer solution a mixture of
A weak acid and its conjugate base
Describe how a solution can act as a buffer
The weak acid dissociates partially whereas the salt dissociates completely generating the conjugate base
The remaining mixture contains high concentrations of the undissociated acid and its conjugate base
The high concentrations of the conjugate base push the concentration of the conjugate base pushes the equilibrium to the left so the concentration of H+ ions is very smal
How does a buffer act to control pH
The weak acid removes added alkali and the conjugate base removes added acid
Describe how equilibrium shifts in a buffer solution when acid is added
The [H+] is increases and the conjugate base reacts with the H+ ions the equilibrium shifts to the left removing most of the H+ ions
Describe how equilibrium shifts when alkali is added in a buffer solution
The small concentration of H+ ions reacts with the OH- ions
The weak acid dissociates shifting the equilibrium to the right to restore most of the H+ ions that have reacted
define equivalence point
point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution
centre of vertical section on curve
Define end point
in a titration where the concentrations of weak acid and conjugate base forms of the indicator are equal
vertical section of the graph
Describe how you would use a pH probe
calibrate with buffers such as pH 7 and rinse with distilled water and dry before use
place it in the storage solution when finished
What happens to the equilibrium of the indicator in a low pH
equilibrium is pushed left and is colour 1
What happens to the equilibrium of the indicator in a high pH
equilibrium is pushed right and is now colour 2
Describe the colours of methyl orange in a reaction
red in a weak acid and yellow in its conjugate base and orange at its end point
How would you pick an indicator for a titration
match equivalence point to indicator end point
Describe the colours of phenolphthalein during a reaction
colourless at low pH and purple at a high pH
What indicator would be suitable for a strong acid and base titration
either indicator can be used as both indicators have end points that lie within the vertical sections of the graph
What indicator would be suitable for a strong acid-weak base titration
vertical section of the graph covers a smaller change in pH and only methyl orange can be used as it has an end point that falls within the range unlike phenolphthalein
