1.7 simple equilibria and acid-base reactions Flashcards
what is dynamic equilibrium?
when the forward rate of reaction is equal to the reverse rate of reaction
what does the ‘dynamic’ mean in dynamic equilibrium?
what does the ‘equilibrium’ mean?
‘dynamic’ - means that the reaction has not stopped; it is simply moving in both direction at the same rate
‘equilibrium’ - means that the amount of reactants and products (ie conc) in the system is staying the same
what is a closed system?
one from which reactants and products cannot escape
in closed systems, the forward and reverse reactions continue until when?
until dynamic equilibrium is reached
(so all reactions in a closed system are reversible in theory, although theybare onlu considered as such is both forward and reverse reactions occur to a significant extent)
are all reactions in a closwd system reversible?
yes in theory
although they are only considered as such is both forward and reverse reactions occur to a significant extent
what is an open system?
one from which reactants and products can escape
(e.g in a fume cupbord)
why can’t reverse reactions takr place in an open system?
the products are removed as soon as theu are formed, so reverse reactions are not allowed to take place
so these reactions never reach equilibrium, but proceed until all the reactions have been converted into products
if a reaction is represented by an equilibrium sign, what is often assumed?
- the system is closed
- the reverse reaction is significant
what does Le Chatelier’s principle state?
if a chemical system at equilibrium experiences a change in concentration, temperature or pressure, the equilibrium will shift in order to minimize that change
what does le chat’s principle predict?
the direction in whcih the system will move to reestablish equilibrium
what are the 3 factors that can affect the position of equilibrium?
- concentration
- temperature
- pressure (gases only)
le chat’s principle predicts that if a reactant’s concentration in a system is increased, the equilibrium will shift to the ____ in order to decrease the concentration of that reactant
opposite side
e.g H2 (g) + I(g) <-> 2HI (g)
if more hydrogen (or iodine) is added, thr system will move to the ___
right
in order to remove the extra hydrogen
e.g H2 (g) + I(g) <-> 2HI (g)
if iodine is removed, the system will move to the ___
left
to replace the lost iodine
e.g H2 (g) + I(g) <-> 2HI (g)
if hydrogen iodide is added, the system will move to the ___
left
to remove the hydrogen iodide
e.g H2 (g) + I(g) <-> 2HI (g)
if hydrogen iodide is removed, thr system will move to the ___
right
to replace the lost hydrogen iodide
if the forwards reaction is exothermic, then the temperature of the system will rise if the forward reaction takes place. what does this make the reverse reaction?
endothermic, and the temperature of the system will fall if the reverse reaction takes place
le chatelier’s principle predicts that an increase in temperature will favour the (endothermic/exothermic) reaction?
endothermic
le chatelier’s principle predicts that a decrease in temperature will favour the (endothermic/exothermic) reaction?
exothermic
if the ∆H=0, what effect will a change in temperature have on the position of equilibrium?
no effect
2SO2 (g) + O2 (g) ⇌ 2SO3 (g) ∆H=-ve
an increase in temperature will favour the direction which decreases the temperature - ie the (exothermic/endothermic) direction
endothermic
2SO2 (g) + O2 (g) ⇌ 2SO3 (g) ∆H=-ve
(when increasing temperature) as the forwards reaction is exothermic, the reverse reaction must be endothermic, and so the reaction will proceed to the ___?
left
2SO2 (g) + O2 (g) ⇌ 2SO3 (g) ∆H=-ve
a decrease in temperature will favour the exothermic direction, so it can increase the temperature. this is the forward reaction so the reaction will proceed to the ___?
right
what does the pressure in a system depend on?
the number of gas molecules in the system
le chatelier’s principle predicts that is the pressure of the system is increased, the system will move towards the side which has (fewer/more) gas moles
has the fewer gas moles
if the pressure of thr system is decreased, the system will move towards the side which has (more/less) gas moles?
more
if the number of gas moles on both sides is the same, then what affect does pressure have on the equilibrium position?
has no effect
PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)
if the pressure is increased, the system will move to the ___
left
where there are fewer moles
PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)
if the pressure is decreased, the system will move to the ___
right
where there are more moles
what affect does the addition of a catalyst have on the position of equilibrium?
no effect on the position of the equilibrium
it will increase rhe rate of the forwards and reverse reactions, but by the same amount (equally) - the position of equilibrium will be unchanged
what do catalysts do to reactions?
it increases the rate of reaction by lowering the activation energy
however it decreases the activation enrrgy of the forward and reverse reactions by the same amount so has no effect on the position of equilibrium
what do square brackets in chemistry indicate?
using concentration
A + 3B ⇌ C + 2D
what is the equilibrium constant Kc?
Kc = [C][D]^2 / [A][B]^3
at dynamic equilibrium, the rates of the forward and reverse reactions are equal:
[A][B]^3 = [C][D]^2
if Kc>1, what direction will the equilibrium be shifted?
to the RHS
if Kc<1, what direction will the equilibrium be shifted?
to the LHS
can Kc be dimensionless?
yes
what is Kc affected by?
temperature
(it will be different at different temperstures)
(but unaffected by any change to concentration and pressure)
if you introduce a change in temp, and it favours the product side, Kc will (increase/decrease)
increase
if you introduce a change in temp, and it favours the reactant side, Kc will (increase/decrease)
decrease
what are acids defined as?
proton donors (H+ donors)
what is a proton defined as (in this topic)?
H+ (aq) ions
what are acids that can donate 1 proton per molecule called?
protic
e.g HCl ⇌ H+ + Cl-
what are acids that can donate 2 proton per molecule called?
diprotic
e.g H2SO4 ⇌ 2H+ + SO4 2-
what are acids that can donate 3 protons per molecule called?
triprotic/tribasic
e.g H3PO4 ⇌ 3H+ + PO4 3-
what are Lowry-Bronsted Acids?
type of acids
what substances can behave as an acid?
any substrance which contains hydrogen bonded to a more electronegative element can behave as an acid
what are bases defined as?
proton acceptors (H+ acceptors)
what species can behave as a base?
any species which has a lone pair of electrons can behave as a base
(via a coordinate/dative bond)
base:
NH3 + H+ ⇌
NH3 + H+ ⇌ NH4 +
base:
CO3 2- + H+ ⇌
CO3 2- + H+ ⇌ HCO3 -
base:
N2H4 + H2O ⇌
N2H4 + H2O ⇌ N2H5 + + OH-
write an equation for the reaction of ammonia acting as a base with a water molecule
NH3 + H2O -> NH4 + + OH-
write an equation for the reaction of a hydronium ion H3O +, acting as an acid with a water molecule
H3O + + H2O -> H2O + H3O +
acid base base acid
what are strong acids?
acids that fully ionise/dissociate
what is the equation for strong acids?
HA -> H+ + A-
what are some examples of strong acids?
- hydrochloric acid (HCl)
- sulfuric acid (H2SO4)
- nitric acid (HNO3)
what are weak acids?
acids that are only partially ionised/dissociated
(mainly do not ionise)
what is the equation for weak acids?
HA ⇌ H+ + A-
what does the degree of dissociation of a weak acid depend on?
(NOT AS)
its acid dissociation constant, Ka
what are some examples of weak acids?
any carboxylic acid
(ethanoic acid, citric acid)
to calculate pH, what do we need to know?
the concentration of hydrogen ions, H+ (aq) in a solution
lower temperature favours (endothermic/exothermic)?
exothermic
higher temperature favours (endothermic/exothermic)?
endothermic
if Kc>1, is there more products or more reactants?
more products
if Kc<1, is there more products or more reactants?
reactants
what is the unit for concentration?
moldm^-3
in a reaction with two acids, which acid acts as an acid and which one acts as a base?
the strongest acid acts as an acid
the weaker acid acts as a base
the more H+, the (higher/lower) the pH?
lower
to how many decimal places should you quote pH values to?
2 dps
what is the equation for working out pH?
pH = -log10[H+ (aq)]
[H+ (aq)] = 10^-pH (antilog(-pH))
what is the pH of a solution of a 0.3M (moldm^-3) solution of HNO3?
(bc strong acid)
HNO3 —> H+ + NO3 -
0.3M 0.3M 0.3M
pH = -log10(0.3)
= 0.52
what is the pH of a 0.134moldm^-3 solution of sulfuric acid?
H2SO4 -> 2H+ + SO4 2-
0.134 —> (x2) 0.268
pH = -log10(0.268)
= 0.57
what is the term ‘position of equilibrium’ used to describe?
the equilibrium state attained under a particular set of conditions
what are some important features of the equilibrium state?
- a stable state of equilibrium is only possible in a closed system
- the equilibrium state can be reached from either direction. the reaction is reversible
- equilibrium is a dynamic state - at a molecular level change is constantly taking place in both directions at equal rates. temp, pressure mass and volume are constant
does Kc tell us about how quickly equilibrium is reached?
no
it only gives information on concentrations achieved at equilibrium
what are compounds or molecules that can act as both an acid and a base called?
amphoteric
for acid equilibria, what is the equilibrium constant called? (Kc but for acid equilibria)
the acid dissociation constant (Ka)
how can equilibrium and le chat’s principle be used in industry?
so that reaction conditions are chosen to maximise the yield of a desired product
what is titration used to determine?
the unknown concentration of an acid or base by finding its neutralisation point
what can a simple titration be carried out between?
a strong acid and a strong base
what can be used to help determine the neutralisation point of the reaction in titration?
indicators
why do specific indicators have to be used for specific reactions?
as they can only indicate a pH change within a certain range
what are the 2 most common indicators used at A-Level?
- methyl orange
- phenolphthalein
when is the indicator methyl orange used for reactions?
(for a more acidic or basic neutralisation point)?
used for reactions with a more acidic neutralisation point
when is the indicator phenolphthalein used for reactions?
(for a more acidic or basic neutralisation point)?
used for reactions with a more basic neutralisation point
what is the colour change for the indicator methyl orange?
orange in acids and turns yellow at the neutralisation point
what is the colour change for the indicator phenolphthalein?
pink in alkalis and turns colourless at the neutralisation point
what does a pH titration curve show?
- how pH of a solution changes during an acid-base reaction
(pH can be measured and recorded throughout the reaction and used to plot a pH titration curve)
on a pH titration graph, when the species react, what is the neutralisation point identified as?
a large vertical section on the pH curve
what must you use in order to know the exact concentration of one of the solutions in titration?
a standard solution must be used or one that has been standardised using standard solution
what is a primary standard?
- a reagent that can be weighed easily
- it is so pure that its mass is truly representative of the number of moles contained in it
- the reagent is dissolved in a known accurate volume of water
what are the features of a primary standard?
- extremely pure
- very stable (low reactivity)
- does not absorb water easily
- high Mr in order to minimise weighing errors
what concept does double titration use?
that different indicators change colour at different values of pH and can be used to calculate the concentration of two different bases in a solution
how many indicators are used in double titration?
it’s performed in 2 stages and using 2 different indicators
- 1 indicator is added at the first stage and the 2nd indicator added at the second stage
how does double titration work? (e.g the concentrations of a mixture of NaOH and Na2CO3 can be determined by titrating with HCl) - describe the experiment
- in the first stage (phenolphthalein) titration:
NaOH + HCl -> NaCl + H20
Na2CO3 + HCl -> NaHCO3 + NaCl - in the second stage (methyl orange) titration:
NaHCO3 + HCl -> NaCl + CO2 + H2O
the second titre gives moles of NaHCO3, which is equal to the moles of Na2CO3 bc that is where the NaHCO3 came from
the moles of NaOH = moles of HCl in first titre - the moles of Na2CO3
the concentrations of both can then be calculated
how can you tell if a questions is asking about back titration?
- if 1 chemical is added in excess (lots of one)
- then with leftover doing a second reaction
name a piece of apparatus that you could use to collect and measure the volume of gas produced from a reaction?
gas syringe/ burette/ measuring cylinder
how can you tell, from the properties of the system, that equilibrium has been reached?
- when there is no change in the amount of each species present
- there is no change in the concentrations present
- the physical properties are constant
carbon dioxide is an acid gas
by considering its interaction with water, explain how carbon dioxide can behave as an acid
- (although CO2 doesn’t contain any hydrogen) it reacts with water to produce H+ ions
apart from errors in reading the burette, suggest one reason why incorrect titres may be obtained when carrying out titration. explain the effect of this error on the valur of the titre obtained
- funnel kept in burette so value of titre less, since more acid dropped into burette from funnel
- difficult to see when indicator changed colour so value of titre more, since end point overshot
- air bubble in burette so value of titre more, since acid used to fill bubble
- burette not rinsed with acid beforehand so value of titre is more, since acid solution is more dilute
Cl2 + H2O ⇌ H+ + Cl- + HClO
pale green colourless
use le chat’s principle to explain why the pale green colour disappears if sodium hydroxide solution is added
- NaOH reacts with the H+ / concentration of H+ decreases
- equilibrium shifts to right and concentration of Cl2 decreases/ concentration of HClO increases
what is the pH scale?
- a measure of acidity/alkalinity
- values below 7 are acidic, pH7 is neutral, above 7 are alkaline
an equation for the reaction of sulfur dioxide with water is shown below:
SO2 (aq) + H2O (l) ⇌ H+ (aq) + HSO3 - (aq)
use the equation to explain why sulfur dioxide is described as an acidic oxide? [1]
(when sulfur dioxide reacts with water) hydrogen ions / H+ (aq) are produced
standard solution definition
a solution whose concentration is known accurately
