Unit 3: Topic 1 Flashcards
Reactions that can only occur in the forwards direction are called…
Irreversible reactions
Define equilibrium.
Equilibrium occurs when the rate of the forwards and reverse reactions are equal.
What does the position of equilibrium describe?
The ratio of reactants to products
Why are some chemical reactions irreversible?
They have an extremely high activation energy
Describe an ‘open system’.
Allow matter and energy to be exchanged with surroundings
Describe a ‘closed system’.
Allow energy, but not matter, to be exchanged with surroundings
Explain why equilibrium is considered “dynamic”.
Dynamic means constant change. At equilibrium reaction is still occurring.
What type of system (open/closed) can equilibrium occur in?
Closed system
What is happening to the concentration of reactants and products in a system at equilibrium?
There’s no change in their concentration
If a system is at equilibrium, describe what you would macroscopically observe.
There would be no observable change
Describe what is occurring on an atomic/molecular level during dynamic equilibrium.
Forwards and reverse reaction occurring
Constant molecular change
Explain why the vapour pressure of a liquid (e.g. water) is at equilibrium in a closed system.
In an open system gas would escape meaning equilibrium wouldn’t be reached
On a concentration-time graph, what is plotted on the x-axis and what is plotted on the y-axis?
x-axis = time y-axis = concentration
As a system approaches equilibrium, does the rate of the forwards increase or decrease? Why?
Decrease, because the concentration of reactants is decreasing
As a system approaches equilibrium, does the rate of the reverse increase or decrease? Why?
Increase, because the concentration of products is increasing
On a concentration-time graph, what feature indicate that a system is at equilibrium?
The straight line which shows no change in concentration
List three factors that, if altered, can shift the position of equilibrium.
Concentration, temperature, volume/pressure
State Le Chatelier’s Principle.
If an equilibrium system is subjected to a change, the system will adjust itself to partially oppose the effect of the change
In a reversible reaction, increasing the concentration of reactants will shift equilibrium to the right. Explain why using collision theory.
It increases the number of collisions that occur in the reaction
On a concentration-time graph, an increase (or decrease) in ALL species indicates that ______ was changed.
Volume
Changing the pressure of a gaseous system can shift equilibrium. Explain what feature of a reaction determines whether it shifts left or right.
The number of gas particles in the reactants/products. The number of moles of gas of either side of the equations shows this.
How will the addition of an inert gas affect the equilibrium of a gaseous system?
No effect on the equilibrium, it doesn’t change concentrations
How will diluting an aqueous system affect the position of equilibrium? (What determines whether it subsequently shifts left or right)
It decreases concentration of all species
Position of equilibrium would move to the side with more particles
How does the addition of a catalyst affect the position of equilibrium?
Catalyst doesn’t affect the position of equilibrium
If the temperature of a system is changed, what feature of a chemical reaction will determine whether the equilibrium shifts left or right?
Whether or not its exothermic or endothermic
Explain why increasing the temperature of an exothermic reaction will shift equilibrium to favour the reverse direction.
There’s more activation energy available to complete the reverse reaction
On a concentration-time graph, what type of ‘change’ causes a gradual shift in the concentration of reactants/products?
Change in temperature
State the formula for the equilibrium expression (aka equilibrium law).
aA+bB⇌ cC+dD
Kc= ([C]^c [D]^d
[A]^a [B]^b )
Define a heterogeneous equilibrium system.
Reactants and products are a mixture of phases
Hetero = different states of matter
Define a homogenous equilibrium system.
Reactants and products are in the same phase
Homo = same states of matter
In a heterogeneous equilibrium system, which ‘states’ are included in the equilibrium expression?
Gas and aqueous
solids and liquids aren’t included
In a homogeneous equilibrium system, which ‘states’ are included in the equilibrium expression?
ALL states. (Every species).
If K»_space; 1, what does this indicate about the position of equilibrium?
Much bigger (10^4) Reaction favours the products (right)
If K = 1, what does this indicate about the position of equilibrium?
Same amount of products and reactants (can be slightly different)
If K «_space;1, what does this indicate about the position of equilibrium?
Much smaller (10^-4) Reaction favours the reactants (left)
What is the “reaction quotient, Qc” used for?
It’s the value of the equilibrium expression calculated at any time during the reaction
What is the formula for Qc?
aA+bB⇌ cC+dD
Qc= ([C]^c [D]^d
[A]^a [B]^b)
What concentrations can be substituted into Kc?
The concentration at equilibrium
What concentrations can be substituted into Qc?
The concentrations from any time in the reaction
If Qc < Kc, which direction will the system proceed in to reach equilibrium?
System shifts to the right to produce more products and reach equilibrium
If Qc = Kc, what does this tell us about the system?
System is at equilibrium
If Qc > Kc, which direction will the system proceed in to reach equilibrium?
System shifts to the left to produce more reactants and reach equilibrium
What factor/s affect the value of Kc?
Temperature
If a chemical equation is reversed, how is the value of KC affected?
The Kc value is inverse
If the coefficients of a chemical equation are doubled, how is the value of KC affected?
The Kc value is squared
If the coefficients of a chemical equation are halved, how is the value of KC affected?
The Kc value is the square root of the original value
What does a RICE table stand for? (R = …, I = …)
R = Reaction I = Initial Concentration C = Change/Coefficients E = Equilibrium Concentration
What assumption can be made when the value of KC is very small?
[Reactants]initial ≈ [Reactants]equilibrium
Define a Bronsted-Lowry acid.
A substance behaves as an acid when it donates a proton
Define a Bronsted-Lowry base.
A substance behaves as a base when it accepts a proton
Define a monoprotic acid.
Donates only one proton per molecule on dissociation
Give 2 examples of a monoprotic acid.
- HCl
- HBr
- HNO3
- CH3COOH (acetic acid/ethanoic acid – the acid in vinegar)
Define a polyprotic acid.
Donates more than one proton per molecule
- Diprotic acids donate 2 (H2SO4)
- Triprotic acids donate 3 (H3PO4)
Define a strong acid.
Fully ionised (dissociates into H+ and anion)
Define a weak acid.
Partially ionised
Define a strong base.
Fully dissociates
Define a weak base.
Partially dissociates
When writing the dissociation reaction of a strong acid, what arrow is used in the equation?
–>
When writing the dissociation reaction of a weak acid, what arrow is used in the equation?
⇌
Does a strong or weak acid have a higher electrical conductivity? Why?
Strong acids have a higher electrical conductivity, its fully dissociated meaning more ions present
Distinguish between concentration and strength.
Concentration:
- Amount (in moles) of solute per unit of volume (litres)
- concentrated and dilute
Strength:
- Its ability to ionise in a solution
- Strong and weak
Does low pH mean an acid is strong?
Low pH doesn’t mean it’s a strong acid, it could be a weak acid but concentrated
What are the strong acids?
Important: Hydrochloric acid Hydrobromic acid Hydroiodic acid Sulfuric acid Nitric acid
Others:
chloric acid
perchloric acid
What are the strong bases?
Potassium hydroxide (KOH) Sodium hydroxide (NaOH) Barium hydroxide (Ba(OH)2) Caesium hydroxide (CsOH) Sodium hydroxide (NaOH) Strontium hydroxide (Sr(OH)2) Calcium hydroxide (Ca(OH)2) Lithium hydroxide (LiOH) Rubidium hydroxide (RbOH)
What are the rules for naming acids?
AHHHHHHHHHHHHHHH
What is a conjugate acid?
The acid that is formed when a base accepts a proton from an acid
What is a conjugate base?
The base that is formed when an acid donates a proton to a base
What is the conjugate base of HCl?
Cl^-
What is the conjugate base of HNO3?
NO3^-
What is the conjugate acid of OH-?
H2O
If I have a strong acid, is the conjugate base strong or weak?
Weak
If I have a weak base, is the conjugate acid strong or weak?
Strong
Which acid would produce a stronger conjugate base: Nitric acid or methanoic acid?
Methanoic acid (a weak acid) would produce a stronger conjugate base
Define “amphiprotic”.
A substance that can gain or lose a hydrogen ion to act as an acid or base
Explain how H2PO4- is amphiprotic.
It can give or gain a proton
Acid: H2PO4^- + H2O ⇌ HPO42^- + H3O^+
Base: H2PO4^- + H2O ⇌ H3PO4 + OH^-
Explain what a buffer solution is.
A solution containing both a weak acid/base and its conjugate; a solution that can partially resist changes in pH
A buffer solution is an aqueous solution consisting of a mixture of either ___ or ____.
A weak acid and its conjugate base
A weak base and its conjugate acid
Explain how a buffer solution is able to resist changes in pH.
The reaction will produce whatever side of the reaction which has the opposite (eg acid = base, base = acid)
Explain how the H2CO3/HCO3- buffer system acts as a buffer.
H2CO3 is a weak acid and HCO3^- is its conjugate base
What is the generalised formula for reactions of acids and water?
HA + H2O ⇌ A^- + H3O^+
Would a strong acid have a small or large Ka value?
Large (products favoured)
Would a weak acid have a small or large Ka value?
Small (reactants favoured)
Recall the formula for the acid dissociation constant.
Ka=[H3 O^+][A^-]
[HA]
Is hydrochloric acid a strong or weak acid?
Strong
Ka = 1.3 x 10^6
Is nitric acid a strong or weak acid?
Strong
Ka = 2.4 x 10^1
Is sulfuric acid a strong or weak acid?
Strong
Ka = 1.0 x 10^3
Are carboxylic acids strong or weak acids?
Weak
Ka = 1.0 x 10^-4
Is carbonic acid a strong or weak acid?
Weak
Ka = 4.4 x 10^-7
Recall the formula for the base dissociation constant.
Kb =[BH^+ ][OH^-]
[B]
Would a strong base have a small or large Kb value?
Large (reactants favoured)
Would a weak base have a small or large Kb value?
Small (products favoured)
Is sodium hydroxide a strong or weak base?
Strong
Are group 1 hydroxides strong or weak base?
Strong
Is barium hydroxide a strong or weak base?
Strong
Is ammonia a strong or weak base?
Weak
Are amines a strong or weak base?
Weak
Recall the formula for the percentage ionisation of an acid.
= [A^-]
[HA] x 100%
Recall the formula for convert a Ka value into a pKa value.
pKa=-log10 (Ka)
Would a strong acid have a small or large pKa value?
Small
Would a weak acid have a small or large pKa value?
Large
Recall the formula for convert a Kb value into a pKb value?
pKb=-log10(Kb)
Recall the formula that relates pka to pkb.
14= pKa + pKb
