QCE Senior Chemistry UNIT 3 & 4 ULTIMATE

What can be changed in a reaction to alter the equilibrium position?

Concentration, temperature, pressure/volume (only applies to gases)

What happens to the equilibrium position when the product concentration is increased?

The reverse reaction increases and equilibrium moves to the left

What does Le Chatelier's principle state?

If an equilibrium system is subjected to a change, the system will adjust itself to partially oppose the effect of the change

What happens to the equilibrium position when the reactant concentration is increased?

The forward reaction increases and equilibrium moves to the right

On an equilibrium graph how do you know the reaction has reached equilibrium?

When equilibrium is established the concentration lines are horizontal

What is meant by an "open system"?

An open system allows matter and energy to be exchanged with the surroundings

Are all reactions reversible?

No, for example due to the formation of energetically stable products or loss of products into the environment

What is a "closed system"?

A reaction vessel that does not allow the reactant or product particles to escape

Give an example of an "open" system.

A flask with no lid that allows heat and gas products escape

What type of system is it when energy can escape but not matter?

Closed system

What is meant by the term "reversible reaction"?

Reactions in which the conversion of reactants to products and products to reactants occur simultaneously.

Are physical changes reversible?

Generally they are reversible as chemical bonds remain unchanged

Why is a substance in a system more likely to react when the temperature is higher?

The particles of that substance have more kinetic energy, causing them to collide more often with the other particles in the system, making the reaction occur

How are equilibrium reactions symbolised?

Equilibrium reactions are symbolised by ⇌

What is activation energy?

The minimum amount of energy required in the collision between reactants for a reaction to occur

What can you measure using observation in equilibrium reactions?

You can observe whether the forward or reverse reaction has occurred if the reactants and products have different colours

In a reaction where a gas is produced what type of system is required for equilibrium to be reached?

A closed system

What does Kc represent?

The equilibrium constant

When is a state of dynamic equilibrium reached?

A state of equilibrium is reached when the rate of the forward and reverse reactions are equal

How can you measure the concentration of an equilibrium using pH?

You can measure the change in pH and calculate the H3O+ concentration, then use simple stoichiometry

Can forward and reverse reactions occur at the same time?

Yes, a forward and reverse reaction can occur at the same time if the reaction occurs within a closed system

What is the impact on the equilibrium position when a catalyst is added to a reaction in a closed system?

There is no impact on the equilibrium position. Equilibrium will only be reached faster

What concentrations are not included when calculating the equilibrium constant?

Concentrations of pure liquids and solids are not included, water is only included if it is a product such as in condensation reactions

Where can the elements with the lowest ionisation energy be found on the periodic table?

"Bottom left e.g. Cs, Rb, Ba"

What kind of ionisation energy makes elements lose electrons more readily?

Low ionisation energy

Is the equilibrium constant value largest when the concentration of reactants is larger than the products?

No

What energy is required to remove an electron from an element?

First ionisation energy

"True or False? The stronger the electronegativity, the more readily the element will accept electrons."

TRUE

If Kc is larger than 104 is there significantly more reactant or product?

Product

"True or false? Is the Kc of a reaction temperature dependent?"

TRUE

What does RICE stand for?

Reaction, initial concentration, change in concentration, equilibrium concentration

What happens to the equilibrium position for an endothermic reaction when heat is added?

The equilibrium position will move towards the product side as heat is stored in the molecule

What can you do to increase the concentration of gas in a closed system?

Decrease the volume

What happens when the pH of a solution is equal to the pKa of an indicator?

The indicator will change colour

What type of substance does this equation represent? BOH → B+(aq) + OH-(aq)

A strong base

What is the definition of a weak base in terms of its Kb?

Low Kb - Weak bases

What is the relationship between the pH range of an acid-base indicator and its pKa value?

A higher pKa correlates to a higher end point of the indicator

What is the definition of a weak acid in terms of its Ka ?

Ka < 1 Weak acids

What is a conjugate acid?

An acid formed when a base accepts a proton (H+) B: + H2O → B:H+ + OH- (B:H+ is the conjugate acid)

What is an amphiprotic substance?

A substance that can act as either an acid or a base

What is an example of a common amphiprotic substance?

Examples could include - water, amino acids and hydrogen carbonate ions (HCO3- )

What is a conjugate pair?

Two species that differ by a proton (H+)

Identify the conjugate acid produced in this reaction: NH4+ + OH- ⇌ NH3 + H2O

H2O

What is a conjugate base?

A base formed by removing a proton (H+) from an acid

Identify the conjugate base produced in this reaction: NH4+ + OH- ⇌ NH3 + H2O

NH3

How does the Brønsted-Lowry model define an acid and a base?

Acids as any species that can donate protons (H+). Bases as any species that accept protons (H+)

What is the conjugate base of CH3COOH?

CH3COO-

What is the conjugate acid of the following base: OH- ?

H2O

Why is water a weak electrolyte?

It can self-ionise, but equilibrium lies to the left H2O ⇌ H+(aq) + OH-(aq)

What is Kw?

Kw is the ionic product constant of water. It is the equilibrium constant for the self-ionisation of water. Kw can be used to calculate the concentration of hydrogen ions from the concentration of hydroxide ions in a solution

How is pH calculated?

The pH of a solution can be calculated from the concentration of hydrogen ions using the relationship pH = -log10 [H+ ]

How is pOH calculated?

pH = -log10 [OH-]

Under what circumstances could the numerical value of Kw differ?

Different temperatures. Kw is 10-14 at just below 25oC

What is self-ionisation of water?

Self-ionisation of water refers to the reaction in which a water molecule loses a hydrogen ion (leaving OH-) and the hydrogen ion immediately reacts with another water molecule to form a hydronium ion (H3O+ )

What is pH?

pH is a figure expressing the acidity or alkalinity of a solution on a logarithmic scale

How is [H+ ] calculated?

[H+ ]= 10^(-pH)

How is [OH- ] calculated?

[OH-] can be obtained using the pH or pOH value: pH + pOH = 14 pOH = 14 - pH pOH= --log10 [OH- ] [OH-] = 10^(-pOH)

pH + pOH = ?

14 or Kw (at just below 25oC)

How do we calculate Kw?

Kw = Ka x Kb

How do we calculate concentration?

C = n / V

What are acid-base indicators?

Weak acids or bases, which dissociate slightly in water and form ions

How are strong acids defined?

Strong acids are defined fully ionising / dissociating (Ka > 1 )

How are weak bases defined?

Weak bases are defined as having a low Kb

How do you calculate Ka?

Ka = [H3O+ ] x [A-] / [HA]

How do you calculate Kb?

Kb = [BH+ ] x [OH-] / [B]

How are weak acids defined?

Weak acids are defined as having a Ka < 1

How are strong bases defined?

Strong bases are defined as having a high Kb

What type of substance does this indicator represent? HIn ⇌ H+(aq) + In-(aq)

A weak acid

What are examples of Strong Acids?

Hydrochloric Acid (HCl) Nitric Acid (HNO3) Sulfuric Acid (H2SO4)

What are examples of Strong Bases?

All group 1 hydroxides (LiOH, NaOH, KOH, RbOH, CsOH) Barium hydroxide (Ba(OH)2)

What is the dissociation expression for a base?

Kb = [products] / [reactants]

What is the definition of a strong acid in terms of its Ka?

Ka > 1 is a strong acid

Which Acid and Bases (strong or weak) completely ionise in water?

Strong acids and bases

What are examples of Weak Acids?

Carboxylic acid (R-C(=O)-OH), carbonic acid (H2CO3), aqueous carbon dioxide (CO2 (aq))

What are examples of Weak Bases?

Ammonia (NH3), amines

What substance states are included in a dissociation expression?

Aqueous (aq)

What is the definition of a strong base in terms of its Kb?

High Kb - Strong bases

The strength of an acid is explained by the degree of ...

dissociation of the acid at equilibrium in an aqueous solution

What is the Bronsted-Lowry definition of an acid?

A proton donor

What is a diprotic acid?

An acid that can donate two protons per molecule

What is a polyprotic acid?

An acid that can donate more than one proton per molecule

Name an example of a triprotic acid

H3PO4 - Phosphoric acid

What is a monoprotic acid?

An acid that can donate one proton per molecule

Name an example of a monoprotic acid

HCl - hydrochloric acid HBr - hydrogen bromide HNO3 - nitric acid CH3COOH - ethanoic acid

Name an example of a diprotic acid

H2SO4 - sulphuric acid H2CO3 - carbonic acid

What is meant by the "strength" of an acid or a base

The level of dissociation that occurs in the acid or base.

If an acid or a base completely ionises in water what is it called?

Strong acid or base

What is hydronium?

H3O+

What does it mean if an acid is dilute?

It means it has a small number of moles of the acid in a litre of water

Why are acids and bases good electrolytes?

They can dissociate into positive and negative ions which means they can conduct an electrical charge

What type of substance has a pH of 1?

A strong acid

If an acid or a base only partially ionises in water what is it called?

A weak acid or base

What does it mean if an acid is concentrated?

It means it has a large number of moles of the acid in a litre of water

What can be tested experimentally to determine the strength of an acid or base?

The electrical conductivity of the acid or base

Which have better electrical conductivity, strong or weak acids (or bases)?

Strong acids (or bases)

What type of substance has a pH of 7?

A neutral substance

What type of substance has a pH of 13?

A strong base

What is a conjugate acid?

An acid formed when a base accepts an H+

What is the conjugate base of ammonium (NH4+)?

NH3

What does amphiprotic mean?

A molecule or ion that can donate of accept a proton. Therefore it acts as both an acid and a base.