Physical Chemistry

What types of substances do lattice enthalpies apply to?

Ionic substances

Define lattice dissociation enthalpy.

The enthalpy change when breaking apart an 1 mole of an ionic lattice into its constituent ions in a gaseous state under standard conditions

Define lattice enthalpy of formation.

The enthalpy change when 1 mole of a solid ionic lattice is formed from its constituent ions in a gaseous state under standard conditions 

Define atomisation enthalpy. 

The enthalpy change when a mole of gaseous atoms is formed under standard conditions 

Define enthalpy of electron affinity?

The enthalpy change when one mole of electrons is added to a mole of gaseous atoms under standard conditions 

How does enthalpy change differ based on the direction of the arrow needed?

Changing the direction of the arrow changes the sign of the enthalpy change

Define enthalpy of solution.

The enthalpy change when one mole of ionic solid is dissolved in water to infinite dilution so that the ions no longer attract, under standard conditions 

Define enthalpy of hydration.

The enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions under standard conditions

Why is enthalpy of hydration normally negative?

Water has a delta positive region that naturally attracts negative ions

State the equation for a solution cycle.

lattice enthalpy = total hydration enthalpies - solution enthalpy

What is the perfect ionic model?

A theoretical model assuming that all ions are perfectly spherical and they have no covalent character

What 2 factors influence how easily an ionic compound dissolves?

The size and charge of the ions

Why do some ionic bonds have more covalent character than others? 

Smaller ions with a higher charge will pull electrons closer to their nucleus, distorting the electron cloud

Define entropy.

A measure of the disorder of a system

What happens to entropy as temperature increases and why?

It increases as the particles gain energy and move further apart, becoming less ordered

What state has the greatest entropy? 

Gas 

Why is the entropy change during evaporation greater than during melting?

Gases are much more disordered than liquids

What is entropy change measured in?

JK^-1mol^-1

State the equation for overall change in entropy.

Change in entropy = entropy of products - entropy of reactants

Why do all spontaneous reactions have a positive entropy?

All things tend towards a natural state of disorder 

What three factors determine whether a reaction is feasible?

Enthalpy

Entropy

Temperature

State the equation for gibbs free-energy.

change in G = (change in H) - (T x change in S)

What are the standard units for G?

kJmol^-1

What should be done to the entropy value before substituting it into the equation for G?

It should be converted into kJK^-1mol^-1

How can it be determined from the equation for G whether a reaction is spontaneous?

change in G is negative for all spontaneous reactions

How can the temperature at which a reaction becomes spontaneous be found?

Put change in G equal to 0, T = change in H/ change in S

What is the purpose of an electrochemical cell?

They use redox reactions to create a flow of electrons around a “circuit”

What is produced by an electrochemical cell?

A potential difference

What are most electrochemical cells made up of?

2 solutions with metal electrodes and a salt bridge

What is a salt bridge?

A tube of unreactive ions that can move between solutions to carry the flow of ions and balance the charge

What solution is a salt bridge normally made of?

KCl or KNO₃

Give 2 essential properties of a salt bridge?

Unreactive

Allows movement of ions

What is a cell potential?

A value that indicates whether the half-cell will react by oxidation or reduction

State the 4 rules for drawing cell diagrams.

The half-cell with the most negative potential (oxidation) goes on the left

The most oxidised species goes next to the salt bridge

A salt bridge is shown using a double-line

Always include state symbols

What is the standard hydrogen electrode (SHE)?

The measuring standard for half-cell potentials

What is the cell potential of the SHE?

0.00V

What are the standard conditions to measure the SHE potential?

1 moldm^-3

298K

100kPa

What solution is used to measure the SHE?

HCl with platinum electrodes

Why are platinum electrodes used for electrochemical cells?

They are inert but conduct electricity

What does a negative cell potential mean?

The substance is more easily oxidised and will lose electrons

What does a positive cell potential mean?

The substance is more easily reduced and will gain electrons

Give the equation to calculate the EMF of a cell composed of 2 given half cells.

EMF = E_right - E_left

Which half cell is always on the left in a conventional cell?

The one where oxidation occurs

What is an easy way to remember the emf equation?

Positive potential - negative potential

What does a positive emf value mean about the reaction taking place?

It is spontaneous and favourable

How does a more positive emf effect the favourability of the reaction?

More positive = more favourable

Describe how to use the anticlockwise rule?

1. Write the most negative emf equation on the top

2. Draw anticlockwise arrows around the equations

3. Balance

4. Write out the cell reaction

What electrode potential signifies a better oxidising agent?

A more positive one

What electrode potential signifies a better reducing agent?

A more negative one

How will an increase in concentration of solutions in a cell effect the EMF?

It will be higher as fewer electrons are produced in the reaction

What effect does an increase in pressure have on the EMF of a cell?

EMF will be more negative as more electrons are produced

Give the 3 types of commercial cells.

Non-rechargeable

Rechargeable

Fuel cells

State briefly how a rechargeable cell works?

It uses a reversible chemical reaction to produce energy, which can be reversed when an external current is applied to recharge the cell

What electrodes are used in a Lithium-ion cell?

Lithium cobalt oxide and carbon

What electrolyte is used in a lithium ion cell?

A lithium salt electrolyte

Give the half-equation for the negative electrode of a lithium ion cell.

Li —> Li⁺ + e-

Give the half-equation for the positive electrode of a lithium ion cell.

Li⁺ + e- + CoO₂ —> Li⁺[CoO₂]^-

How is a rechargeable cell recharged?

A current is applied to it forcing the electrons back in the opposite direction

Give the 2 electrode equations for a zinc-carbon cell.

negative = Zn²⁺ + 2e- —> Zn

positive = 2NH₄⁺ + 2e- —> 2NH₃ + H₂

Why are E values of commercial cells approximate?

They are not measured under standard conditions

Give the overall equation for a zinc-carbon electrode.

Zn(s) + 2NH₄⁺(aq) —> Zn²⁺(aq) + 2NH₃(g) + H₂(g)

What are some disadvantages of zinc-carbon batteries?

Ammonia and hydrogen gases cause battery to expand

Zinc casing thins

How are gaseous ammonia and hydrogen removed from non-rechargeable batteries?

MnO₂ is used to oxidise H₂ to water which dissolves the ammonia

How does a hydrogen fuel cell work?

A constant supply of H₂ and O₂ is supplied to the cell, producing a constant current flow

What electrodes are used in a hydrogen fuel cell and why?

Pt as it is inert and conducts electricity

What are the 2 main advantages of hydrogen fuel cells?

Water is the only waste product

It is more energy efficient than a combustion engine

Give the equation for the negative electrode of a hydrogen fuel cell under alkaline conditions.

H₂(g) + 2OH^-(aq) —> 2H₂O(l) + 2e-

Give the equation for the positive electrode of a hydrogen fuel cell under alkaline conditions.

O_2(g) + 4e- + 2H₂O(l) —> 4OH^-(aq)

Give the overall equation for hydrogen fuel cells.

2H₂(g) + O₂(g) —> 2H₂O(l)

What are the disadvantages of hydrogen fuel cells?

Hydrogen is flammable

The cells are expensive to produce

What are the features of a chemical reaction at equilibrium?

Forward and reverse reactions occur at the same rate
The concentrations of reactants and products remain constant

Why can dynamic equilibrium only occur in closed systems?

No substances can get in to influence the reactions

What is Le Chatelier’s principle?

If a change is made to a system in dynamic equilibrium, the position of equilibrium will move to counteract the change

How does a catalyst effect the position of equilibrium?

It has no effect, but allows it to be reached faster

How does an increase in temperature effect the yield of the reaction?

The position of equilibrium moves in the endothermic direction, increasing the yield of the endothermic products

How does an increase of the reactants’ concentration effect the yield of the reaction?

Increasing the concentration of the reactant will increase the yield of the products

When can pressure effect the position of equilibrium in a reaction?

If there are different numbers of moles on each side of the equation 

How does an increase in pressure effect the yield of a reaction using gases?

It favours the side of the reaction with fewer moles, increasing the yield of these products

Why must a compromise of pressure and temperature be reached in an industrial reaction?

A high temperature is needed to give a high rate of reaction, but can be expensive and decrease the yield in an endothermic reaction

A high pressure is needed to give a high rate of reaction and may increase yield, but can be expensive

What are the compromise conditions for the haber process?

400*

200atm

What is Kc?

An equilibrium constant that indicates the position of equilibrium for a reaction at a certain temperature

How is concentration represented in a Kc equation?

[X]

State the Kc formula for the reaction aA + bB —> cC + dD

[C]^c x [D]^d / [A]^a x [B]^b

How do concentration and catalysts effect Kc?

They have no effect

How does change in temperature effect Kc?

If a temperature increase causes an increase in products, Kc increases

How are units calculated for a Kc value?

Substitute the concentration units into the Kc expression

What condition is required for Kp to be calculated for a reaction?

All reactants and products must be in the gaseous state

Describe how to calculate the partial pressure of a substance in a reaction.

1. Find the total number of moles at equilibrium

2. Find the mole fraction of the desired substance by dividing the number of moles of it by the total number of moles

3. Multiply the mole fraction of the substance by the total pressure of the reaction 

How should partial pressure be shown in a Kp expression?

(pA)

What unit are partial pressures measured in?

Pa/KPa

State the Kp expression for the following reaction:

2A_(g) + 3B_(g) —→ Y_(g) + 2Z_(g)

Kp = (pY)(pZ)²/ (pA)²(pB)³

State what is meant by thermochemistry.

The study of heat changes during chemical reactions

What is meant by an exothermic reaction?

A reaction where energy is released to the surroundings

What is meant by an endothermic reaction?

A reaction where energy is taken in from the surroundings

If a reaction is endothermic in one direction what must happen in the other direction?

It must be exothermic

What is enthalpy change measured in?

kJmol^-1

What is the purpose of a fuel?

To provide a large heat output when burned

What is enthalpy?

Measuring a heat change at a constant pressure

What is the symbol for energy change?

(delta) H

What are the standard conditions for measuring enthalpy change?

100kPa pressure

298K temperature