Energetics

5.1 Endothermic and Exothermic ReactionsEndothermic rxn heat is taken in from the surroundings (rxn vessel gets cooler)

Exothermic rxn rxn that result in the release of heat (rxn vessel gets warmer)

Enthalpy change amount of heat energy taken in/ given out in a chemical rxn (ΔH)

Positive for endo. Rxn

Negative for exo. rxn

Exothermic Rxn

Endothermic Rxn

Stability

Exo. produces a more stable productEx. Cdiamond Cgraphite ΔH= -1.9 kJ mol-1

Kinetic vs. Thermodynamic stabilityGraphite is more thermodynamically more stable than diamondDiamond is kinetically stableWhat does this mean?

Activation Energy

Define Activation Energy

Activation energy

High AE= faster or slower rxn?

Can the conditions be altered?

If yes, in what ways?

Does how endo- or exothermic a rxn is tell us how quickly the rxn will run?

Ex. Which rxn will run faster? -52 kJ mol-1

-16 kJ mol-1

Do endo. or exo. rxns usually occur spontaneously? (under normal conditions)

5.2 Calculations of enthalpy changes from

experimental dataSpecific heat capacity (c) energy required to raise the temp. of 1 g of substance by 1 K (1°C) or, the energy to raise 1 kg of substance by 1 K.

Units-> J g-1 K-1

J g-1 °C-1

kJ kg-1 K-1

J kg-1 K-1

Calculating c

q= mcΔTq= heat energym= massΔT= change in temperature

How does heat capacity affect how easily a substance can be heated?

Can c be calculated for a substance undergoing cooling?

Measuring enthalpy change of combustion

rxnsWorked ex. Page 185

Literature value for ΔH ethanol is -1371 kJ mol-1

What are some reasons the calculated value is different?

A bomb calorimeter could have been used so that the system was heavily insulated and provide a plentiful supply of oxygen

Enthalpy changes in solution

General method for measuringMeasure known amounts of reagentsRecord initial tempsMix in a polystyrene cupRecord max/ min temperatures observed

Assume that c for the final solution is the same as water

ΔH of Solutions:Definitions

Enthalpy change of neutralisation (ΔHn) enthalpy change when 1 mol of water molecules are formed when acid reacts with alkali under standard conditions

H+(aq) + OH-(aq) H2O(l)

Enthalpy change of solution (ΔHsol) the enthalpy change when 1 mol of solute is dissolved in excess solvent to form a solution of ‘infinite dilution’ under standard conditions

NH4NO3(s) NH4+(aq) + NO3

-(aq)

5.3 Hess’s Law

The enthalpy change accompanying a chemical reaction is independent of the pathway between the initial and final states

What does this mean?

Ex. Find ΔHr for the reaction of AB

Knowns:ΔHr= ΔH1 +ΔH2

AC =ΔH1

BC= ΔH2

What is C?What needs to change about the BC step?

Hess’s law:Definitions

State function pathway does not matter

Standard conditions pressure= 1 atm (or, 1.01E5 Pa), 298K (or, 25°C)

Standard enthalpy change (ΔHrΘ) the

enthalpy change when molar amounts of reactants as shown in the stoichiometric equation react together under standard conditions to give products (Θ= under standard conditions)

Working out enthalpy changes

Hess’s law can be used to determine enthalpy changes of unknows from knowns

P. 194 Worked example

5.4 Bond enthalpies

The enthalpy change when 1 mole of covalent bonds, in a gaseous molecule, are broken under standard conditions (aka bond energy)

Ex. Then enthalpy of H-H bond is 436 kJ mol-1

H2(g) 2H(g) ΔHΘ= +436 kJ mol-1

How many H-H bonds were broken?

Bond Enthalpy

What state must a substance be in to calculate bond enthalpy?

Consider this process:Br2(l) 2Br(g) Br-Br= 193 kJ mol-1

What is the ΔHΘ? +224 kJ mol-1

Why is this higher than the bond enthalpy?The reactants are not in a

gaseous stateWe must also account for the energy required for vaporisation of the reactantsThis process is called atomisation

Bond breaking

Bond breaking is…

endothermic or exothermic?

Endothermic! What does that mean about ΔH?

Positive! What will bond making be?

Exothermic with a negative ΔH

Average bond enthalpy

The average amount of energy required to break 1 mole of covalent bonds, in a gaseous molecule under standard conditions

These are the values used to calculate bond enthalpies

Using bond enthalpies to work out enthalpy

changes in a rxn1. Must draw out the structural formulas for rxn

2. Imagine the rxn happening and ALL bonds being broken1. Add up the total energy for all broken bonds

3. Draw in all the bonds formed in products1. Add up the total energy of all bonds made

4. Determine signs for the total enthalpy changes 1. Broken positive2. Made negative

5. Add the changes to get the overall enthalpy change of the rxn

Example

Consider the rxn between ethene and bromine, to produce 1,2-dibromoethane,

C2H4(g) + Br2(g) C2H4Br2(g)

What bonds are broken?

What bonds are made?

Follow your steps!

Using a cycle

Same concept as previous calculations, but a process is drawn out to see all the steps

P. 203

HL2

5.5 Calculating enthalpy changes:

DefinitionsStandard enthalpy change of combustion (ΔHc

Θ) the enthalpy change when 1 mole of a substance is completely burnt in oxygen under standard conditions.

If ΔHcΘ is always negative, what does this

mean?

Standard enthalpy change of formation (ΔHf

Θ) the enthalpy change when 1 mole of the substance is formed from its elements in their standard states under standard conditions

Endo and exo rxns are dependent on the type of substanceΔHf

Θ for any element in its standard state is zero

Using ΔHcΘ to calculate

enthalpy changeMethod 1: Construct an enthalpy cycle

P. 208

Method 2: rearrange the equations to give the overall equations related to the enthalpy change

P. 209

Method 3: use an enthalpy level diagram for calculations

P. 210

Method 4: use the equation,

ΔHr = ΣΔHc (reactants)- ΣΔHc (products)

Using ΔHfΘ to calculate

other enthalpy chages Method 5: similar to method 1, but used for formation rather than combustion

P. 214

Method 6: refer method 2 (be sure equations are running in the correct direction)

Method 7: draw enthalpy level diagram for formation (method 3)

Method 8: use the equation,

ΔHr = ΣΔHf (products)- ΣΔHf (reactants)

Choosing your method

Choose a method based on the data you are given, NOT on what needs to be found

If needing the enthalpy of combustion and given the enthalpy of formation, use one of the methods 5-8

Once the basic principle of the methods are understood, there is no need to have any distinctions between them

5.6 Enthalpy changes for ionic compounds

First ionisation energy

Second ionisation energy

First electron affinity enthalpy change when one electron is added to each atom in 1 mol of gaseous atoms under standard conditions (always EXOTHERMIC)

X (g) +e- X- (g)

Second electron affinity (always ENDOTHERMIC) why?

Lattice enthalpy(ΔHΘlatt) the enthalpy change

when 1 mol of an ionic compound is broken apart into iest constituent gaseous ions under standard conditions

Born-Haber cyclesEnthalpy level diagram breaking down the formation of an ionic compound into a series of simpler steps

1. put the equation for the enthalpy of formation

2. add lattice enthalpy

3. convert to gaseous form (why?)Two steps

Must convert ALL reactants to gaseous form

Connect the cycle by adding the electrons removed from one reactant to the more electronegative reactant

Draw a Born-Haber cycle

P. 219-221

Na and Cl example

Comparisons of lattice enthalpy

P. 224

What is lattice enthalpy the result of?

Electrostatic attractions of + and – ions

If the attractions of great, will more or less energy need to be supplied to break the bonds?

More

Effect of charge and size

How does the charge of the ions effect lattice enthalpy?

The higher the ion charge, the greater the lattice enthalpy

• Does NaCl or MgCl2 have great lattice enthalpy?

MgCl2

• How does size effect lattice enthalpy?

The larger the ions the weaker the forces, the smaller the lattice enthalpy

• Which has the larger lattice enthalpy, CsCl or NaCl?

NaCl

Theoretical vs. experimental

Theoretical assumes a totally ionic model

What is this?Bonding is solely due to attractive forces between oppositely charged ions

Experimental use the Born-Haber cycle to find

These are compared to determine how ionic a particular compound is

How to use theoretical and experimental

valuesIf values are exactly the same, complete ionic bonding is suggested

If values are significantly different, it is suggested that the bonding has a significant degree of covalency

Ex. Silver iodide

Theoretical value/ kJ mol-1 736

Experimental value/ kJ mol-1 876

What do the values suggest?

Covalent character

What is covalent character the result of?

Polarisation of the negative ion by the positive one

How does size of the anion effect this?

The polarisation effect is greater

5.7 ENtropY A measure of randomness or disorder of a system

Especially significant in the case of endothermic processes occurring at standard conditions (ice melting at room temp, water evaporating, NaCl dissolving in water etc)

Endo rxns can only occur if there is an increase in entropy

Represented by S

Units: J K-1 mol-1

Standard entropy

Represented by SΘ

Positive ΔSΘ indicates increased entropy

Less order

Negative ΔSΘ indicates decreased entropy

More order

Predicting sign of entropy change

Which state of matter has the higher entropy?

Gases

Which state has the least entropy?

Solids

Period 2 entropies

Element

Li Be B C N2 O2 F2 Ne

State

Solid

Solid

Solid

Solid

Gas

Gas

Gas

Gas

SΘ / J K-1 mol-1

29 10 6 6 192

205

203

146

Predicting sign

Must consider whether the system’s disorder increases or decreases

Good to consider whether moles of gas have increased or decreased

What would an increase in moles of gas mean?

If moles of gas remain constant, our prediction of a change in entropy would be approximately zero

Calculating entropy change for a rxn

ΔSΘ= ΣSΘproducts -- ΣSΘ

reactants

5.8 Spontaneity

Spontaneous reaction one that occurs without any outside influence

Predicting spontaneity

A reaction being spontaneous does not mean it will run quickly!

Whether a rxn is spontaneous or not under a certain set of conditions can be deduced by looking at the change in the “entropy of the Universe”

ΔSUniverse = ΔSsuroundings + Δssystem

If ΔSUniverse is positive, the entropy of the universe increases and the rxn occurs spontaneously

When heat is given out in a rxn, the entropy of the surroundings get hotter

Gibbs free energy

Represented by ΔGAlso called free energy change

ΔG = ΔH – TΔS

ΔH and ΔS are referring to the system

Under standard conditions this symbol is used: ΔGΘ

For a reaction to be spontaneous, ΔG for the rxn must be NEGATIVE

Units kJ mol-1

Calculating ΔG

Method 1 use ΔGΘ = ΔHΘ – TΔSΘ

Method 2 use ΔGΘ = ΣΔGfΘ(products) –

ΣΔGfΘ(reactant)

Standard free energy of formation the free energy change for the formation of 1 mol of substance from its elements in their standard state under standard conditions

Temp must be in K

If no temp is given and it is standard conditions, assume 298K

Non-spontaneous rxns

If a rxn is non-spontaneous, does that mean it will never happen?

NO

What will make it run?

Outside influence such as; temp, catalysts, etc.

Effects of temp on spontaneity

Refer to the equation:

ΔG = ΔH – TΔS

If ΔS is positive, temp must be high or low to be spontaneous?

High

ΔG must be negative to be spontaneous, thus TΔS must be higher than ΔH

Temp and spontaneity

If ΔS is negative --TΔS will be positive and the rxn cannot be spontaneous

Endo rxns will only occur spontaneously in entropy is increased and temp is significantly high

Exo rxns will always be spontaneous at some temp

If the rxn involves a decrease in entropy the rxn will be spontaneous at a lower temp; becomes less spontaneous as temp increases

Signs ΔH ΔS --TΔS ΔG Spontaneou

s?-- + -- Negative At all temps

+ + -- Becomes more neg as temp increases

Becomes more spontaneous as temp increases

-- -- + Becomes less neg as temp increases

Becomes less spontaneous as temp increases

+ -- + Positive Never