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Energetics. What happens during a chemical Reaction?. Molecules collide with energy > than E A. Energy used to break bonds Break IN. Energy released when new bonds are made Make OUT. Exothermic Energy Diagram. Intermediate. Energy. Bonds Break Energy In. Make Bonds Energy Out.

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What happens during a chemical reaction
What happens during a chemical Reaction?

Molecules collide with energy > than EA

Energy used to break bonds

Break IN

Energy released when new bonds are made

Make OUT


Exothermic energy diagram
Exothermic Energy Diagram

Intermediate

Energy

Bonds Break

Energy In

Make Bonds Energy Out

EA

Reactants

ΔH -ve

Products


Enthalpy change h o
Enthalpy Change, ΔH O

  • The enthalpy change of a system, ΔH, is the heat energy change at constant pressure.

  • ΔH values depend on the temperature & pressure of the system so enthalpy changes are measured under standard conditions O

  • Temperature 298K (25°C)

  • Pressure 100kPa

  • EA = Activation Energy – minimum energy needed to break bonds and start reaction


Endothermic energy diagram
Endothermic Energy Diagram

Intermediate

Make Bonds Energy Out

Bonds Break

Energy In

EA

Products

ΔH +ve

Reactants


Calcuate h for the combustion of methane
Calcuate ∆H for the combustion of Methane

CH4 + 2O2→ CO2 + 2H2O

O=C=O

O=O

H – O – H

O=O

H – O – H

4 x C-H = 4 x 413

2 x O=O = 2 x 498

Total In = 2648

4 x O-H = 4 x 464

2 x C=O = 2 x 805

Total Out = 3466


O=C=O

O=O

H – O – H

H – O – H

O=O

Bonds Made

4 x O-H = 4 x 464

2 x C=O = 2 x 805

Total Out = 3466

Bonds Broken

4 x C-H = 4 x 413

2 x O=O = 2 x 498

Total In = 2648

  • (enthalpy of bonds broken)

(enthalpy of bonds formed)

H =

H = 2648 - 3466

Exothermic H –ve

H = - 818 kJmol-1


Calculating h values
Calculating ΔH values

  • Calculate the energy needed to break bonds

  • (enthalpy of bonds broken)

  • Calculate energy released when bonds are formed

  • (enthalpy of bonds formed)

  • (enthalpy of bonds broken)

(enthalpy of bonds formed)

H =

-


Experimental vs calculated h values
Experimental vs Calculated ∆H values

  • ∆H combustion of CH4

  • Calculated value = -818 kJmol-1

  • Accurate experimental value = -890 kJmol-1

  • Why different values????


Bond dissociation energy
Bond Dissociation Energy

  • The energy needed to break a particular bond (or the energy released when a bond is formed) is called the bond dissociation energy.

  • e.g. Cl – Cl → 2Cl +242kJmol-1

  • The standard molar enthalpy change of bond dissociation is the enthalpy change when one mole of the same type of bonds are broken in gaseous molecules under standard conditions


Mean bond enthalpies
Mean Bond Enthalpies

  • The Cl – Cl bond is only found in Cl2 but other bonds like C-C & C-H are found in lots of different compounds

  • For these bonds we use mean bond enthalpy values in calculations

  • Mean bond enthalpy is the mean (average) value of the bond dissociation energy of a particular type of bond over a range of different compounds.


Standard enthalpy of combustion
Standard Enthalpy of Combustion

  • ΔH Oc is the enthalpy change

  • when 1 mole of a substance is completely burnt in oxygen

  • under standard conditions

  • (298K & 100kPa)

  • All reactants & products being in their standard states (At 298K water is liquid)


Equations for h o c
Equations for ΔH Oc

ΔH Oc CH4

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

ΔH Oc C(graphite)

C(graphite) + O2(g) → CO2(g)

ΔH Oc H2

H2(g) + ½ O(g) → H2O(l)


Standard enthalpy of formation
Standard Enthalpy of Formation

  • ΔH Of is the enthalpy change

  • when 1 mole of a compound is formed,

  • under standard conditions

  • (298K & 100kPa)

  • from its elementsin their standard states.


Equations for h o f
Equations for ΔH Of

ΔH Of CH3OH

C(graphite) + 2H2(g) + ½ O2(g) → CH3OH

ΔH Of CH4

C(graphite) + 2H2(g) → CH4

ΔH Of Na2CO3

2Na(s) + C(graphite) + 3/2O2(g) → Na2CO3


Equations for h o f1
Equations for ΔHOf

  • State symbols are essential

  • Elements are in their standard states

  • Where allotropes exsist – e.g. Carbon

  • The particular allotrope should be stated

  • e.g. diamond or graphite

  • If allotrope not specified it is assumed to be the more stable form (graphite)


H o f of elements
ΔHOf of Elements

  • The standard enthalpy of formation of an element in its standard state is zero

  • Write equations for the ΔHf of the following

  • CH3CH2OH(l)

  • NH4NO3(s)

  • H2O(l)


Equations for h o f h o c
Equations for ΔH Of & ΔH Oc

ΔH Of H2O

H2(g) + ½ O2(g) → H2O(l)

ΔH Oc H2

H2(g) + ½ O2(g) → H2O(l)

The same reaction represents the

  • Enthalpy of formation of water &

  • Enthalpy of combustion of hydrogen


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