Unit 4
Physical Chemistry
, Section 1
Thermodynamics
, Energetics
• Enthalpy ( ) – Heat energy transferred in a reaction at a constant pressure.
Units are kj mol-1. Reactants and products are in their standard states, and
measurements are taken under standard conditions (298K, 100kPa).
• Exothermic – Bonds are formed, giving energy out to the surroundings, causing
the temperature to increase. Products will have less energy than the reactants.
Enthalpy change is negative. An example is respiration (oxidation of glucose).
• Endothermic – Bonds are broken, energy is taken from the surroundings, causing
the temperature to decrease. Products will have more energy than the reactants.
Enthalpy change is positive. An example is photosynthesis (energy from sun).
,Enthalpy Diagrams
Taken from Theory pages - Labster
, Bond Enthalpy
• We know that atoms within molecules are held together by strong covalent bonds.
Energy is required to break those bonds, and energy is released when new bonds are
formed.
• Bond enthalpy is the energy required to break 1 mol of a certain covalent bond in the
gaseous state.
• However, bond enthalpies can alter for a specific bond depending on where that bond
is (C-C bond in methane will require a different amount of energy to break than in
ethane, propane, etc.).
• Therefore, we use mean bond enthalpies, which is the energy needed to break 1 mol of
a particular bond, averaged over a range of compounds, in their gaseous states.
, Enthalpy Change
• When reactions occur, bonds are broken and bonds are formed. Therefore, there
will be both endothermic and exothermic reactions.
• Whether the enthalpy change is positive or negative depends on which reaction
requires more energy.
• 1)
Taken from CGP Books
, Calculating Enthalpy Change Using
Mean Bond Enthalpies
• Mean bond enthalpies are used to measure the enthalpy change for a reaction.
• There are 2 things to note;
• Reactants/products must be in their gaseous state
• The value will be less accurate than if we were to use enthalpy of formation
data.
• Equation is = Sum of (enthalpies of bonds broken) – Sum of (enthalpies of
bonds formed).
, Worked Example
First step, work out the bonds in each molecule (draw molecules out).
Then, work out the enthalpies of the bonds broken and bonds formed.
Finally, use the equation to work out the enthalpy change.
Is the reaction endothermic or exothermic?
Taken from CGP Books
Equation is = Sum of (enthalpies of bonds broken) – Sum of (enthalpies of bonds formed).
,
, Questions
1) Work out the enthalpy change for the reaction
H2 + Cl2 → 2HCl
2) NOT TO COMPLETE, just to see the types of questions further
into A-level Chemistry that require drawing out!!!
CH3CH3 + 2Cl2 → CH2ClCH2Cl + 2HCl
Taken from CGP Books
Physical Chemistry
, Section 1
Thermodynamics
, Energetics
• Enthalpy ( ) – Heat energy transferred in a reaction at a constant pressure.
Units are kj mol-1. Reactants and products are in their standard states, and
measurements are taken under standard conditions (298K, 100kPa).
• Exothermic – Bonds are formed, giving energy out to the surroundings, causing
the temperature to increase. Products will have less energy than the reactants.
Enthalpy change is negative. An example is respiration (oxidation of glucose).
• Endothermic – Bonds are broken, energy is taken from the surroundings, causing
the temperature to decrease. Products will have more energy than the reactants.
Enthalpy change is positive. An example is photosynthesis (energy from sun).
,Enthalpy Diagrams
Taken from Theory pages - Labster
, Bond Enthalpy
• We know that atoms within molecules are held together by strong covalent bonds.
Energy is required to break those bonds, and energy is released when new bonds are
formed.
• Bond enthalpy is the energy required to break 1 mol of a certain covalent bond in the
gaseous state.
• However, bond enthalpies can alter for a specific bond depending on where that bond
is (C-C bond in methane will require a different amount of energy to break than in
ethane, propane, etc.).
• Therefore, we use mean bond enthalpies, which is the energy needed to break 1 mol of
a particular bond, averaged over a range of compounds, in their gaseous states.
, Enthalpy Change
• When reactions occur, bonds are broken and bonds are formed. Therefore, there
will be both endothermic and exothermic reactions.
• Whether the enthalpy change is positive or negative depends on which reaction
requires more energy.
• 1)
Taken from CGP Books
, Calculating Enthalpy Change Using
Mean Bond Enthalpies
• Mean bond enthalpies are used to measure the enthalpy change for a reaction.
• There are 2 things to note;
• Reactants/products must be in their gaseous state
• The value will be less accurate than if we were to use enthalpy of formation
data.
• Equation is = Sum of (enthalpies of bonds broken) – Sum of (enthalpies of
bonds formed).
, Worked Example
First step, work out the bonds in each molecule (draw molecules out).
Then, work out the enthalpies of the bonds broken and bonds formed.
Finally, use the equation to work out the enthalpy change.
Is the reaction endothermic or exothermic?
Taken from CGP Books
Equation is = Sum of (enthalpies of bonds broken) – Sum of (enthalpies of bonds formed).
,
, Questions
1) Work out the enthalpy change for the reaction
H2 + Cl2 → 2HCl
2) NOT TO COMPLETE, just to see the types of questions further
into A-level Chemistry that require drawing out!!!
CH3CH3 + 2Cl2 → CH2ClCH2Cl + 2HCl
Taken from CGP Books
