12.4A Energetics & Thermodynamics Flashcards
Bond energy/Bond dissociation energy
— the enthalpy change when 1 mole of covalent bonds between two atoms are broken in gaseous molecules under standard conditions of 298K and 1atm
- Bond energy is always endothermic (E > 0) as energy must be absorbed in order to break bonds
- Bond energy is a measure of covalent bond strength
calculate Enthalpy change (ΔH)
∆𝐻_𝑟 = ∑ enthalpy change for bonds broken + ∑ enthalpy change for bonds formed
∆𝐻_𝑟 = ∑ [bond energy of products] − ∑ [bond energy of reactants]
∆𝐻_𝑟 = – Q / n
Q: State the meaning of the term mean bond enthalpy for the X—X bond
The enthalpy change to break 1 mol of X—X bonds, averaged over a range of compounds/molecules
Enthalpy, H
- The total heat content of a substance
- It cannot be measured directly
- Energy is used (absorbed from the surroundings e.g. melting, boiling) when bonds are broken
- Energy is released (into the surroundings) when bonds are formed
Enthalpy change, ∆H
— The amount of heat absorbed or evolved in a chemical reaction
∆𝐻 = (Total enthalpy of products)−(Total enthalpy of reactants)
∆H = - Q / n
Calorimetry
— is a technique used to measure enthalpy change (by using calorimeters ex. polystyrene cup, a vacuum flask or metal can)
Calorimetry – apparatus & substances required:
- Known amounts of reactants
- Known volume of liquids
- Thermometer (accurate to 0.1°C or 0.2°C)
- Insulated container (with lid if necessary)
- Stopwatch (sometimes)
- Glass rod (sometimes)
Calorimetry – underlying principles
Calorimetry relies on the fact that 4.18 J of energy is required to increase the temperature of 1 g of water by 1°C (specific heat capacity, c J g-1 K-1)
amount of heat Q = mc∆T
∆H = - Q / n
Examples of reasons for the different values obtained during calorimetry
- heat loss/not all heat transferred to the apparatus/heat absorbed by the apparatus
- incomplete combustion/not completely burned/reaction is not complete
- reactants and/or products may not be in standard states
- MBE do not refer to specific compounds/MBE values vary with different compounds/molecules
- are average/mean values taken from a range of compounds/molecules
- Standard enthalpy change of FORMATION, (∆𝑯_𝒇)^𝜽
— the enthalpy change when 1 mole of a pure compound is formed from its constituent elements in their standard states, under standard conditions of 298K and 1atm
!!! (∆𝑯_𝒇)^𝜽 of elements are assigned value of 0 !!!
H2 (g) + ½ O2 (g) → H2O (l)
(∆𝑯_𝒇)^𝜽
(∆𝑯_𝒇)^𝜽 of compounds represents the energy transferred to and from the surroundings when chemical bonds in the elements are broken and new bonds are formed in the compound.
Hence it measures the stability of the compound relative to its constituent elements
Q: (∆𝑯_𝒇)^𝜽 of CO2 and CO is -394 kJ mol-1 and -110 kJ mol-1 respectively.
Comment on the relative stability of these oxides with respect to their elements.
- The oxides are more stable relative to the elements as their formation are exothermic reactions, lowering the energy of the system.
- However, CO2 is the preferred product since its formation is more exothermic than that of CO.
- Standard enthalpy change of COMBUSTION, (∆𝑯_𝒄) ^𝜽
— the enthalpy change when 1 mole of a substance is completely burnt in excess oxygen under standard conditions of 298K and 1atm
(∆𝑯_𝒄) ^𝜽 always exothermic (< 0) as heat is always evolved during combustion
C (s) + O2 (g) → CO2 (g)
- Standard enthalpy change of ATOMISATION, (∆𝑯_𝒂𝒕) ^𝜽
— the enthalpy change when 1 mole of isolated gaseous atoms is produced from the element in its standard state under standard conditions of 298K and 1 atm
(∆𝑯_𝒂𝒕) ^𝜽 is always endothermic (> 0) as energy is always absorbed since the bonds in the elements must first be broken before atoms can be formed
C (s) → C (g)
- Standard enthalpy change of NEUTRALISATION, (∆𝑯_𝒏) ^𝜽
— the enthalpy change when 1mole of water is formed when an acid neutralizes a base.
The reaction is carried out in an infinitely dilute solution under standard conditions of 298K and 1 atm
- (∆𝑯_𝒏) ^𝜽 the same for all reactions involving a strong acid and a strong base (ex. HCl, HNO3, H2SO4, NaOH)
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
5.1. Enthalpy change of HYDRATION, (∆𝑯_𝒉𝒚𝒅) ^𝜽, of an anhydrous salt
— the enthalpy change when 1 mole of a hydrated salt is formed from 1 mole of anhydrous salt under standard conditions of 298 K and 1 atm
- (∆𝑯_𝒉𝒚𝒅) ^𝜽 is always exothermic (< 0) as energy is always released when bonds are formed between the salt and water molecules
Na2S2O3 (s) + 5 H2O (l) → Na2S2O3 * 5 H2O (s)
5.2. Enthalpy change of HYDRATION, (∆𝑯_𝒉𝒚𝒅) ^𝜽, of gaseous ions
— the enthalpy change when 1 mole of a gaseous ions is hydrated
- The ions are dissolved in a large amount of water such that further addition of water produces no further heat changes
Na+ (g) + aq → Na+ (aq)
- Enthalpy change of solution, (∆𝑯_𝒔𝒐𝒍) ^𝜽
— the enthalpy change when 1 mole of a solute is completely dissolved in a solvent to form an infinitely dilute solution under standard conditions of 298 K and 1 atm
Na2SO4 (s) + aq → Na2SO4 (aq)
Hess’s Law states that…
— the enthalpy changes in a chemical reaction is independent of the pathway which the reaction takes place, provided that the initial and final states of the system remains the same
!!!ENERGY CYCLE DIAGRAM!!
the concept was later developed into the Law of Conservation of Energy, also the First Law of Thermodynamics – energy can be transformed from one form to another, but cannot be destroyed
Bond energy is the enthalpy change when one mole of covalent bonds between two atoms are broken. This does not exist in ionic compounds e.g. NaCl
For ionic compounds, we need to introduce:
1) Enthalpy change of atomisation
2) First ionisation energy
3) First electron affinity
4) Lattice energy
- Enthalpy change of ATOMISATION, (∆𝑯_𝒂𝒕) ^𝜽
— the enthalpy change when 1 mole of isolated gaseous atoms are produced from the element in its standard state under standard conditions of 298K and 1 atm
!!! Always endothermic (> 0) as energy is always absorbed so that bonds within the elements have to be broken to release the atoms
½ Cl2(g) → Cl(g)
- First ionisation energy
— energy required to remove 1 mole of outermost electron from 1 mole of gaseous atoms to produce 1 mole of gaseous ions each with +1 charge
- Always endothermic (> 0) as energy is always absorbed to break bonds between electron and the nucleus
Cs(g) → Cs+(g) + e–
3.1. ELECTRON AFFINITY, (∆𝑯_𝒆𝒂) ^𝜽
The FIRST electron affinity, (∆𝑯_𝒆𝒂𝟏) ^𝜽,
— is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of gaseous 1– ions under
standard conditions of 298 K and 1 atm
Cl (g) + e– → Cl – (g)
3.2. The SECOND electron affinity, (∆𝑯_𝒆𝒂𝟐) ^𝜽,
— is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous 1– ions to form 1 mole of gaseous 2– ions under standard conditions of 298K and 1 atm
Note that:
2nd electron affinities are always endothermic ( (∆𝑯_𝒆𝒂𝟐) ^𝜽 is positive), and so are 3rd electron affinities.