Reactivity 1.2 Flashcards
Bond breaking… what energy?
Absorbs
Bond forming… what energy?
Releases
Energy is required to…
break a chemical bond (endothermic)
Energy is given out when..
chemical bonds are formed (exothermic)
Bond enthalpy values only refer to…
The gaseous state
Bond enthalpy units
kJmol-1
Bond breaking
Seperating atoms that are attracted by an electrostatic force
Bond making
Involves bringing together atoms that are attracted by an electrostatic force
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Strength of bonds
Are influenced to some degree by the nature of other atoms attached to one or both of the bonding atoms
Average bond enthalpy
Average energy required to break one mole of the bond in similar compounds all in the gaseous state
Average bond enthalpy strength order
Single covalent < double covalent < triple covalent
When can you calculate enthalpy changes?
When all reactants and products are in the gaseous phase
Enthalpy change for a reaction is…
Total bonds enthalpies in reactants - total bond enthalpies in products
Why is calculated change in enthalpy different to that in the data booklet?
Values used are average values in similar compounds
Hess’s law of constant heat summation
The total energy change in a chemical reaction depends only upon the initial and final states and is independent of the reaction pathway
Energy cycle - what side is worth what?
Clockwise = anti clockwise
Standard enthalpy of atomisation
Enthalpy change when one mole of gaseous atoms are formed from the element in its standard state
Enthalpy of formation
Enthalpy change when one more of product is formed from its elements in their standard state
Standard state rules (4)
- HOFBrINCl are all diatomic, everything else in monotomic
- Br2 and Hg are gases
- Noble gases are all gases
- Everything else is solid
First ionisation energy (IE)
Minimum energy required to remove one mole of electrons from one mole of gaseous atoms
First affinity energy (EA)
Energy released when one mole of gaseous atoms gain an electron to form one mole of gaseous ions
Lattice enthalpy (lat)
The energy needed to convert one mole of ionic solid into gaseous ions under standard conditions (ionic solid to gaseous ions)
Enthalpy of atomisation (atm)
The enthalpy change that occurs when one mole of gaseous atoms is formed from the element in its standard state (standard state to a gas
Enthalpy of sublimation (sub)
Enthalpy change when one mole of gaseous atoms is formed from the element in solid state
Enthalpy of solution (sol)
The enthalpy change when 1 mole of solute is dissolved in solvent to infinite dilution at STP.
Enthalpy of solution (sol) steps
1 - lattice enthalpy when lattice breaks into gaseous ions
2 - enthalpy of hydration, hydrates the gaseous ions to make them aqueous
Enthalpy of hydration (hyd)
The enthalpy change when 1 mol of anhydrated substance is converted to 1 mol of hydrated substance
Enthalpy of dissociation (dis)
Amount of energy required to break a chemical bond between two species
How to calculate enthalpy of dissociation in diatomic molecules?
Use bond enthalpy from data booklet and half (Cl has half Cl2 enthalpy)
Does the route taken affect the total enthalpy change?
No, Hess Law states it is independent (special case of law of conservation of energy)
Why are the enthalpies of reactions only valid for reactions in the gaseous state?
It does not include intermolecular interactions
What do energy cycles allow for?
The calculation of values corresponding to certain enthalpy changes which cannot be determined directly
Average bond enthalpy
Energy required to break one mole of the same type of bonds in the gaseous state averaged over a variety of similar compounds
Enthalpy of combustion (c)
The enthalpy change when one mole of substance in its standard state is completely combusted in oxygen under standard conditions
Standard enthalpy of formation of an element in its normal standard state
Must be 0 (not for allotropes though)
Hess’s Law of Constant Heat Summation
Total energy change in a chemical reaction depends only on the initial and final states and is independent of the reaction pathway
Enthalpy of combustion cycle
H= Hc reactants - Hc product
Enthalpy of formation cycle
H = Hf products - Hf reactants
Ionisation enthalpy (IE) equations
M(g) -> M+(g) + e-
Always positive, endothermic
Electron affinity (EA) equation
M (g) + e- = M- (g)
Always negative, exothermic
Lattice enthalpy (latt) notes for equation
g + g -> s (Exothermic in formation)
s -> g + g (Endothermic in breakdown)
Atomisation enthalpy (atm) equation and note
M (s) -> M (g)
Always positive H (endothermic)
Covalent bond enthalpy equation and note
X (g) + Y (g) -> X-Y (s)
Exothermic (bond formation)
What can a Born Haber cycle tell you?
The energy changes that occur when an ionic compound is formed
What factors affect the size of the lattice enthalpy?
Ionic radius and charge of ion
Ionic radius as a factor of lattice enthalpy
The smaller the ions, the closer they are together and the stronger the forces of attraction (larger lattice enthalpy)
Ionic charge as a factor of lattice enthalpy
The greater charge, the stronger the forces of attraction, the greater the lattice enthalpy
What are the lattice enthalpy values in a born haber cycle?
Experimental values as they are based on experimental data
Born haber as a special case of Hess’s Law
It allows a lattice enthalpy to be calculated from other enthalpy changes
