Thermochemistry Flashcards
Ionisation energy
(e.g. Cl (g) —> Cl+ (g) + e)
- energy required to remove one mole of electrons from one mole of atoms in the gaseous state
First ionisation energy
- energy required to remove the 1st valence electron
Standard enthalpy of formation
enthalpy change when 1 mole of a compound is formed in its standard state from elements in their standard states
Standard enthalpy of combustion
enthalpy change when 1 mole of a substance is combusted in oxygen with all reactants and products in their standard states (products usually CO2 and H2O)
Standard enthalpy of vaporisation
enthalpy change when 1 mole of a substance is vaporised at its boiling point (l) to (g)
Standard enthalpy of fusion
enthalpy change when 1 mole of a substance is melted at its melting point (s) to (l)
Standard enthalpy of sublimation
enthalpy change when 1 mole of a substance sublimes at its sublimation point (s) to (g)
Exothermic
transfers heat energy from the system into the surroundings (warm to touch)
Endothermic
takes heat from the surroundings into the system (cool to touch)
Atomic radius
- decreases as you go across the period
- increases as you go down a group
Amount of shielding?
the number of electron shells shielding the attractive force of the nucleus from the valence shell
How does nuclear charge affect atomic radius?
e.g. “although they have the same amount of shielding, __ has a greater nuclear charge because more protons are attracting the valence shell. This means the valence shell is pulled in closer = smaller radius”
Ionic radius (between Na and Na+)
- Na+ has lost an entire energy shell, making it physically smaller
- this means it experiences less shielding, so the valence shell is attracted more strongly to the nucleus
Electronegativity
- a measure of attraction an atom has for electrons in a bond
- weaker electrostatic attraction = lower electronegativity
- top right in the periodic table is most electronegative
More protons in the nucleus (electronegativity) =
greater electrostatic attraction for electrons in a bond
More protons in the nucleus (ionisation energy) =
attracts the electrons in its valence shell more strongly, thus taking more energy to remove an electron
Temporary dipole forces
- very weak bond between neighboring molecules
- caused by uneven distribution of electrons in one end of a molecule
- this causes electrons in neighboring molecules to be repelled, creating a temporary force of attraction
Permanant dipole forces
- stronger type of intermolecular bond (but still weak)
- caused by polar molecules with permanent dipoles being attracted to one another
- permanent dipoles = attraction always maintained
Hydrogen bond
- the strongest of the intermolecular forces
- only occurs in specific molecules (H-F, H-O, H-N bond)
- F, O, N are especially electronegative; thus forming especially strong dipoles
- high boiling point (high heat energy to break the intermolecular forces)
Heating curve of water
- shows how the temperature of water changes as it is heated over time
- it involves the standard enthalpy of fusion and vaporisation (solid to liquid, liquid to gas)
Slope on the graph (heating curve)
- increasing temp, increasing kinetic energy (particles vibrate more quickly)
- NO change in state occurs (e.g. solid stays solid)
Flat line on the graph (heating curve)
- change in state (solid turns to liquid)
- NO temperature change
- heat energy is used to break the intermolecular forces (IMF) between water molecules and the potential energy of the molecule increases
- (s) to (l) = some IMF break, (l) to (g) = ALL IMF break
Entropy (symbol s)
a measurement of the disorder of particles in a system
Low entropy
particles are ordered in a SOLID state
High entropy
- particles become more disordered during a reaction, either DISSOLVED (aq) or in a GAS STATE (g)
What causes a higher entropy?
- more moles = more chances for particles to become disordered
- faster moving particles = more likely to be disordered
What does spontaneous mean (entropy)
the reaction/process just happens (or carries on once it starts) e.g. salt readily dissolves, alcohol burns
Why is a reaction spontaneous (in terms of entropy)
∆S(total) must be posi�ve
∆S(total) = ∆S(system) + ∆S(surroundings)
- talk about the system and the surroundings and whether entropy increases or not
- since total entropy change is positive, the increase in entropy of the system must be greater than the decrease in entropy of the surroundings (and vice versa)
Transition metals
- lie from Sc to Zn
- form ions by losing their 4s electrons before their 3d
- 4s electrons are lower energy level than 3d so they are easier to remove
What does the abbreviation VSEPRT stand for in relation to identifying shapes from lewis diagrams?
Valence Shell Electron Pair Repulsion Theory
(repel for maximum separation)
Why is Fluorine the most electronegative (reference the factors affecting periodic trends)
- Fluorine is at the top of Group 17
- going up a group, the atomic radius decreases, as does shielding from the inner energy levels
- this means there is a strong electrostatic attraction between the positive nucleus and any bonding electrons
