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UNIT 1 HIGHER CHEMISTRY (STRUCTURE AND BONDING (VAN DER WAALS FORCES (LDF-…
UNIT 1 HIGHER CHEMISTRY
CONTROLLING THE RATE
COLLISION THEORY
CONCENTRATION- When increasing conc. there is less space for particles to move and therefore there are more collisions increasing the reaction rate.
PARTICLE SIZE- Breaking up large surfaces into smaller pieces increases the overall surface area leading to an increase in reaction rate as there are more surfaces in which for reactions to take place on.
PRESSURE- If pressure is increased there is less space for molecules to move in therefore causing an increase in chance of collision, increasing reaction rate.
TEMPERATURE- An increased temperature makes particles move faster causing more collisions therefore increasing reaction rate.
COLLISION GEOMETRY- If positioned wrong molecules won't react, so molecules must face the correct way to increase the reaction rate.
CATALYST- Orientates the substrate so more collisions occur, therefore increasing reaction rate.
Substances will only react if they collide successfully with enough energy and the correct collision geometry.
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REACTION PROFILES
For a reaction to occur particles must have a minimum amount of kinetic energy called activation energy.
If Ea is high only a small number of particles will have enough energy for a reaction to occur, so reaction rate will be slower.
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PERIODICITY
BONDING
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COVALENT BOND- The eletrostatic attraction between the positive nuclei and negative shared pair of electrons
COVALENT MOLECULAR- Electrically neural, held together by covalent bonds, have low melting and boiling points
COVALENT NETWORK- Infinite 3D networks. E.g Boron, Carbon, Silicon
FULLERENES- composed of an even number of carbon atoms, covalently bonded to each other
MONATOMIC- Full outer shell of electrons. Do not form any covalent or ionic bonds. Consist of only 1 atom.
COVALENT RADII- Size of atom from the nucleus to the outer energy level. DECREASES ACROSS, due to increased nuclear charge pulling electrons closer. INCREASES DOWN, due to increased no. of e- shells.
IONISATION ENERGY- The energy required to remove one mole of e- from one mole of atoms in the gaseous state. INCREASES ACROSS, due to increased nuclear charge pulling e- closer so more energy needed to remove them. DECREASES DOWN, as more e- shells cause a larger shielding effect so outer e- less strongly held
ELECTRONEGATIVITY- Measure of an atom's attraction for the shared pair of e- in a bond. INCREASES ACROSS, due to increased nuclear charge. DECREASES DOWN, more e- shells causing a larger shielding effect.
STRUCTURE AND BONDING
POLAR COVALENT BONDING- Some ionic character, small difference in elecronegatives. Electrons pulled closer to the slightly electronegative atom causing slight negative and positive charge.
PURE COLVALENT BONDING- No ionic character, electronegatives the same or very similar. Electrons equally shared.
IONIC BONDING- Most ionic character. Big difference in electronegatives. Electrons not shred but ions form.
INTERMOLECULAR FORCES- weaker, break when a substance melts or boils.
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VAN DER WAALS FORCES
LDF- Electrostatic attration between tempory dipole and tempory induced dipole. Exisit between all atoms much weaker.
PD-PD- If poolar bonds are present pd-pd is present. permanent attraction between positive and negative poles of ajacent molecules. No symmetry
HYDROGEN BONDING- Attraction between hydrogen and either oxygen, nitrogen or flurine.
VISCOSITY is the measure of how thick or runny a liquid is. The STRONGER the strength of intermolecular forces the more viscous.
As molecules get BIGGER, MISCIBILITY DECREASES.