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Negative change in enthalpy
Positive change in enthalpy
Requirements for a reaction to occur
- Molecules/atoms must collide
- Must collide with correct orientation for that reaction
- Must collide with specific energy (EA)
What is activation energy?
Energy required to initiate a successful chemical reaction. In a reaction there often needs to be transition states that contain a level of enthalpy greater than both the products and reactants. These transition states occur as old bonds are broken and new bonds are formed. The materials at this point are referred to as an activated complex.
Increasing reaction rate:
- Catalysts- create activation complex with a lower required EA, correctly orientate molecules.
- Increase temperature- Increase # collisions, increase # collisions with sufficient energy
- Increase concentration- More reacting particles
- Increase pressure- Brings particles closer together->more likely to collide
- Increase surface area- Reduce particle size, more particles exposed, more possible collisions.
Same formula, different structure
Properties of metallic substances:
- Good conductors in both solid and liquid states
- Hard, tough and dense
- High melting and boiling points
- Positive metal ions surrounded by a sea of delocalised electrons
Properties of ionic compounds
- High melting and boiling points-strong electrostatic attraction between oppositely charged ions in 3d lattice
- Made of hard crystals
- Brittle-strong electrostatic attraction between oppositely charged ions in 3d lattice
- Non-conductors in solid state- occupy fixed positions
- Good conductors in liquid state- ions are able to move through liquid
Properties of covalent substances
- Low melting and boiling points-weak attractive forces
- Nonconductors as liquid and solid, sometimes aqueous- neutral particles and localised electrons
- Many insoluble
- Organised in neutral lattice structure
Properties of covalent network substances:
- Very high melting and boiling points- strong bonds in rigid lattice.
- Non-conductors in solid and liquid states-localised and no electrons
- Hard and brittle