Unit 1 - Chemical Changes and Structure Flashcards
Rate of Reaction
rate = change/ time
rate = 1/ time
Rate of reaction can be monitored by measuring:
- change in concentration
- change in mass
- change in volume
- colour change
Rate of reaction can be increased by:
- increasing temperature (particles have more kinetic energy)
- increasing concentration (more particles = more likely to react)
- increasing surface area (decreases particle size)
- adding a catalyst (reduces activation energy)
Collision theory
Collision theory states that for a chemical reaction to occur successfully particles must collide with sufficient energy to overcome the activation energy and collide with the correct collision geometry/ orientation
Catalysts
Catalysts speed up the rate of chemical reactions without being used up or altered
Enzymes are biological catalysts
Homogenous catalyst: catalyst is the same state as reactants
Heterogeneous catalyst: catalyst is a different state as reactants
Action of a catalyst:
- Reactant molecules adsorb (form temporary bonds) onto the catalyst surface, making the bonds within the reactants weaker, reducing the requires activation energy
- more successful collisions occur and reactants form products faster
- product molecules desorb (break temporary bonds) from the catalyst surface, leaving the catalyst unaltered and ready to be reused
Activation energy
Activation energy, Ea, is the minimum kinetic energy required by colliding particles for the reaction to occur. Only molecules with a kinetic energy greater than the activation energy are able to overcome it and successfully collide.
Enthalpy
Enthalpy change (delta H) is the change innerly per mole. Measured in kilojoules, kJ.
Exothermic reactions give off heat energy to their surroundings and have a negative enthalpy value
Endothermic reactions take in heat energy from their surroundings and have a positive enthalpy value .
REDOX
Oxidation reaction: involves the loss of electrons (OIL)
Reduction reaction: involves the gain of electrons (RIG)
Redox reaction: when an oxidation and reduction reaction occur simultaneously
Oxidising agent:
- causes the oxidation reaction to occur
- is reduced in the reaction
- has a high electronegativity
- uses: to kill bacteria and fungi (i.e. biocides), to break down coloured compounds (i.e. bleach), for water purification
Reducing agent:
- causes the reduction reaction to occur
- is oxidised in the reaction
- low electronegativity
Electronegativity
Electronegativity is the attraction elements have for bonding electrons
An atom with high electronegativity will attract bonding electrons whereas an atom with low electronegativity will have a weak attraction for electrons
Electronegativity decreases going down a group as atomic size increase (so further from pull of nuclear charge) and number of shells increases (as they shield nuclear charge)
Electronegativity increases going across a period as nuclear charge increases
Covalent radius
Covalent radius is half the distance between the nuclei of two covalently bonded atoms
Covalent radius decreases going across a period because the increased nuclear charge (due to increased number of protons) attracts the outer electrons more strongly
Covalent radius increases going down a group because there are more occupied electron shells (which physically takes up more space, plus the shells ‘shield’ some of the nucleus’s charge so the electrons in the most outer shell aren’t attracted as strongly)
Metallic bonding
Metallic bonding:
- Is the electrostatic force of attraction between positively charged ions and delocalised electrons
- High melting and boiling points
- Have electrons that can move freely throughout metal structure (are delocalised)
- Lattice structure
- Good conductors of heat and electricity
Ionic bonding (+ ions)
Ionic bonding:
- Involves the transfer of electrons between atoms so they form a stable electron configuration (i.e. creates ions,)
- Metals form positive ions ans non-metals form negative ions
- Ionic bonding is the electrostatic attraction between positive and negative ions
- Solid ionic compounds have a lattice structure
- When molten, the lattice structure is broken and the ions are free to move, therefore allowing it to be a good conductor of electricity
- When solid the ions are trapped in their lattice and cannot conduct
- Have high MP + BP
Ionisation
Ionisation is the process whereby an atom is converted into an ion, or an ion becomes increasingly charged (endothermic process)
Ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atom (/ion). Units: kJ mol-1
One electron is removed per each ionisation energy
Forces
Intramolecular forces:
- Exist between atoms and create bonds
- Involve sharing/ transfer of electrons
- Stronger than intermolecular forces
- These bonds are broken/ formed in a chemical reaction
- High MP + BP
- e.g. covalent, ionic
Intermolecular forces:
- (aka. Van der Waals forces)
- Exists between molecules/ atoms
- Creates forces of attraction
- Weaker than intramolecular forces
- These forces are overcome when the substance is melted/ boiled
- Low MP + BP
Van der waals forces
London dispersion forces:
- form as a result of temporary/ induced dipoles
- when the electrons in an atom/ covalent molecule are unevenly distributed, temporarily creating small regions of charge (𝛿+, 𝛿-), can then induce further dipoles
- occur between atoms and molecules
- weak intermolecular forces
Permanent dipolen-dipole interactions:
- form as a result of polar bonds (due to bonding of atoms of different electronegativity) causing permanent dipoles
- molecules with polar bonds (therefore polar dipoles) are only polar if the molecule is unsymmetrical, if the polar bonds/ permanent dipoles are symmetrically opposed the molecule is non-polar
- occur between polar molecules
- stronger intermolecular forces than LDFs
Hydrogen bonding:
- form as a result of hydrogen covalently bonding to fluorine, oxygen or nitrogen
- occur between positive and negative dipoles of certain neighbouring polar molecules
- stronger intermolecular forces than LDFs and permanent dipole permanent dipole interactions
Solubility
- ‘like dissolves like’ (compounds of similar intermolecular bonding/ polarity dissolve one another)
- ionic compounds can dissolve in water as they are polar and the polar water molecules hydrate the ions (they surround the ions and pull them from the lattice)
- when liquids mix thoroughly they are miscible (this happens when the intermolecular forces between the substances are fairly similar)
- when liquids do not mix they are immiscible (e.g. oil and water, as one is non-polar and they other is polar)
Viscosity
- viscosity is a measure of how easily liquid/ gas flows - how easily the molecules in a substance are able to move past each other (the more viscous it is, the thicker the fluid, the less easily it flows)
- the stronger the intermolecular forces between molecules in fluids the more viscous it is
