Unit 1 - Chemical Changes and Structure Flashcards
What is Collision theory?
The collision theory states that for a chemical reaction to occur particles must collide with a minimum of kinetic energy (called the activation energy) and the correct orientation of geometry
What is the activated complex?
The activated complex is an unstable arrangement of atoms formed at the maximum of the potential energy barrier during a reaction. It is where the bonds between the reactants have not fully broken and the new bonds have not fully formed.
Using collision theory to explain the effect of particle size
Decreasing the particle size of a solid reactant increases the number of possible collisions (increasing the surface area). This increases the number of successful collisions and increases the reaction rate.
Using the collision theory to explain the effect of concentration
Increasing the concentration of a reactant increases the number of particles in a given volume, which means there are more successful collisions. This increases the reaction rate.
Using the collision explain the effect of pressure on a gaseous reactant
Increasing the pressure of a gaseous reactant increases the concentration of the gas (more particles are squashed into a smaller volume) so there are more successful collisions and the reaction rate increases
Using collision theory to explain whey a 10C rise in temperature doubles the rate
When the temperature rises by 10C, the rate of the reaction increases because the average kinetic energy of the colliding particles increases. This means that the more particle collide with the activation energy, resulting in more successful collisions.
What is a catalyst?
Catalysts increase the rate of a reaction by providing an alternative reaction pathway with a lower activation energy.
What is a homogenous catalyst?
Homogenous catalyst exist in the same state as the reactants
What is a heterogenous catalysts?
Heterogenous catalysts exist in a different state to the reactants
Name the three steps of the active site of a heterogenous catalyst
1) ADSORPTION
Molecules of one or both reactants form bonds with the catalyst. This weakens the bonds within the molecules
2) REACTION
The molecules react on the catalyst surface. The angle of collision is more likely to be favourable since one of the molecules is fixed
3) DESORPTION
The product molecules leave the catalyst and the vacant site can be occupied by another reactant molecule
Why is there an energy change when chemicals react?
The reactants and products have different energies
Explain an exothermic reaction
The products have less energy than that of the reactants therefore energy is lost to the surroundings in the form of heat e.g burning a fuel
Energy change -VE
Explain an endothermic reaction
The products have more energy than the reactants therefore energy is gained from the surroundings
Energy change +VE
What is the atomic radius?
The atomic radius of an element is half the distance between the nuclei of two bonded atoms of the element.
What is the pattern in atomic size going across a period?
As we move across a period the covalent atomic radius decreases (the atoms get smaller) because the nuclear charge increases, attracting the outer electrons more strongly, pulling them in closer; without any additional shielding.
What is the pattern in atomic size going down a group?
As we move down a group the covalent radius increases (the atoms get larger) because of the addition of extra electron shells
More shielding of outer electrons from the nuclear attraction
What is electronegativity?
Electronegativity is a measure of the attraction of an atom for the electrons in a bond
What happens to electronegativity looking across a period?
Across a period electronegativity increases because the nuclear charge (the number of protons) increases so the attraction of the nucleus for bonding electrons increases.
What happens to the electronegativity looking down a group?
Down a group electronegativity decreases because atoms have a bigger radius (more electron shells) so the bonding electrons are further away and more shielded from the nuclear charge.
Why do noble gases have extremely high ionisation energies?
They have a full outer shell and so are very stable and resistant to the removal of an electron.
What is the structure and properties of a Covalent network?
STRUCTURE - a giant lattice of covalently bonded non-metal atoms
PROPERTIES -
What is the structure and properties of a discrete covalent?
STRUCTURE - discrete molecules held together by weak intermolecular forces
PROPERTIES - covalent discrete molecules have much lower melting and boiling points because it is weak forces (intermolecular forces) between the molecules which break when a covalent discrete substance melts or boils. The strong covalent bonds are NOT broken.
What is ionic character?
The greater the difference in the electronegativity value of the elements in a bond, the more ionic character it has.
What is a pure covalent bond?
The electronegatives of both atoms are identical and the bonding electrons are shared equally between both atoms.
What is a polar covalent bonds?
When there is a small difference between the electronegativities of both non-metal atoms in the covalent bond, the bonding electrons are pulled more closely to the more electronegative atom.
What is a pure (non-polar) covalent bonds?
The pair of electrons is shared equally in a pure covalent bond because the electronegativity of both atoms is the same. The C-H bond is classed as non-polar because there is only an electronegativity difference of 0.3, so all the bonds in alkanes and alleges are classed as non-polar e.g. CH4
What is a non-polar molecule?
Symmetrical arrangement of polar bonds - the dipoles cancel out and overall polarity is zero therefore non-polar
What is Polar molecule?
Unsymmetrical arrangement of polar bonds - the dipoles do not cancel
A polar molecule also has a permanent slight positive charge at one side and a negative at the other
Experiment to identify whether a molecule is polar or non-polar
If a thin stream of a liquid is attracted to a charged plastic rod, the liquid contains polar molecules.
A jet of water from a burette is deflected when a charged rod is held close to it.
What is LDF?
London dispersion forces are the result of electrostatic attraction between temporary dipoles and induced dipoles and are caused by the continual movement of electrons in atoms and molecules.
What are the properties of LDF?
Found BETWEEN all molecules and are the only bond found between non-polar
What is permanent dipole - permanent dipole?
Permanent dipole - permanent dipole interactions are additional electrostatic forces of attraction between polar molecules
Properties of permanent dipole - permanent dipole
- Polar molecules are attracted to one another by forces called permanent dipole - permanent dipole interactions in addition to LDF (the negative end of the molecule is attracted to the positive end)
- high m.pt and b.pt
What is hydrogen bonding?
Hydrogen bonds are intermolecular forces which are formed between molecules in which hydrogen is bonded to a strongly electronegative atom like fluorine, oxygen or nitrogen
Properties of hydrogen bonding
- highly polar
- high m.pt and b.pt
- strongest intermolecular bond
