kinetics and oxidation, reduction, and redox equations (A1 physical chemistry) Flashcards
define rate of reaction and give the rate calculation
the change of concentration/amount of a reactant or product per unit time
rate = amount of reactant used/product made / time
collision theory
for a reaction to occur, the particles must collide in the right direction and must also have a minimum amount of kinetic energy
explain an energy profile diagram
as we reach the peak, molecule bonds are stretching as they have more kinetic energy and thus are moving around more
the activation energy can be calculated as the difference in energy between the reactants line and the peak
at the peak, bonds have sufficient energy to break
explain what a Maxwell-Boltzmann distribution shows
shows the energy in gas particles as gas particles move at different speeds as they have different kinetic energies
the graph starts at 0,0 as no particles have zero kinetic energy, the line never touches the x-axis
the area under the curve is equal to the total number of molecules
the peak shows the most likely energy of a particle in a sample
the mean energy the particles have is slightly to the right of the most probable energy
the activation energy is usually far to the right and significantly far from the peak - the majority of particles do not have enough energy to successfully react, moving at slow to moderate speeds and therefore not colliding frequently enough
explain what changes in a Maxwell-Boltzmann distribution curve if temperature increases, and if a catalyst is involved
curve shifts to the right
peak is lower
area under curve is the same
area under curve beyond activation energy increases
activation energy will shift to left increasing area under curve beyond activation energy, so more particles have sufficient energy to react
why does a small increase in temperature lead to a large increase in rate?
the combination of more frequent collisions and more energetic collisions
ways to measure rate in experiments
we can see how long it takes for a precipitate to form by timing how long it takes for a cross drawn underneath a solution in a flask to disappear, however this is prone to human error as it is difficult to know when exactly the cross disappears, we can use the same observer every time to attempt to reduce these errors
in reactions that produce a gas, we can place the reaction on a balance and measure the mass loss as gas is lost and time this, fairly accurate method however we should use a fume cupboard if the gas is toxic or harmful
we can also measure the rate by measuring the amount of gas produced using a gas syringe measured over a specific time
what are the priority oxidation numbers?
fluorine always takes priority, with an oxidation state of -1
aluminium’s oxidation state is always +3
oxygen’s oxidation state changes to -1 in peroxides (e.g. H₂O₂) and to +2 in OF₂
chlorine‘s oxidation state is -1 unless in a compound with oxygen or fluorine, where it has a positive value
hydrogen’s oxidation state changes to -1 in hydrides (metal bonded to H e.g. NaH)
balancing and combining half equations e.g. combine the half equations for the conversion of Fe²⁺ to Fe³⁺ and MnO₄⁻ to Mn²⁺ to form the overall ionic equation for this redox reaction
- write down the species before and after the reaction
- balance any atoms apart from oxygen and hydrogen
- balance any oxygens with H₂O
- balance any ions with H⁺ ions
- balance charges with electrons
Fe²⁺ → Fe³⁺
Fe²⁺ → Fe³⁺ + e⁻ (oxidation)
MnO₄⁻ → Mn²⁺
MnO₄⁻ → Mn²⁺ + 4H₂O
MnO₄⁻ + 8H⁺ → Mn²⁺ + 4H₂O
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (reduction)
now combine the equations, cancelling out all electrons
5Fe²⁺ → 5Fe³⁺ + 5e⁻
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
