Rates of Reaction Flashcards
Rate of Reaction
the change in concentration per unit of time of any one product or reactant
Instantaneous rate of reaction
the rate of reaction at any one particular time during a reaction
average rate of reaction
total gas produced over total time
Factors Affecting RoR
-nature of reacants
-particle size
-concentration
-temperature
-catalyst
Nature of Reactants
-ionic reactions are fast because ions are free to move in solution.. no bonds break (sodium chloride and silver nitrarte–> a percipitate of silver chloride forms instantly)
-covalent reactions are slower than ionic as covelent bonds need to be broken before the reaction can occur (hcl and sodium thiosulfate–> pale yellow percipitate of sulphur forms slowly)
Particle Size
the rate of reaction increases as the particle size decreases becasue the smaller the particle size, the large the surface area therefore the greater number of collisions, the greater number of successful collisions
( eg. Marble Chips)
Concentration
the greater the concentration, the greater the rate of reaction (directly proportional). if the concentration is increaded the number of collisions is also increased
(eg sodium thiosulfate and hydrochloric acid)
Temerapture
the greater the temberature the greater the rate (exponential relationship) because as the temperature increases, the more if the colliding molecules have the minimum activation energy needed to react.
(eg sodium thiosulfate and hcl)
Catalyst
a substance that alters the rate of a chemical reaction but is not used up/consumed in the reaction itself. a catalyst reduces the amount of energy needed for an effective collision
(MnO2 is a catalyst in the breaking down of H2O2 to water and oxygen)
Properties of Catalysts
- specific - each catalyst only acts on one type of reaction (eg enzymes, a bio catalyst made by living cells how this as as the enzyme catalase only breaks down hydrogen peroxide)
- only needed in small amounts
- catalyst alters both the forward rate and the reverse rate by the same amount if the reaction is reversable
- can be poisoned by certain substances
Types of Catalyst
- Homogeneous catalyst: the same phase as the reactants
- Heterogeneous catalysts: in a different phase to the reactants
- Autocatalysis: a product of the reaction that acts as a catalyst
Mechanism of Catalysts
the decomposition of hydrogen peroxide catalysed by the resence of I ions illustrated the formation of an intermediate
1. H2O2 + I- –> H2O +IO-
2. H2O2 +IO- –>H2O + O2 + I-
Surface Adsorption Theory
- reactants are adsorbed onto the surbace of the catalyst, forming a temp bond
- reactants are close together at high concentration on the surface and are therefore more likely to collide with each other and react
- the products diffuse away from the surface of the catalyst
Adsorption
moleculed adhere to the surface of the phase
Absorption
moleecules are drawn into the buld of the phase