Chemical Kinetics Flashcards
What is chemical kinetics?
It is the branch of chemistry, which deals with the study of reaction rates and their mechanisms.
Does diamond last forever?
Thermodynamic data indicates that diamond shall convert to graphite but in reality the conversion rate is so slow that the change is not perceptible at all. Therefore, most people think that diamond is forever, which is untrue.
What is rate of reaction? What are the 3 types of reactions based on rate?
The speed of a reaction or the rate of a reaction can be defined as the change in concentration of a reactant or product in unit time.
Reactions can be of 3 types based on rate :
- FAST - Ionic reactions. Eg. precipitation of AgCl occurs instantaneously by mixing of aq. solutions of AgNO3 & NaCl
- SLOW - Eg. rusting of iron in the presence of air & moisture
- MODERATE - Eg. inversion of cane sugar & hydrolysis of starch
What is the formula for avg rate of reaction?
Average rate of reaction = Rate of disappearance of Reactants = Rate of appearance of Products
Avg rate of reaction
= Decrease in concentration of R / Time taken = -∆[R] / ∆t
= Increase in concentration of P / Time taken = +∆[P] / ∆t
What are the units of rate of a reaction?
Units of rate are concentration/time
Eg. if concentration is in mol/L and time is in seconds then the units will be mol/Ls
In gaseous reactions, when the concentration of gases is expressed in terms of their partial pressures, then the units of the rate equation will be atm/s
What is the formula for instantaneous rate of reaction?
How can it be determined graphically?
Instantaneous rate = -d[R] / dt = +d[P] / dt
It can be determined graphically by drawing a tangent at time (t) on either of the curves for concentration of [R] and [P] vs time (t) and calculating its slope
What is the formula for rate of reaction when stoichiometric coefficients of reactants or products are not equal to 1?
Rate of disappearance of any of the reactants or the rate of appearance of products is divided by their respective stoichiometric coefficients.
Rate of reaction
= -∆[R] / ∆t × (1 / stoich coeff.) = +∆[P] / ∆t × (1 / stoich coeff.)
Rate of gaseous reaction can also be expressed as ________
Answer : Rate of change in partial pressure of the reactant or the product
Reason : For a gaseous reaction at constant temperature, concentration is directly proportional to the partial pressure of a species.
Formula : k = (2.303 / t) log Po / (2Po - Pt)
What is rate law?
What is rate expression?
The representation of rate of reaction in terms of concentration of the reactants is known as rate law. It is also called as rate equation or rate expression.
Rate ∝ [A]^x [B]^y
Rate = k [A]^x [B]^y (where k is rate constant)
Definition :
Rate law is the expression in which reaction rate is given in terms of molar concentration of reactants with each term raised to some power, which may or may not be same as the stoichiometric coefficient of the reacting species in a balanced chemical equation .
How is rate law predicted?
Rate law for any reaction cannot be predicted by merely looking at the balanced chemical equation, i.e., theoretically but must be determined experimentally.
What is order of a reaction?
The sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction.
Order of a reaction tells us how many reactants’ concentrations the rate of that reaction is dependent on.
What are the units of rate constant for different orders of reaction?
Zero order reaction - mol / L s
First order reaction - 1 / s
Second order reaction - L / mol s
What are elementary and complex reactions?
The reactions taking place in one step are called elementary reactions. For elementary reactions, order of reaction is equal to its molecularity (∴ non-zero & non-fraction)
When a sequence of elementary reactions (called mechanism) gives us the products, the reactions are called complex reactions.
What is molecularity of a reaction?
The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.
What are the types of molecularity?
- Unimolecular :
When one reacting species is involved
Eg. decomposition of ammonium nitrite
NH4NO2 → N2 + 2H2O - Bimolecular :
Involves simultaneous collision between two species
Eg. dissociation of hydrogen iodide.
2HI → H2 + I2 - Trimolecular :
Involves simultaneous collision between three reacting species. Aka termolecular reaction.
2NO + O2 → 2NO2
The probability that more than three molecules can collide and react simultaneously is very small. Hence, reactions with the molecularity three are very rare and slow to proceed.
