Chem Chapt 14: Reaction Kinetics

Define rate of reaction

It is the change in concentration of a reactant or product per unit time

Define instantaneous rate

It is the measurement of the rate of reaction at a particular time. It is obtained by finding the gradient of the tangent to the concentration-time curve at the particular time

Define average rate

It is the measurement of the change in concentration of a reactant or product over a specific time period

Define initial rate

It is the instantaneous rate at the start of the reaction, when an infinitesimally small amount of the reactant of the reactant has been used up

Initial rate be approximated by average rate as long as:

The time interval is small enough such that at most 10% of reactants have been used and the time interval starts from t=0

Define rate equation

It is defined as an experimentally derived mathematical equation between reaction rate and concentration of reactants at a particular temperature

Define order of reaction

Order of reaction with respect to a particular temperature is the power to which the concentration of that reactant is raised in the experimentally derived rate equation

Define rate constant (k)

The rate constant is a proportionality constant for a given reaction at a specified temperature, and it is only affected by temperature and catalysts

State the units for k for zero, first and second order

For zero order, units is mol dm-3 S-1, for first order, units is S-1, for second order, units is mol-1 dm3 S-1

Describe the shape of the rate against [reactant] graph for zero, first and second order reactions

For zero order, the graph is horizontal as rate is constant even when concentration of reactant increases. For first order, an increases in the concentration of reactant is proportionate to the increase in rate. For second order, the graph increases exponentially as concentration of reactant increases

Describe the [reactant]-time graph for zero order, first order and second order graphs

For zero order graphs, the graph decreases linearly over time, which shows that there is no change in gradient over time, which is also rate. For first order reactions, the graph decreases exponentially over time which indicates that gradient and rate decreases too, half time of the graph is also constant. The graph for second order will also have a exponential decrease over time but half life will not be constant

What is a graph that differentiates second order reactions between other reactions

The rate against [reactant] square graph that will indicate a linearly increasing graph

Define half life

It is the time taken for the concentration of a reactant to decrease to half its initial value

state two equations relating to half lives

Half life=ln2/rate constant and concentration at time t/concentration at time 0=(1/2)^n

Define pseudo first order reaction

It is when one of the reactant is in such large excess that the concentration of that reactant remains almost constant throughout the reaction

State 3 scenarios for pseudo first order

Presence of large excess of reactant, about 10 times more. The solvent is also a reactant. The presence of a catalyst

Why must quenching be done in aliquots?

To slow down or stop the reaction so that the composition of the reaction mixture in the aliquot stops changing

State how quenching can be done

By adding a large volume of cold solvent to dilute the sample, by adding a quenching agent that reacts with one of the reagents, or by using an inhibitor

Describe the process of conductometric method

Mix various specified volumes of the reagents thoroughly and measure the electrical conductivity of the mixture at regular time intervals. As the electrical conductivity decreases with time due to decrease in the number of ions, the gradient of the graph of electrical conductivity against time would be the rate

Define what are discontinuous methods and when they are used

They are methods that measures initial rates using appropriation of average rates. If the experiment involves using fixed concentrations of reactants to form fixed concentration of products, it is a discontinuous experiment

Explain why in discontinuous methods, varied volumes of water are used

They are varied to keep total volume of reaction mixture constant across different experiments. This way, initial concentration of each reactant in the reaction mixture is directly proportional to its volume needed

Define rate determining step

It is the slowest step in a sequence of elementary reactions. It also possesses the highest activation energy amongst all the steps involved

Define activation energy and state the collision theory

Activation energy is the minimum amount of energy that the reactant particles must have to collide effectively to form products. The collision theory states that reactant particles must have energy equal to or greater than activation energy for collisions to be effective.

Describe how smaller particle sizes affect rate of reaction

The smaller the particle size, the larger the surface area per unit volume and hence the more frequent the effective collisions between reactant molecules and hence higher rate of reaction

Describe how increased concentration of reactants affects rate of reaction

As concentration increases, the reactant particles are closer together and number of molecules per unit volume increases, thus increasing frequency of effective collisions between reactant molecules, increasing rate of reaction

Describe how increase in pressure of a system affects rate of reaction(gaseous only)

As partial pressure of a gaseous reactant increases, there is an increases in the concentration of the reactant and hence increase in frequency of effective collisions between reactant molecules and hence higher rate of reaction

Describe how increased temperature affects rate of reaction and rate constant!!!!! (need draw maxwell-boltzman curve)

When temperature increases, reactant particles gain more kinetic energy and hence more molecules have energy that is greater than or equal to the activation energy and hence the frequency of effective collisions between reactant molecules increases, alongside with rate constant. Thus increasing rate of reaction

Describe how a catalyst affects rate of reaction

A catalyst increases the rate of chemical reaction by providing an alternative pathway with lower activation energy and remains chemically unchanged at the end of the reaction. It will not affect yield of reaction