Chemistry - Topic 6 Kinetics

Reaction rate

Reaction rate

The change in concentration of reactants or products per unit time.

Instantaneous rate of reaction

The rate of reaction at a specific stage, can be calculated using the tangent at a point of a graph of reaction rate.

Units of rate of reaction

mol per dm cubed per seconds

Collision theory

The theory that states that for a chemical reaction to occur, particles must collide with sufficient energy and proper orientation.

Activation energy

The minimum amount of energy required for a chemical reaction to occur from a collision between particles. Necessary for overcoming repulsion between molecules and breaking some bonds in the reactants for a reaction to occur.

Collision geometry

Collision geometry refers to the shape or structure of objects involved in a collision; the physical arrangement and dimensions of the objects that come into contact during a collision. Only certain arrangements lead to a reaction.

Effect of concentration on rate of reaction

Increasing number of particles in a fixed volumes leads to a greater collision frequency

Effect of pressure on rate of reaction

For gasses, equivalent to increasing concentration (increased pressure leads to greater frequency of collision)

Effect of surface area on rate of reaction

Reactions generally occur at the surface of a solid, therefore increasing the surface area of a solid (dividing), means the number of effective particles has increased and hence the rate of reaction

Effect of temperature on rate of reaction

For an ideal gas, the average KE of particles is proportional to its temperature (K). The rate of rxn increases exponentially with an increase in temperature because of an increase in KE, which increases collision frequency and means more particles have an energy greater than the activation energy

Maxwell-Boltzman distribution curve

Allows us to picture the relative energy levels of particles in a sample of gas at a particular temperature by plotting the energy of particles against number of particles.

Effect of catalysts on rate of reaction

Lowers activation energy by providing an alternative reaction pathway without itself being consumed. Therefore reaction occurs rapidly in the presence of a catalyst

Measuring rate of reaction by volume of gas produced

If a gas is a product, RoR can be measured by determining gas produced at regular intervals using an inverted burette or gas syringe.

Measuring rate of reaction by change in mass

If gas (other than hydrogen, too light) is a product, RoR can be measured by documenting mass of reactants and products as the reaction takes place. if plugged into a computer, can collect continuous data

Measuring rate of reaction by transmission of light

If a reactant or product is coloured, its transmission of light can be measured using a colorimeter or spectrophotometer with a pre-selected wavelength. As the concentration of a coloured substance changes, so will its absorbance of specific wavelengths. If plugged into a computer, can collect continuous data

Measuring rate of reaction by titration

A solution can be titrated at regular intervals against a standard to determine the concentration of product or reactant. A technique called quenching is used to pause the neutralization reaction.

Measuring rate of reaction by conductivity

A conductivity meter which involves inert electrodes being immersed in a solution, can measure presence of ions. Works if ions are a reactant or product

Measuring rate of reaction by ‘clock reactions’

Non-continuous, an arbitrary endpoint with some kind of observable factor is used to compare RoR of different reactions. Can only give an average rate of reaction over an interval of time.