chem chapter 6

reaction rate

the change in concentration of a reactant or product over time.

Reactants will have a negative reaction rate, as they are consumed

Products will have a positive reaction rate, as they are produced.

average rate of reaction

the average change in concentration over a given time interval

represented by the slope of a line that is drawn between 2 points on the curve (secant line)

instantaneous rate of reaction

the rate of the reaction at a particular time

represented by the slope of a tangent line to the curve at that time

how to measure reaction rates

a chemist can monitor the rate of appearance of a product or disappearence of a reactant by monitoring observable changes

measuring reaction rates- monitoring colour

• in solutions that involve coloured reactants or products, colour changes can be observed, or its intensity can be measured with a spectrophotometer

e.g. The “iodine-clock reaction” is a reaction with a dramatic colorless-to-blue color change.

The color change occurs when iodine reacts with starch to form a dark blue iodine/starch comple

measuring reaction rates- monitoring mass

the decrease in mass can be measured in a reaction when a gas is produced in an open vessel

measuring reaction rates- measuring pH

changes in pH can be measured in reactions where an acid or base are involved

When a reactant is being consumed, the solution will become less acidic and pH will increase

measuring reaction rates- monitoring pressure and volume

when a reaction involve gases, the change in volume or pressure can be measured

collision theory

a reaction occurs between 2 particles if they collide at the correct orientation and with certain minimum energy

What is needed for a effective collision

1. molecules must be in the correct orientation

2. there must be sufficient collision energy

transition state

as a chemical reaction occurs, there exists a transition state, where chemical bonds are partially broken and partially formed

this transition is very unstable

activation energy

the energy required to reach the transition state

the minimum energy required for a successful collision to take place

potential energy diagram

shows the relative potential energies of reactants, transition states, and products as a reaction progresses with time

• the enthalpy change, ΔH, is the difference in the energy between the reactants and the products

• the activation energy, Ea, is the difference in the energy between the reactants and the transition state

activated complex

the highly unstable structure that forms at the transition state

• results at the maximum energy point along the reaction path

• it is neither a reactant nor product

endothermic reactions

in an endothermic reaction, the reactants are at a lower energy level compared to the products

exothermic reactions

• in an exothermic reaction, the reactants are at a higher energy

level compared to the products

5 factors that affect the rate of a reaction

1. concentration

2. temperature

3. surface area

4. chemical nature of the reactants

5. catalysts

concentration

as concentration increases, the rate increases

when more particles are present, there will be more collisions

as reactants are used up, the reaction slows down

temperature

when temperature increases, the average kinetic energy of the particles increases

more particles have the minimum energy (activation energy) required to react

surface area

with greater surface area, there are more particles accessible for collisions

chemical nature of the reactants

a more complicated reaction will react slower than a simple one

catalysts

substances that increase the rate of reaction without being consumed

• enzymes are biological catalysts (e.g., lactase, protease) catalysts hold molecules in the correct orientation

potential energy diagram- reactions with a catalyst

What is the rate law

The rate law is a mathematical expression that relates the rate of a reaction with the concentration of reactants.

The rate of a reaction is exponentially proportional to the initial concentrations of the reactants

reaction mechanism

a series of steps that make up an overall reaction

reaction intermediates

molecules that are formed in an elementary step and consumed in a subsequent elementary step

rate law for an elementary step

rate law for an elementary step is determined by its molecularity

requirements for a plausible reaction mechanism

the sum of the elementary steps must give the overall balanced equation for the reaction

the rate-determining step must predict the same rate law that is determined experimentally