Physical Chemistry

Exothermic

Chemical reaction in which heat energy is given out (temperature rises)

Endothermic

Chemical reaction in which heat energy is taken in (temperature lowers)

Displacement reaction example

Mg displacing Copper from Copper (II) Sulfate

Copper (II) Sulfate measured and transferred into a polystyrene cup

Initial temp of the solution is measured

Mg added and the maximum temp is measured and recorded

The temp rise is then calculated

Q =

m x c x Δt

Molar enthalpy change (ΔH)

Amount of energy transferred (Q) in kJ / Number of Moles

Exothermic energy level diagram

In an exothermic reaction reactants have more energy than the products

Energy is given out in the form of heat which warms the surroundings

ΔH is given as a negative sign because the reactants are losing energy

Endothermic energy level diagram

In an endothermic reaction, the reactants have less energy than the products.

Energy is taken in which cools the surroundings

ΔH is given as a positive sign because the reactants are gaining energy

Bond breaking is an ___________ process

Endothermic

Because breaking bonds requires energy

Bond making is an __________ process

Exothermic

Because energy is released when new bonds are made

Therefore when the energy needed to break the bonds is less than the energy released in making new bonds

The reaction is exothermic

Enthalpy change ΔH =

Energy needed to break all the bonds - Energy released to make all the new bonds

Rate of reaction =

Amount of reactant used or amount of product formed / time

Rate of reaction is measured by

How quickly reactantsa re used up or how quickly the products are formed.

Investigating the reaction between marble chips and HCl

As the marble chips react with the acid, CO2 is given off.

The purpose of the cotton wool is to allow CO2 to escape, but to stop any acid from spraying out

The mass of CO2 lost is measured at intervals.

Investigating the effects of change in surface area of solid between marble chips and HCl

This time the chips are larger (have a smaller surface area)

Since the surface area is smaller, the rate of reaction is less

Same amount of CO2 produced

Investigating the effects of changes in concentration of solutions between marble chips and HCl

This time the there is half the concentration of acid
The marble chips however must be in excess

The reaction with half the concentration of acid happens slower and produces half the amount of CO2

Investigating the effects of changes in temperature on the rate of a reaction

Same quantities of everything but at a higher temperature

Higher rate of reaction

Investigating the effects of the use of a catalyst on the rate of a reaction

Rate of reaction increases because an alternative pathway with a lower activation energy is provided by the catalyst

Increasing the surface area of a solid (particle collision theory)

More particles exposed

Higher frequency of collisions

Increased rate of reaction

Increasing the concentration of a solution or pressure of a gas (particle collision theory)

More particles in the same space

Higher frequency of collisions

Increased rate of reaction

Increasing the temperature

Particles have more KE

Higher frequency of collisions

Higher proportion of those collisions are successful because the collision energy is greater or equal to the Activation Energy

Increased rate of reaction

Catalyst definition

A catalyst is substance that increases the rate of a reaction, but is chemically unchanged at the end of the reaction

How does a catalyst work?

Catalyst is not used up in a reaction

Speeds up a reaction by providing an alternative pathway with lower activation energy

Activation energy

The minimum amount of energy required for the reaction to be successful

Reaction profile diagram

the larger the chips the….

smaller the surface area

practical question: surface area is bigger (smaller marble chips)

faster rate of reaction

same amount of CO2 produced

practical question: solution concentration is half, marble chips are in excess

slower rate of reaction

half the amount of CO2 produced

practical question: catalytic decomposition result if substance is not a catalyst (liver is a very good catalyst)

no bubbles of oxygen produced

⇌

reversible reaction symbol

hydrated copper sulfate colour

blue

anhydrous copper sulfate colour

white

A reversible reaction can only reach dynamic equilibrium

In a sealed container

Characteristics of a reaction at dynamic equilibrium

forward and reverse reactions occur at the same rate

concentrations of reactants and products remain constant

Why does a catalyst not affect the position of equilibrium in a reversible reaction

A catalyst speeds up both the forward and the backward reactions.

Dynamic equilibrium is reached quicker, therefore the addition of a catalyst does not affect the position of equilibrium.

Equilibrium shift when the temperature is increased

Moves in the endothermic reaction

Equilibrium shift when pressure is increased

To the side with fewer molecules