Fast and Slow ChemistryFast and Slow ChemistryChapter 15

Fast and Slow ChemistryFast and Slow ChemistryRead page 247What would happen if this was a

slow reaction???

Chemical Energy – Chemical Energy – What is chemical energy?What is chemical energy?All substances have chemical

energy.

The chemical energy of a substance is the sum of its potential energy (store energy) and kinetic energy (energy of movement).

What is chemical energy?What is chemical energy?These energies result from such

things as:◦Attractions between electrons and

protons◦Repulsions between nuclei◦Repulsions between electrons◦Movement of electrons◦Vibrations of and rotations around bonds

The chemical energy of a substance is sometimes called its heat content or enthalpy.

It is given the symbol H

Energy changes during Energy changes during chemical reactionschemical reactionsDuring a

chemical reaction, the atoms in the reactants are arranged into products with different chemical energies.

Energy changes during Energy changes during chemical reactions – chemical reactions – Exothermic Exothermic The total chemical energy of the

products is less than the energy of the reactants.

Since energy is never lost, the difference in energy between reactants and products is released into the environment.

It is often released as heat energy.

Energy changes during Energy changes during chemical reactions – chemical reactions – Endothermic Endothermic The chemical energy of the

products is greater than the energy of the reactants.

Energy must be absorbed from the environment around the reactants in order for the reaction to occur.

Energy ChangesEnergy ChangesThe energy released or absorbed

during a chemical reaction is called the heat of reaction.

Since the heat of reaction is equal to the difference in enthalpy between the products and the reactants, it is given the symbol ∆H, where:◦∆H = H(products) – H(reactants)

∆∆H H For exothermic reactions, ∆H will be

negativeFor endothermic reactions, ∆H will be

positiveThis depends on whether H(products)

is greater than or less than H(reactants).

Most reactions we encounter are exothermic.

Thermochemical Thermochemical EquationsEquationsThermochemical equations show

the energy released or absorbed during a chemical reaction.

Energy is measured in Joules (J) or kilojoules (kJ).

6CO2(g)+6H2O(l)→C6H12O6(aq)+6O2(g); ∆H=+2803 kJ mol-1

C6H12O6(aq)+6O2(g)→ 6CO2(g)+6H2O(l); ∆H=-2803 kJ mol-1

Activation EnergiesActivation EnergiesCH4(g)+2O2→ CO2(g)+2H2O(g); ∆H=-890 kJ mol-

1

This is an exothermic reaction. The energy of the reactants is higher

than the energy of the products.Why doesn’t natural gas burst

immediately into flame and release energy when it comes into contact with air?

To start a gas oven, why must we use a match or a spark?

Activation EnergiesActivation EnergiesWell what happens to chemical

bonds during a reaction?The bonds between the atoms in

reactants must first be broken.◦For this to occur energy must be

absorbedThe new bonds form as the

products are created◦Energy is released as this happens

Activation EnergyActivation EnergyThe energy required to break the

bonds of reactants is called the activation energy.

A diagram showing this is called an energy profile.

Your TurnYour TurnPg 250Questions 1 and 2

Making Reactions Go Making Reactions Go FasterFasterThe rate at which chemical

reactions occur is an important consideration for industrial chemists and chemical engineers.

Some reactions take less than 10-

11 seconds, while others take years.

Considerable effort is directed towards maximising reaction rates in industry.

Collision TheoryCollision TheoryFor a chemical reaction to occur,

the particles involved must collide with each other with sufficient energy to overcome the activation energy ‘barrier’

The rate of reaction can depend on the number of collisions as well as the energy of the collisions being greater than the activation energy.

Factors that affect ratesFactors that affect ratesThere are four main ways in which

reaction rates can be increases:◦Increasing the surface area of solids◦Increasing the concentration of

reactants in solution (or pressure of gases)

◦Increasing the temperature◦Adding a catalyst

Explain to me how these factors can increase the rate of reaction?

Extending Collision TheoryExtending Collision Theory

CatalystCatalystMany reactions occur more rapidly

in the presence of particular elements or compounds.

These substances, known as catalysts, are not consumed during the reactions and therefore do not appear as either reactants or products in reaction equations.

CatalystsCatalystsMany catalysts have been

discovered by simple trial and error.

CatalystsCatalystsThere are two types of catalysts

◦Homogenous catalysts – these are in the same state as the reactants and products

◦Heterogeneous catalysts – these are in different states from the reactants.

Chemists prefer to use Heterogeneous catalysts as they are more easily separated from the products of a reaction

How do catalysts workHow do catalysts workParticles tend to adsorb to the surface of the

catalyst.Adsorption distorts bonds in the reactants

allowing the reaction to proceed more easily than it would if the catalyst was absent.

Essentially a catalyst lowers the activation energy required to break the bonds of the reactants.

The relative energies of the reactants and products are unaffected by the presence of the catalyst.

This means ∆H is not changed.

How do catalysts workHow do catalysts work

Over to youOver to youRead the Extension task on page

257Over to youPage 258Question 3 and 4