CHEMICAL ENERGETICS

ENERGY CHANGES IN CHEMICAL RXNS

In chemical rxns, energy is always given out or taken in. This energy is usually in the form of HEAT!

Exothermic reactionsEnergy is GIVEN OUT!Temperature of the surroundings RISES! Container feels hotter.

Products have LOWER energy than the reactants!

Energy level diagram of exothermic reactions

Energy changes in reactions are caused by the breaking & making of chemical bonds.

Bond Making –> Exothermic Process Bond Breaking –> Endothermic Process

Bonds have to be broken 1st, then new bonds are made!

Activation Energy

In order to break the bonds, the reactant particles must collide with each other. However, these collisions must happen with sufficient energy in order to break the bonds in the reactants.

The minimum amount of energy required to break the bonds in the reactant (or start the reaction) is called “activation energy.”

Examples of exothermic reactions:

Neutralization (acid-base) reactions Combustion reactions (burning fuels like natural gas, ethanol, and hydrogen)RespirationMetal+acid rxns

Endothermic reactionsEnergy is TAKEN IN!Temperature of the surroundings GOES DOWN! Container feels cold.

Products have HIGHER energy than the reactants!

Energy level diagram of endothermic reactions

Examples of endothermic reactions:

PhotosynthesisReactions happening in cooking

890.3 kJ

energy is

released

25.5 kJ energy

is gained

ENERGY FROM FUELS

Fuel…

Any substance we use to provide energy.The burning of fuels is endothermic / exothermic.

The fossil fuels…

Coal, petroleum (oil), natural gas (methane)

Two fuels growing in importance…

Ethanol (made from sugar cane and corn----mixed with petrol or used in car engines)Hydrogen (used in fuel cells)

Equations of the fuels for burning…

CH4(g ) +

C2H5OH (l) +

H2 (g) +

Which of these

Nuclear fuels…do not burn.Have unstable radioisotopes. These break down naturally into new atoms, producing a lot of energy.No CO2 or air pollutants released.Finding a safe place for the radioactive waste and risk of explosions (spreading the radioactive material)are problems.

SIMPLE CELLS TO PRODUCE ENERGY (ELECTRICITY)

How to make a simple cell?

Two different metals (electrodes) are dipped into an electrolyte and connected externally with a conductive wire.Electricity is ….

-Zn is more active metal than Cu. Therefore, it loses electrons and becomes oxidized. Zn (s) Zn2+(aq) + 2e-

-Electrons flow along the wire to the Cu strip, as a current.

-H+ ions in the electrolyte gain the electrons on the Cu strip and form Hydrogen gas.

-2H+(aq) + 2e- H2(g)

So;-Electricity is produced as a results of constant motion of electrons in the external circuit (wire).-A redox rxn is happening to produce the electricity in the simple cells.

At –

pole

,

e- a

re

LOST!

Example1) Draw the diagram of a cell formed with Mg and Silver metals dipped into aqueous solution of AgNO3.2) Identify the substances oxidized and reduced.

Differences between electrolysis and voltaic cells:1)

2)

THE HYDROGEN FUEL CELL

- Electrodes are C. - Not running out of H2.- Only water is formed (no pollutants)!- Plenty of energy (2.5 times as much as methane)

H2 is very flammable! Risk of explosion…