chemical energetics

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CHEMICAL ENERGETICS ENERGY CHANGES IN CHEMICAL RXNS

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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 reactions. Energy is GIVEN OUT! Temperature of the surroundings RISES! Container feels hotter. - PowerPoint PPT Presentation

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Page 1: CHEMICAL  ENERGETICS

CHEMICAL ENERGETICS

ENERGY CHANGES IN CHEMICAL RXNS

Page 2: CHEMICAL  ENERGETICS

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

Page 3: CHEMICAL  ENERGETICS

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

Page 4: CHEMICAL  ENERGETICS

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

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

Page 5: CHEMICAL  ENERGETICS

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

Page 6: CHEMICAL  ENERGETICS

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.”

Page 7: CHEMICAL  ENERGETICS

Examples of exothermic reactions:

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

Page 8: CHEMICAL  ENERGETICS

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

Page 9: CHEMICAL  ENERGETICS

Examples of endothermic reactions:

PhotosynthesisReactions happening in cooking

Page 10: CHEMICAL  ENERGETICS

890.3 kJ

energy is

released

25.5 kJ energy

is gained

Page 11: CHEMICAL  ENERGETICS

ENERGY FROM FUELS

Page 12: CHEMICAL  ENERGETICS

Fuel…

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

Page 13: CHEMICAL  ENERGETICS

The fossil fuels…

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

Page 14: CHEMICAL  ENERGETICS

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)

Page 15: CHEMICAL  ENERGETICS

Equations of the fuels for burning…

CH4(g ) +

C2H5OH (l) +

H2 (g) +

Which of these

Page 16: CHEMICAL  ENERGETICS

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.

Page 17: CHEMICAL  ENERGETICS

SIMPLE CELLS TO PRODUCE ENERGY (ELECTRICITY)

Page 18: CHEMICAL  ENERGETICS

How to make a simple cell?

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

Page 19: CHEMICAL  ENERGETICS

-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!

Page 20: CHEMICAL  ENERGETICS
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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.

Page 22: CHEMICAL  ENERGETICS
Page 23: CHEMICAL  ENERGETICS

Differences between electrolysis and voltaic cells:1)

2)

Page 24: CHEMICAL  ENERGETICS

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…