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!

Energy 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 1 st, then new bonds are made!

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.”

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

Energy 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

Photosynthesis Reactions happening in cooking

890.3 kJ energy is released 25.5 kJ energy is gained

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

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

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

CH 4 (g ) + C 2 H 5 OH (l) + H 2 (g) + Which of these

do not burn. Have unstable radioisotopes. These break down naturally into new atoms, producing a lot of energy. No CO 2 or air pollutants released. Finding a safe place for the radioactive waste and risk of explosions (spreading the radioactive material)are problems.

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)  Zn 2+ (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 -  H 2 (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 - are LOST!

1) Draw the diagram of a cell formed with Mg and Silver metals dipped into aqueous solution of AgNO 3. 2) Identify the substances oxidized and reduced.

1) 2)

-Electrodes are C. - Not running out of H 2. -Only water is formed (no pollutants)! -Plenty of energy (2.5 times as much as methane) H 2 is very flammable! Risk of explosion…