Chemistry - Energy changes

Exothermic reactions

-reactions that release energy to the surroundings

-the products have less energy than the reactants

Why do exothermic reactions release energy?

Because more energy is released when new bonds form than is needed to break old bonds

Examples of exothermic reactions

combustion, respiration, neutralisation

Where are exothermic reactions used

-Hand warmers

-self heating cans

-burning fuels

Exothermic reaction change diagram

Endothermic reaction

-A reaction in which energy is absorbed from the surroundings

-the products have more energy thaan the reactants

Why do endothermic reactions absorb energy?

More energy is needed to break the bonds than is released when new ones form

Examples of endothermic reactions

Thermal decomposition, photosynthesis

Where are endothermic reactions used

-ice packs

-evaporation

Endothermic reaction change diagram

activation energy

The minimum amount of energy required to start a chemical reaction.

breaking bonds

endothermic process absorbing energy

making bonds

Exothermic process releasing energy

energy change formula

Overall energy change = energy put in - energy taken out

negative overall energy change

exothermic reaction

positive overall energy change

endothermic reaction

chemical cells

a device that produces energy from a chemical reaction

How chemical cells work

-two metals act as electrodes

-the more reactive metal loses electrons and becomes the anode

-the less reactive one gains ean lectron and becomes the cathode

non-rechargeable batteries

Once the reactants are used up the battery is dead

rechargeable batteries

Chemical reactions can be reversed when an external circuit is supplied

Fuel cells advantages

higher efficiency, lower emissions, reduced noise pollution

Fuel cells disadvantages

high cost, limited fuel availability, efficiency losses, environmental impacts, durability, safety concerns

fuel cells

a type of electrochemical cell which provides electricity continuously, as long as it is supplied with a fuel (like hydrogen) and oxygen