CHEMISTRY TOPIC 5

Energy changes

Exothermic

transfer energy to the surroundings, so surrounding temperature rises

Exothermic reaction examples/ uses

combustion

neutralisation

oxidation

uses - self heating cans and hand warmers

Endothermic reaction

take in energy from surroundings, so surrounding temperature falls

Examples of endothermic reactions + uses

Thermal decomposition

citric acid

Use - sports injury

Energy conservation

energy is transferred in chemical reactions, not created or destroyed, the overall energy in the universe stays the same

Reaction profiles

show relative energies of reactants and products and how energy changes in the reaction as it progresses over time.

Activation energy

the minimum amount of energy that reactants need to react

ENDO THERMIC GRAPH (draw it)

( check flash card )

Exothermic graph

check flash card

Bond breaking is

endothermic ( strong bond → ( energy supplied ) → broken bond )

Bond forming is exothermic

C + O → strong bond formed + energy released

In endothermic reactions

energy used to break bonds is greater than energy released by forming new bonds.

In exothermic reactions

energy released by forming bonds is greater than energy used to break existing bonds

Bond energy calculations

overall energy change = total energy needed to break bonds - total energy released by forming new bonds.

Electrochemical cells

use chemical reactions to produce electricity

simple cell

reactions between electrolyte and electrodes set up, charge difference between electrodes. ( how this works, - two different electrodes ( usuallymetals ) placed inside electrolyte solution, at the top of the two oppositley charged electrodes is a voltmetre, both connected to a wire, from this you measure voltage of each cell.

Battery

two or more cells connected in series ( voltage of cells in battery are combined )

Non rechargeable cells

reaction is irreversible, cant be recharged once reactant is used up

Non rechargeable cells

reaction is reversible, can be recharged by connecting to external current,

Fuel cells

uses electrochemical oxidation of a fuel ( e.g. hydrogen ) to produce potential difference

Hydrogen fuel cell

( look at flash card NO.49)

Anode reaction

OXIDATION - h2 → 2h+ + 2e-

Cathode reaction

( reduction ) O2 + 4H+ + 4E- → 2h2o

ADVANTAGES of batteries

less pollution

less frequent replacing

lasts longer

cheaper to make

DISADVANTAGES of batteries

H2 needs storage space, hard to store safely

H2 production often uses fossil fuels