Chemistry Topic 5 - Energy Changes

Exothermic reaction definition

A reaction that transfers energy to the surroundings so the temperature of the surroundings increases

Examples of exothermic reactions

Combustion / Many oxidation reactions / Neutralisation

Everyday uses of exothermic reactions

Self-heating cans / Hand warmers

Endothermic reaction definition

A reaction that takes in energy from the surroundings so the temperature of the surroundings decreases

Examples of endothermic reactions

Thermal decompositions / Reaction of citric acid and sodium hydrogencarbonate

Everyday uses of endothermic reactions

Injury ice packs

Activation energy

The minimum amount of energy that particles must have to react

Exothermic reaction profile feature

Products are at a lower energy level than the reactants because energy is released

Endothermic reaction profile feature

Products are at a higher energy level than the reactants because energy is taken in

Energy balance in bond breaking

Energy must be supplied to break bonds so bond breaking is an endothermic process

Energy balance in bond making

Energy is released when new bonds are formed so bond making is an exothermic process

Exothermic reaction bond energy condition

Energy released from forming new bonds is greater than the energy needed to break existing bonds

Endothermic reaction bond energy condition

Energy needed to break existing bonds is greater than the energy released from forming new bonds

Overall energy change calculation

Sum of energy needed to break bonds minus sum of energy released when bonds are formed

A chemical cell contains

Chemicals which react to form electricity

How a simple chemical cell is constructed

By connecting two different metals in contact with an electrolyte

What determines the voltage produced by a chemical cell

The type of electrode metals used and the type/concentration of electrolyte

Metal reactivity relationship to cell voltage

The greater the difference in reactivity between the two metals / the greater the voltage produced

Battery definition

Two or more chemical cells connected together in series to provide a greater voltage

Non-rechargeable cells and batteries

Stop producing voltage when one of the reactant chemicals is completely used up

Alkaline batteries

A common example of a non-rechargeable battery

Rechargeable cells and batteries

Can be recharged because the chemical reactions are reversed when an external electrical current is applied

Fuel cell definition

An electrochemical cell that is continuously supplied with an external fuel and oxygen to generate electricity

How a fuel cell operates

The fuel is oxidised electrochemically within the cell to produce a potential difference

Hydrogen fuel cell overall reaction product

Water only

Hydrogen fuel cell overall balanced chemical equation

2H2 + O2 -> 2H2O

Half equation at the negative electrode in an alkaline hydrogen fuel cell (HT only)

H2 + 2OH- -> 2H2O + 2e-

Half equation at the positive electrode in an alkaline hydrogen fuel cell (HT only)

O2 + 2H2O + 4e- -> 4OH-

Alternative half equation at the negative electrode in an acid hydrogen fuel cell (HT only)

H2 -> 2H+ + 2e-

Alternative half equation at the positive electrode in an acid hydrogen fuel cell (HT only)

O2 + 4H+ + 4e- -> 2H2O

Major advantage of hydrogen fuel cells over rechargeable batteries

They can be refueled quickly and do not degrade in performance over time

Major disadvantage of hydrogen fuel cells compared to batteries

Hydrogen is a flammable gas that is difficult and dangerous to store safely

Environmental benefit of hydrogen fuel cells

They produce zero greenhouse gases or pollutants at the point of use