Chemistry - 7 Energy Changes

Exothermic reaction

a reaction that releases energy in the form of heat

Endothermic reaction

a reaction in which energy is absorbed

Neutralisation: endo- or exo-?

exothermic

Combustion: endo- or exo-?

exothermic

Examples of exothermic reactions [3]:

• neutralisation

• combustion

• respiration

Thermal decomposition: endo- or exo-?

endothermic

calcium carbonate thermal decomposition

calcium carbonate --> calcium oxide + carbon dioxide

Uses of exothermic reactions [2]:

• hand warmers

• self-heating coffee

Uses of endothermic reactions:

• cold packs

Disposable hand warmer [2]

• iron + sodium chloride --> iron (III) oxide

• lasts for hours

Reusable hand warmer [5]

• often sodium ethanoate (CH3COO-Na+)

• supersaturated solution of dissolved salt

• crystallises when metal disc is pressed

• put in boiling water to re-dissolve

• lasts about 30 minutes

Self-heating can reaction:

calcium oxide + water --> calcium hydroxide

Instant cold packs [3]

• ammonium nitrate and water

• when dissolved, takes in energy

• lasts about 20 minutes

Exothermic reaction profile

High bar, curve, low bar

Endothermic reaction profile

Low bar, curve, high bar

Energy change during reaction =

difference between products and reactants

Activation energy

minimum energy needed to get a reaction started

Activation energy =

reactants to peak of curve

Bond breaking: endo- or exo-?

endothermic

Bond making: endo- or exo-?

exothermic

Bonds in exothermic reactions

making bonds > breaking bonds

Bonds in endothermic reactions

making bonds < breaking bonds

Bond energy

the amount of energy that will break a bond between two atoms

To calculate the energy change for a chemical reaction:

• how much energy is needed to break the bonds

• how much energy is released when bonds are formed

If the overall energy change is negative, the reaction is...

...exothermic

To find the overall energy change:

add the energy required to break bonds and the energy released by their formation

Alkaline fuel cell: hydrogen half equation

H₂ + 2OH⁻ → 2H₂O + 2e⁻

Alkaline fuel cell: oxygen half equation

O₂ + 2H₂O + 4e⁻ → 4OH⁻

Acidic fuel cell: hydrogen half equation

H₂ → 2H⁺ + 2e⁻

Acidic fuel cell: oxygen half equation

O₂ + 4H⁺ + 4e⁻ → 2H₂O

Alkaline fuel cell: full equation

2H₂ + O₂ → 2H₂O

Acidic fuel cell: full equation

2H₂ + O₂ → 2H₂O

Advantages of hydrogen fuel [3]:

• no electrical recharging

• no pollutants

• range of sizes

Disadvantages of hydrogen fuel [3]:

• highly flammable

• produced by non-renewable resources

• difficult to store