Studied by 3 people
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
Note 
Flashcard (34)