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Energy
Changes (Reaction
Profiles (Exothermic
Diff. between start,…
Energy
Changes
Reaction
Profiles
Exothermic
Diff. between start, peak temp = activation energy
Diff. between reactant, product temp, given to surroundings
Endothermic
Diff. between start, peak temp = activation energy
Diff. between reactant, product temp, taken by surroundings
Bond breaking & making
Chem. react = old bonds break, new one made
Breaking - Endothermic; Making - Exothermic (simultaneous)
Energy change - Energy comparison in breaking, making
Fuel Cells
Hydrogen power
2 H2 + O2 = H2O; burns well, produces no pollutants
Safety, storage problems; supply making needs fossil fuels
Fuel cell - same reaction; still needs constant H supply
Trying to match fossil performance, convenience, price
Hydrogen fuel cell
H gas supplied to -ve, diffuses through; react w/ OH-
O gas supplied to +ve, reacts to form OH-
-ve gives e- to external unit; +ve accepts e-
-ve = 2 H2(g) + 4OH-(aq) = 4 H2O(l) + 4e-
+ve = O2(g) + 2 H2O(l) + 4e- = 4OH-(aq)
Overall (cancel out some chemicals) leads to same react
2 H2(g) + O2(g) = 2 H2O(l)
Pros & Cons:
+ Different sizes
+ No electric recharge, pollutants
- H is flammable, difficult store
- sometimes produced non-renewable
Exothermic,
Endothermic
Exothermic reactions
Energy transfer to surroundings; E.g. metals & acids
Fuel (Methane) burning - oxidised; Neutralisation etc.
Products have lo-er energy content ; Diff. in kJ/mol
Methane burning = 890 kJ/mol energy transfer
Endothermic reactions
Less common - citric acid + sodium hydrogen-carbonate
Thermal decomposition (CaCO3) - becomes CaO + CO2
Reaction needs constant energy source (Bunsen flame)
Product has hi-er energy content (Thermal = 178kJ/mol)
Energy
transfer
Warming up
Hand warmers - cold weather, aches, pains
Disposable - Fe oxidation into hydrated Fe(III) oxide
(Similar to rusting) NaCl catalyst; lasts hours
Reusable - Crystallisation from salt solutions
Often Sodium ethanoate - supersaturated solution
Dissolve in hot water, cool; metal scrapings start react
Self-heat cans - CaO + H2O = Ca(OH)2 - took yrs, Ms of $
Third of can for reactants to mix; early vers. didn't heat enough
Cooling down
Chem. cold packs - Ammonium nitrate + Water
Treat sport injuries - reduce swell, numbs pain
Sometimes in gel; instant take 20 mins
Often in first aid; can also be used to chill cans
Bond Energy
Calculations
Making and Breaking
Balance between energy release and gain
Exothermic - more energy released than gained
Endothermic - more energy gained than released
Bond energy
Energy needed to break the bond between 2 atoms
Measured in kJ/mol; calc. chem. react energy changes
Cl2 = 243 kJ/mol; HCl = 432 kJ/mol; N2 = 945 kJ/mol
Calculation = bond energies & energy released in making
Making & Breaking have same amount of energy involved
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