C7 - Energy Changes
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Exothermic
-transfers energy to surroundings -increase temp of surroundings -more energy in reactants than in products
Exothermic eg
-combustion -neutralisation
Endothermic
–take in energy from surroundings -drop in temp of surroundings -more energy in products than in reactants
Endothermic eg
thermal decomposition
Hand warmers - disposable
-exothermic -oxidation of iron
Hand warmers - reusable
-place pack into boiling water to re dissolve crystals
Self heating can
-calcium oxide + water —> calcium hydroxide -exothermic
Cold packs
-endothermic -ammonium nitrate + water -pack is compressed + seal is broken -reaction occurs -non reusable
activation energy
minimum energy needed for colliding particles of reactants to have enough energy to cause a reaction
Less energy of particle collisions causes
-bounce off each other -do not react
Exothermic reaction profile
-high reactants at the start -low products at the end
Endothermic reaction profiles
-low reactants at the start -high products at the end
Breaking bonds
-endothermic -energy is needed to overcome bonds from surroundings
Making bonds
-exothermic -energy is transferred to surroundings
Exothermic bond energy calculations
-more energy needed to break bonds in reactants
Endothermic bond energy calculations
-more energy needed to form bonds in products
bond energy
energy needed to break the bond between 2 atoms
bond energy units
kJ/mol
overall energy change for exothermic
-negative value
overall energy change for endothermic
• positive value
simple electrical cells
-2 metals dipped in salt solution -connected by wire
simple electrical cells - electrons
more reactive metal donates electrons to the less reactive metal
the greater the difference in reactivity between the 2 metals
the higher the voltage produced by the cell is
current
flow of electrons