Energy from chemicals Flashcards
Energy law
Cannot be created or destroyed but it can be transformed from one form to another
Phase transition
change in state
Why steam burn worse than boiling water burn
The condensation of a vapour to a liquid is an exothermic process, and energy flows out of the system to the surroundings
What is the enthalpy change for endo and exo reactions (+/-)
Exo: < 0
Endo: > 0
change in H = H (products) - H (reactants)
Definition of exothermic change
A change in which heat is given out to the surroundings
Does temperature of reaction mixture for exo reactions increase/ decrease/ remain the same?
increase as heat is given out to the surroundings
what happens to the temperature of the reaction mixture after a while (exo)
it increases till the highest temperature is reached
when the reaction is complete, the temperature of the reaction mixture decreases till it reaches room temperature
What is a energy level diagram
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What are exothermic reactions
- condensation
- freezing
- dissolving an anhydrous salt in water
- dissolving an acid in water
- combustion of fuels (addition of O2)
- respiration
- neutralisation
- corrosion of metals (eg. rusting)
Definition of endothermic change
is a change in which heat is absorbed from the surroundings
Temperature of the reaction mixture for endothermic change
the temperature of the reaction mixture falls to the lowest temperature is reached before rising till it reaches room temperature
Temperature falls and container feels cooler
What is a endothermic change
- Evaporation
- Melting
- Dissolving some ionic compound and ammonium salts in water
- Photosynthesis
- Action of light on silver bromide in photographic film
- Thermal decomposition
Endo or exo?
1. Dissolving naoh in water
2. Precipitation
3. Harber process
all exo
Energy level diagram vs energy profile diagram
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Activation energy definition
the minimum energy that reacting particles must gain in order for a chemical reaction to occur (Ea)
Axis of energy profile diagram
Y axis is Energy/kJ
X axis is progress of reaction
Enthalpy diagram vs energy profile diagram
they r the same heehee
Bond breaking and bond forming
exo/endo?
bond breaking is endothermic
bond forming is exothermic
Bonds in water
2O-H bonds
Bonds in oxygen and hydrogen
O=O
and
H-H
Phrasing for exo
The total energy released from forming the ( ) bonds in ( ) is greater than the total energy absorbed for the breaking of ( ) bonds in ( ) and ( ) bonds in ( ).
Bond energies + SI Unit
energy required to break the covalent bonds in one mole of gaseous molecules into its atoms
SI Unit: kJ mol -1
Why bond energy can vary
values used are AVERAGE bond energies
how to calculate bond energies
total energy absorbed for bond breaking - total energy released for bond formation
