Chapter Ten Flashcards
Energy changes in chemical reactions
Where is chemical energy stored
In between the bonds between atoms and molecules
Chemical energy results from
- Attraction between e- and p+ in atoms
- Repulsions between nuclei
- Repulsions between e-
- Movement of electrons
- Vibrations and rotations around bonds
What is the SI unit for energy
Joule
Additional SI units for energy
- Kilojoules
- Megajoules
What is the relationship between Kj, Mj and J
1 MJ = 1000kJ = 1 000 000 J
Law of conservation of energy
Energy cannot be created or destroyed but it can change forms
System
The chemical reaction
Surroundings
Usually regarded as everything that is not the chemical reaction
Energy leaves the… into the … or leaves the … into the …
- Surroundings, system
- System, surroundings
What determines whether energy is released or absorbed
The relative energies of the bonds within the reactants and products
Will separated particles have more or less energy than when they were together
More
What happens to the bond when energy is absorbed
The bond breaks
What happens to energy when bonds form
Energy is released
What forms does energy take
- Heat
- Light
- Electricity
- Kinetic
Endothermic reactions
- Energy enters the system
- The total chemical energy of the reactants is less than the products
- Absorbs heat from surroundings so the environment becomes cooler
Exothermic reactions
- Energy is released into the surroundings
- Total chemical energy or reactants is more than the products
- All combustion reactions give off heat energy to the surroundings
Changes of state
- Phase changes
- Physical changes but still involves energy being released/absorbed
Heat taken from environment =
Endothermic
Heat released to environment =
Exothermic
Enthalpy (H)
The stored chemical energy of a substance
Enthalpy change/ heat of reaction (ΔH)
The exchange of heat between the system and its surroundings
Enthalpy change in endothermic reactions
- System has gained energy so ΔH is positive
- ΔH > 0
Enthalpy change in exothermic reactions
- System has lost energy so ΔH is negative
- ΔH < 0
Where are the enthalpy changes written in thermochemical equations
To the right of the equation
What units is enthalpy measured in
kJmol^-1
How do you compare enthalpy changes
Compare equal quantities of the substance
How is ΔH effected by reversing a chemical reaction
Reversing a chemical reaction changes the sign but not the magnitude of the ΔH (- or +)
Activation energy
The energy required to break the bonds of reactants so that a reaction can proceed
