LT#1: Explaining Enthalpy Changes Flashcards
Define Thermodynamics
The study of energy and energy transfer
What is the first law of Thermodynamics?
Law of Conservation of energy.
Energy cannot be created or destroyed; therefore the energy in the universe is always constant.
Define the term System
The part of the universe being studied
Define the term Surroundings
Everything else in the universe not part of the system, including the beaker, any water your chemical are dissolved in, and yourself
Define Enthalpy (H)
Total internal energy of a substance at a constant pressure.
Measured in KJ or KJ/mol
When do Enthalpy changes occur?
When the type and number of bonds in your chemical system change.
When a new bond or IMF is formed energy is …
Released
When a new bond or IMF is broken energy is …
Required/absorbed from surroundings.
Define Exothermic Change
When the amount of energy released forming the bonds in the products is greater than the energy required to break the bonds in the reactants, there is a net release of energy to the surroundings.
Define Endothermic Change
When the amount of energy required to break the bonds in the reactants is greater than the energy released forming the bonds in the products, there is a net absorption of energy by the system.
How do you tell if a reaction is endo or exo if given a thermochemical equation?
If endothermic, energy is a reactant because it is required for the reaction to occur.
If exothermic, energy is a product because it is being released.
How do you tell if a reaction is endo or exo if given a ∆H value?
Endothermic reactions have positive ∆H’s because energy is entering the system.
Exothermic reactions have negative ∆H’s because energy is leaving the system.
How do you tell if a reaction is endo or exo if given an energy diagram?
If exothermic the reactants have higher energy than the products (arrow goes down)
If endo the products have higher energy than reactants (arrow goes up)
Why does changing a chemical’s state create an enthalpy change?
Involves breaking the intermolecular forces (endothermic) between particles going from solid to liquid to gas or forming intermolecular forces (exothermic) going the other direction.
How does dissolving a solute create an enthalpy change?
Depending on the type and number of intermolecular forces involved, dissolving can be either endothermic or exothermic.
