Thermochemistry Flashcards
Enthalpy
Stored chemical energy
only changes in enthalpy can be measured (during chemical reactions)
Heat of fusion
Amt of heat formed as liquid freezes or a solid melts (basically whenever s is involved)
formula: MΔH f
Heat of Vaporization
Heat released a gas condenses or liquid evaporates
formula: M∆Hvap
Energy
Ability to do work (in J)
Heat
Form of energy (som. call thermal energy) that is transferred from hot substances to cold ones
Heat conduction occurs when….
High temp (faster particles) bump into lower temp (slower particles) transferring their kinetic energy
Specific heat
Resistance to temp change
measuring heat exchange
q=mc∆t
q-Heat (j)
m-Mass (g)
c-Specific heat (j/g°C)
∆t-Temp in (k/°C) - dosnt matter bc increments are the same :)
Effect of low specific heat on metals? Include water in answer
Metals have low specific heat so they are able to transfer heat quickly, not taking a lot of effort to change temp
Water has high specific heat (good at absorbing heat)
Melting and boiling point are endothermic
While freezing and condensing point are exothermic
This is bc phase changes release or absorb energy
Why does temp not rise until ice cubes are completely melted?
Room is continually adding heat as ice cubes melt, so the temp does not rise bc this heat is going to the endothermic process of breaking hydrogen bonds b/w molecules
Why does heat stay constant during crystallization?
Crystallization/freezing is exothermic giving off or generating heat so the temp stays constant until the process is done and all that’s left is a solid block of ice
Substance A has a heating curve with a longer plateau at the boiling point than substance B. What (at a molecular level) might account for this difference?
Represents specific heat and shows how it takes longer to boil water, and how resistant liquids are to temp change
Why are heats of fusion generally smaller than heats of vaporization?
Heat of fusion are generally smaller bc it takes more energy to go from liquid to gas
Spontaneity
Reactions that occur without continuous outside help
Determined by 2 factors
- Most are exothermic (give off enthalpy)
2. Most have increase of entropy
Entropy ΔS
Randomness/disorder of the system
Increases when…
- Liquid turns to gas
- Solid into liquid or gas
(through phase changes or reactions) - causes an increase in the number of moles
Entropy reaction
ΔSp- ΔSr = Δ S
Gibbs free eq
Energy available to do work
Determining spontaneity eq
ΔG = ΔH -TΔS,
Reaction rate
Change in concentration of substance over time
Collision theory
- In order to react particles must collide w/ enough KE to break existing bonds (exothermic)
- Collide with correct orientation (creating new bonds endothermic)
Factors Effecting Reaction rate (1)
- Increase in concentration rate
- leads to more collisions or like-hood of collisions
- through higher morality of aq solution, or higher concentration of gases
Factors Effecting Reaction rate (2)
- Increase pressure of gas reactants (more collisions)
Factors Effecting Reaction rate (3)
- Increase temp
- Faster particles move faster as they overcome the activation hill
- Have enough KE (most reactions happen faster at higher hills)
Factors Effecting Reaction rate (4)
- Increase surface area
- Causes more reactant particles to be exposed to one another (more collisions)
Factors Effecting Reaction rate (5)
- Add catalyst
- Substances that lower reaction rate w/ out being used up in reaction
- Creates lower energy activation complex, offering different reaction route w/ less Ea
