Thermochemistry

Question 1

Enthalpy

Answer 1

Stored chemical energy

only changes in enthalpy can be measured (during chemical reactions)

Question 2

Heat of fusion

Answer 2

Amt of heat formed as liquid freezes or a solid melts (basically whenever s is involved)

formula: MΔH f

Question 3

Heat of Vaporization

Answer 3

Heat released a gas condenses or liquid evaporates

formula: M∆Hvap

Question 4

Energy

Answer 4

Ability to do work (in J)

Question 5

Heat

Answer 5

Form of energy (som. call thermal energy) that is transferred from hot substances to cold ones

Question 6

Heat conduction occurs when….

Answer 6

High temp (faster particles) bump into lower temp (slower particles) transferring their kinetic energy

Question 7

Specific heat

Answer 7

Resistance to temp change

Question 8

measuring heat exchange

Answer 8

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 :)

Question 9

Effect of low specific heat on metals? Include water in answer

Answer 9

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)

Question 10

Melting and boiling point are endothermic

While freezing and condensing point are exothermic

Answer 10

This is bc phase changes release or absorb energy

Question 11

Why does temp not rise until ice cubes are completely melted?

Answer 11

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

Question 12

Why does heat stay constant during crystallization?

Answer 12

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

Question 13

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?

Answer 13

Represents specific heat and shows how it takes longer to boil water, and how resistant liquids are to temp change

Question 14

Why are heats of fusion generally smaller than heats of vaporization?

Answer 14

Heat of fusion are generally smaller bc it takes more energy to go from liquid to gas

Question 15

Spontaneity

Answer 15

Reactions that occur without continuous outside help

Question 16

Determined by 2 factors

Answer 16

1. Most are exothermic (give off enthalpy)

2. Most have increase of entropy

Question 17

Entropy ΔS

Answer 17

Randomness/disorder of the system

Question 18

Increases when…

Answer 18

• Liquid turns to gas
• Solid into liquid or gas
  (through phase changes or reactions)
• causes an increase in the number of moles

Question 19

Entropy reaction

Answer 19

ΔSp- ΔSr = Δ S

Question 20

Gibbs free eq

Answer 20

Energy available to do work

Question 21

Determining spontaneity eq

Answer 21

ΔG = ΔH -TΔS,

Question 22

Reaction rate

Answer 22

Change in concentration of substance over time

Question 23

Collision theory

Answer 23

1. In order to react particles must collide w/ enough KE to break existing bonds (exothermic)
2. Collide with correct orientation (creating new bonds endothermic)

Question 24

Factors Effecting Reaction rate (1)

Answer 24

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

Question 25

Factors Effecting Reaction rate (2)

Answer 25

2. Increase pressure of gas reactants (more collisions)

Question 26

Factors Effecting Reaction rate (3)

Answer 26

3. Increase temp
   - Faster particles move faster as they overcome the activation hill
   - Have enough KE (most reactions happen faster at higher hills)

Question 27

Factors Effecting Reaction rate (4)

Answer 27

4. Increase surface area

- Causes more reactant particles to be exposed to one another (more collisions)

Question 28

Factors Effecting Reaction rate (5)

Answer 28

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