Chapter 5

Question 1

Thermochemistry is

Answer 1

the branch of chemistry that studies the E changes in chemical reactions

Question 2

Energy is

Answer 2

the ability to do work (to make things move).
- its the driving force of most chem reactions
- UNIT: Joule (J), Calorie –> used in America & in food

Question 3

Kinetic Energy is

Answer 3

E of motion/speed (of either atoms or molecules). –> cant be negative, no direction rmr
- it’s directly related to the type of motion & the speed at which the atoms or molecules r moving.

Question 4

3 Types of Molecular Motions

Answer 4

1) Vibration–> the movement of the atoms along the bonds in a molecule (in solid, liquid, & gas)
2) Rotation–> When an atom or molecule spins on its axis (in liquids & gas)
–> rmr no empty space between atom molecules between solids & liquids
3) Translation–> When a molecule moves from 1 place to another (only in gas)

KINETIC ENERGY: Vibration < Rotation < Translation

Question 5

Potential Energy

Answer 5

Chemical E stored in the bonds between atoms in a molecule

Question 6

Temperature is

Answer 6

the average KE of the particles in a substance. It tells us how fast the particles in a substance, on average are moving.
- Unit: degrees Celsius, F, Kelvin
–> kelvin is the accurate one cuz Celsius is relative to water –> Ex, -10 degrees Celsius means 10 degrees Celsius below freezing of water –> rmr u can’t have negative KE

Question 7

Thermal Energy is

Answer 7

the sum of ALL kinetic energies of the particles. The thermal E in an object depends on 2 factors
1) # of particles: the more particles there are, the greater the thermal E
2) Speed of particles: The faster the particles r moving, the greater the thermal E.

Question 8

2nd law of thermodynamics

Answer 8

• when 2 objects r in thermal contact, heat is always transferred from the object at higher temp to the object at the lower temp until 2 objects r at the same temp

Question 9

Heat is

Answer 9

the transfer/flow of thermal E from a HOT object to a COLD object.
- Symbol: Q
- Unit: Joules or Calorie
- when the fast-moving particles collide with the slow-moving particles some of the KE is transferred to the slower particles, so the slower ones start moving faster (gain KE) while the fast-moving particles slow down (losing KE)
- Thermal KE has been transferred as heat between the objects

Question 10

Systems

Answer 10

A system refers to all chemical components that r involved in a chem reaction –> the reactants, products & wtv solvents they r in
Surroundings: Everything outside the system we r studying
- OPEN SYSTEM: mass transfer & heat transfer –> open can of cold drink
- CLOSED SYSTEM: heat transfer –> sealed can of cold drink
- ISOLATED SYSTEM: NO mass or heat transfer –> perfectly insulated thermos

Question 11

Specific Heat Capacity is

Answer 11

• symbol: c
  is the amount of E needed to increase the temp of 1 gram of substance by 1 degree Celsius
• its a physical property of a substance
• the smaller the c value, the more quickly a substance changes in temp. –> it will get hotter & colder quickly due to not being able to resist change in E
• Ex, specific heat capacity of water=4.18 J/g degrees C

Question 12

Endothermic & Exothermic reaction

Answer 12

Exothermic reaction = when heat leaves a system, ∆T is negative & thus so is Q

Endothermic reaction = When heat enters a system, ∆T is positive & thus so is Q

Question 13

Bond Energy

Answer 13

• AKA bond dissociation energy
• Bond energy is the measure of bond strength
• it is experimentally determined by measuring the E required to break the atoms involved in a molecular bond into free atoms

Question 14

In a chem reaction, we change both the types of molecules & the types of bonds

Answer 14

• PE is the E stored in chemical bonds. –> If we change the types of bonds from the reactant side to product side, the PE will change –> PE=sum of bond energy
• KE is the E of motion/speed of the molecules. –> If we change the structure (mass, formula) of the molecules, the KE will also change
  –> in addition, cuz the mass of each molecule also changes the reactant side to the product side, the KE also changes
• KE= 1/2mv^2

Question 15

Enthalpy is

Answer 15

the total energy in a system
- Enthalpy= KE + PE
- during a reaction, E is absorbed to break the bonds in the reactants & released when new bonds are formed in the products
–> therefore, ∆H= change in enthalpy (energy) if a system= sum of E required to break old bonds + sum of E required to make new bonds

• if ∆H is negative –> there is a net loss of E in the reaction= EXOTHERMIC REACTION
• if ∆H is positive –> there is a net gain of energy in the reaction= ENDOTHERMIC REACTION

Question 16

if Q is positive… if Q is negative…

Answer 16

if Q is positive, reaction is exothermic
if Q is negative, reaction is endothermic
- rmr if this if Q is calculating heat of system not surrounding

Question 17

Exothermic Reactions

Answer 17

• Bond breaking (energy absorption) < bond forming (energy released)
• the system releases (loses) energy to the surrounding –> surrounding becomes hotter
• ∆H= negative
• in a thermochemical equation, ∆H is written on the products side

Question 18

Endothermic Reactions

Answer 18

• Bond breaking (energy absorption) > bond forming (energy released)
• the system absorbs (gains) energy to the surrounding –> surrounding becomes colder
• ∆H= positive
• in a thermochemical equation, ∆H is written on the reactant side

Question 19

Molar Enthalpy

Answer 19

∆Hm
- the enthalpy change of exactly 1 mole of a substance undergoing a change
- Unit: KJ/mol

Question 20

Reaction Enthalpy

Answer 20

∆Hrxn
- the total (net) enthalpy change of a reaction, for any given amount
- Unit: KJ

Question 21

Calorimetry

Answer 21

• A calorimeter is a device used to measure heat of a reaction
• It can be sophisticated & expensive or simple & cheap (styrofoam cup)
• the reaction is done in water inside an enclosed container (calorimeter)
• A thermometer is used to measure the temp of the solution before & after the reaction

Question 22

Calorimeter: Scenario 1: the reaction is exothermic…

Answer 22

it releases energy (heat) into the surrounding (water)
- the temp of water increases
- the enthalpy lost by the reaction (∆Hrxn) is gained by the water (Q)

Question 23

Calorimeter: Scenario 2: the reaction is endothermic…

Answer 23

it gains E (heat) from the surrounding (water)
- the temp of water decreases
- the enthalpy gained by the reaction (∆Hrxn) is form by the water (Q)

Question 24

Hess’s law of Additivity of Reaction Enthalpies

Answer 24

• the enthalpy change (∆H) if a reaction depends only on the initial and final conditions of the process. ∆H of a multistep process is the sum of the enthalpy changes of its individual steps
• if a chemical equation is reversed, then the sign of ∆H changes
• if the coefficients are altered by a factor, ∆H is altered as well in the same manner.

Question 25

The standard enthalpy of formation

Answer 25

∆H°f
- the change in enthalpy E when 1 mol of a compound is formed directly form its elements in their most stable state at SATP (standard ambient temp 25°C & pressure 1000Kpa)
- CORRECT FORMAT: element + element –> compound
- ∆H°f for elements already in their standard state at SATP is 0
–> Ex, O2 (g), K(s), Mg (l)

Question 26

Heating Curves - Diagonal Parts

Answer 26

temp rises, KE increases
- heat is used to increase the motion & speed of the molecules (vibration, rotation, translation) –> the speed (KE) the water molecules increases –> temp rises
- FORMULA: Q=mc∆T

Question 27

Phase Change- The Flat Parts

Answer 27

temp stays constant (KE is constant, PE increases)–>Ex, E used to break SOME of the H-bonds in water
- A phase change is when matter changes from one state to another
- it occurs when sufficient energy is supplied to the system (of when a sufficient amount is lost), & also occur when the pressure on the system is changed.
- during phase change the temp doesn’t change cuz the heat is used to break the intermolecular interactions between H20 molecules (PE)–> the speed (KE) the waster molecules are moving at doesn’t change.