Unit 2 - Energy Changes and Rates of Reaction

Question 1

Thermochemistry (definition)

Answer 1

The study of energy changes in chemical and physical processes

Question 2

Law of Conservation of Energy

Answer 2

states that the total energy of the universe is constant, energy cannot be created or destroyed, only transferred

Question 3

System and Surroundings (definitions and relationship)

Answer 3

System is part of universe being observed (ex contents of beaker), the reaction

Surroundings is everything else in the universe (ex air, glassware), absorb energy from system or give energy required for system

Energy gained by system is lost by surroundings, energy lost by system is gained by surroundings.
ΔEsys = -ΔEsurr

Question 4

Temperature (definition)

Answer 4

Directly related to the average kinetic energy of the particles of a system
-measuring temp before and after a physical or chemical change helps us calculate the amount of heat that has entered or left the system

Question 5

Heat (definition)

Answer 5

Refers to the transfer of kinetic energy, measured in Joules
Also called enthalpy or thermal energy

Question 6

Molar Enthalpy Change, ΔH (definition)

Answer 6

the amount of heat transferred when one mole of a substance reacts kj/mol

Question 7

Endothermic Reactions

Answer 7

-heat is ABSORBED (into system)
-surroundings get cooler
- positive ΔH
-heat is a REACTANT

Question 8

Exothermic Reactions

Answer 8

-heat is RELEASED (into surroundings)
-surroundings get warmer
- negative ΔH
-heat is a PRODUCT

Question 9

Endothermic vs Exothermic: Changes in State or Temperature

Answer 9

EXOTHERMIC
removing energy: solid←liquid←gas

ENDOTHERMIC
adding energy: solid→liquid→gas

Question 10

Endothermic vs Exothermic: Chemical Reactions (making/breaking bonds)

Answer 10

EXOTHERMIC
making bonds

ENDOTHERMIC
breaking bonds

Question 11

Measuring Temperature of Physical Change (what does thermometer measure, exo and endo rxns)

Answer 11

Thermometer measures temp change of SYSTEM

EXOTHERMIC: decrease in temp, sys losing heat
ENDOTHERMIC: increase in temp, sys gaining heat

Question 12

Measuring Temperature of Chemical Change or Dissolving (what does thermometer measure, exo and endo rxns)

Answer 12

Thermometer measures temp change of SURROUNDINGS

EXOTHERMIC: increase in temp of surr., sys losing heat
ENDOTHERMIC: decrease in temp of surr., sys gaining heat

Question 13

Hess’ Law of Heat Summation (definition/use)

Answer 13

-Enthalpy change of a physical or chemical process depends only on the initial and final conditions of the process;
-enthalpy change of overall process is the sum of the enthalpy changes of its individual steps

Question 14

Reaction Rate (meaning, how to measure)

Answer 14

-The change in concentration of a reactant or product over time
-can measure rate of appearance/disappearance of substance by monitoring observable changes such as: colour, mass, pH, pressure + volume

Question 15

Collision Theory

Answer 15

A reaction occurs between two particles if they collide at the correct orientation and with minimum energy. In order to be effective:
1. molecules must be in the correct orientation
2. must have sufficient collision energy (activation energy - minimum collision energy required for rxn to take place)

Question 16

Factors Affecting Speed of Reaction (5 factors)

Answer 16

1. as concentration ↑, ROR ↑ (more particles, more collisions)
2. as temperature ↑, ROR ↑ (kinetic energy increases, more particles have minimum activation energy)
3. as surface area ↑, ROR ↑ (more particles accessible for collisions)
4. more complicated rxns occur slower than simple ones
5. Catalysts make rxns happen faster (decrease activation energy)

Question 17

Transition State

Answer 17

-Transition state corresponds to the point on the potential energy diagram/graph with the highest potential energy
-point where bonds break and form at the same time

Question 18

Reaction Mechanisms (definition, intermediates, catalysts)

Answer 18

-series of steps that make up an overall reaction
-each step is called an elementary step, involves a single molecular event
-reaction intermediates are molecules formed in the elementary step and consumed in a subsequent step
-Catalysts are used in one elementary step and created in a subsequent step

Question 19

Rate Determining Step

Answer 19

slowest step in the reaction mechanism which determines the overall rate

Question 20

Requirements for Plausible Reaction Mechanism

Answer 20

-Sum of the elementary steps must give the overall balanced equation for the reaction
-The rate determining step must predict the same rate law that is determined experimentally

Question 21

Calorimetry (definition)

Answer 21

The process of measuring energy changes in a chemical system

Question 22

Calorimeter (definition, use)

Answer 22

Used to measure the heat absorbed or released in a reaction by measuring the temperature change of an insulated mass of water
Assume that heat is transferred to/from the water only

Question 23

Specific Heat Capacity (definition and use)

Answer 23

the amount of energy needed to raise the temperature of one gram of a substance at 1°C
Used to determine amount of energy required or released during physical or chemical change

Question 24

Standard Enthalpies of Formation, ΔH°f (definition and use)

Answer 24

the amount of heat absorbed or released when one mole of a compound is formed at SATP from its elements in their standard states
Used to calculate molar enthalpy of a reaction
**enthalpy of formation of an ELEMENT in its standard state (ie. Fe(s)) is defined as ZERO.