Chemistry - Unit 11: Chemical Reactions

heat

amount of thermal energy transferred in or out of a system ( heat energy is removed/added to a system)

thermal energy

the energy an object has due to the movement of its molecules. The faster these molecules move, the hotter the object and the more thermal energy it possesses

temperature

avg kinetic energy (how much atoms are moving) of a substance - Kinetic energy is directly proportional to temperature (more particle movement, higher temperature)

enthalpy

Heat energy absorbed or released by a system during a chemical or physical process

Know: Enthalpy related to temperature change & Enthalpy related to phase change

three different ways of heat transfer

conduction, convection, radiation

conduction

• transfer of energy through direct contact

• usually occurs between solids

convection

• transfer of energy through matter movement

• ONLY occurs w/ liquids and gases

radiation

• transfer of energy through electromagnetic radiation

• doesn’t require matter to move through

liquid

macroscopic level: mold and take the shape of their container.

microscopic level: particles have more movement; packed tight but can move past each other

solid

macroscopic level:

microscopic level: compressed tightly together equating to a lack of molecular movement (no IMFs)

macroscopic level

changes are observed as melting, boiling, or sublimation [solid → gas (skipping liquid)]

microscopic level

involve changes in particle arrangement & motion

gas

macroscopic level: indefinite (will take the shape of the container it is in) shape + volume; particles are far apart and move freely.

microscopic level: particles are widely spaced with high kinetic energy and minimal attractive forces (no IMFs)

conductors

materials where heat can easily be transferred through (metals, water)

insulators

materials that block energy flow & slow heat transfer (air, styrofoam) 

calorimetry

Process of measuring the heat energy released or absorbed in a system. 

The assumption in calorimetry experiments: No energy is lost to the surroundings 

Equation: Q1=-Q2 (energy gained in system = energy lost in surroundings)

intermolecular forces (IMF’s)

fusion

btw solid & liquid (melting/freezing)

Vaporization

between liquid & gas

Phase Diagrams (triple point)

specific temperature & pressure where all three states of a substance can coexist at equilibrium (where the line is together right before it branches out into 2)

Phase Diagram

shows how pressure & temperature influence phase changesof a substance and the conditions under which different phases exist.

Heating & Cooling Curves

• Heat energy is being put into the system over time, changing either the substance’s temperature or phase 

• The system absorbs heat, which becomes either KE/PE → system decreases/increases in KE/PE

  • From A→B, C→D, E→F: changes in temperature → KE because KE is a measure of temperature (KE=32kT)

  • From B→C, D→E: changes in phase → PE because the distance between particles increases, causing an increase in the system’s PE

The energycalculation for each section on the diagram is given by the equation for Q below the x-axis

Exothermic v.s Endothermic Reactions

Exothermic Reaction     —     releases heat into the surroundings

• Enthalpy change (Hrxn) is negative (-)

Endothermic Reaction     —     absorbs heat from the surroundings 

• Enthalpy change (Hrxn) is positive (+)

Equation

Hrxn=Hproducts-Hreactants

Transition State (activated complex)

•  the substance’s state when it has the amount of energy indicated by the peak of the graph (highest energy point)

  • Required state for reactants to start reacting (breaking & forming bonds)

Enthalpy of Transition State (H*):

 energy of the transition state (y-value at the peak)

Activation Energy

The minimum amount of energy required for reactant molecules to undergo a successful chemical reaction (a.k.a to make reactants start interacting). 

• *Activation energy (Ea) is not related to the enthalpy change (H) 

(delta)H

amount of energy released (-) / absorbed (+) in a reaction when the amount of 

moles of a compound indicated in the balanced chemical equation is used. 

Relation to Overall Enthalpy Change of Reaction

Negative Enthalpy Change (exothermic)

• Energy released in bond formation > energy absorbed in bond breaking

Positive Enthalpy Change (endothermic)

• Energy released in bond formation < energy absorbed in bond breaking

Bond Breaking

• Endothermic — energy put in to break the bond; reaction absorbs energy

  • Particles separate, increasing PE (energy needs to be put in)

Bond Formation

• Exothermic — energy released & decreases

  • Particles get closer, decreasing PE (energy needs to be released)

Collision Theory

Three conditions must be met for a collision to be successful: 

1)     Collision Molecules must collide & come in contact with each other

2)     Correct Orientation Molecules must collide in the right orientation

3)     Sufficient KE Molecules must have enough energy to reach activation energy

specific heat

The amount of heat required to raise the temperature of one gram of a substance by one degree Celsius. It is a physical property that varies between different substances and is crucial in understanding heat transfer (material ability to absorb more heat before its temperature rises: higher speific heat = more heat is able to be absorbed)

limited reagent

the reactant that is completely consumed in a chemical reaction and therefore limits the amount of product that can be formed ex.ou have enough bread and cheese to make 10 sandwiches, but only 2 slices of ham. The ham is the limiting reagent because you can only make 2 sandwiches with the available ham, regardless of how much bread and cheese you have (leftover called excess reactant) aka ham