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Gr. 12 Energy Changes and Rates of Reaction Cheat Sheet by

science     chemistry     energy     rates     reactions     highschool     gr-12     thermochemistry     enthalpy

Introd­uction

Therm­och­emi­stry: the study of the energy changes that accompany physical or chemical changes in matter
Types of energy:
Ep (potential energy)
the energy of an object due to its posi­tio­n/c­omp­osi­tion
Ek (kinetic energy)
the energy of an object due to its motion
Thermal energy (Eth): the total quantity of Ek and Ep in a substance; depends on how fast the particles are moving: more energy = more speed = more Eth
Heat: the transfer of Eth from a warm object to a cool object
Tempe­rat­ure: measure of the average Ek of the particles in a substance
Law of Conser­vation of Energy: energy cannot be created or destro­yed, only converted from one form to another
Not­e: Temper­ature ≠ Eth! A cup of water at 90°C has a higher temper­ature than a bathtub of water at 40°C, but the water has more Eth since it has more molecules

System­/Su­rro­undings and Reactions

System: the group of reac­tants and products being studied
Surroundings: all the matter that is not a part of the system
Types of systems:
Open system
both energy and matter are allowed to enter and leave freely
Closed system
energy can enter and leave the system, but matter cannot
Isolated system
neither matter are allowed to leave the system (complete isolation is impo­ssi­ble)
Types of reacti­ons:
Endothermic
energy from the surrou­ndings is abso­rbed by the system
Exot­her­mic
energy from the system is rele­ased into the surrou­ndings

Specific Heat Capacity and Calori­metry

Specific heat capacity: the amount of energy required to raise the temper­ature of 1 g of a substance by 1°C (measured in J/g°C); depends on type and form of substance
Calor­ime­try: the experi­mental process of measuring the ΔEth in a chemical or physical change
Calorimeter: device used to measure ΔEth
Types of calori­met­ers:
Poly­sty­rene (styro­foam)
Reasonably accurate and inexpe­nsive
Bomb
More precise, used for reactions that involve gases
Flame
Used for combustion reactions

Calori­metry Calcul­ations

4 assump­tions when performing calori­metry calcul­ati­ons:
1. Any thermal energy transf­erred from the calori­meter to the outside enviro­nment is negligible
3. All dilute, aqueous solutions have the same density as water (D = 1.00 g/mL)
2. Any thermal energy absorbed by the calori­meter itself is negligible
4. All dilute, aqueous solutions have the same specific heat capacity as water (c = 4.18 J/g°C)
Calorimetry formula:
Q = mcΔT
m = mass of the substance (g)
c = specific heat capacity of the substance ( J/g°C)
ΔT = temper­ature change experi­enced by the system; ΔT = Tfinal - Tinitial (°C)
Q = total amount of Eth absorb­ed/­rel­eased by a chemical system ( J )
Value of Q has two parts:
The numb­er: how much energy is involved
The sign: the direction of the energy transfer (important to show, even if it is positi­ve!)

Because of the law of conser­vation of energy, the total thermal energy of the system and the surrou­ndings remain constant:

Qsy­stem + Qsurr­oun­dings = 0
Qsy­stem = - Qsurr­oun­din­gs

Enthalpy Change (ΔH)

Enthalpy (H): the total amount of Eth in a system; not directly measur­able
Must measure enthalpy change (ΔH) by measuring the ΔT in the surr­oun­dings
Enthalpy change (ΔH): the energy released to/abs­orbed from the surrou­ndi­ngs during a chemic­al/­phy­sical change; can be measured using calori­metry data
As long as pressure is constant, the enthalpy change of a chemical system is equal to the flow of thermal energy in or out of the system
Enthalpy change formula:
ΔH = |Qsys­tem|
ΔH = ±|Qsu­rro­und­ing­s|
If ΔH > 0, the reaction is endo­the­rmic
If ΔH < 0, the reaction is exot­her­mic
If there is more than one substa­nce making up the surrou­ndings (i.e. bomb­/flame calori­met­ers), then
Qsurroundings = Σ Qsubs­tan­ces
 

Molar Enthalpy Change (ΔHx)

Molar enthalpy change (ΔHx): the enthalpy change asso­ciated with a physic­al/­che­mical change involving 1 mol of a substa­nce (J/mol)
x = type of change (vapor­iza­tion, neutra­liz­ation, combus­tion, etc.)
Molar enthalpy change formula:
ΔH = nΔHx

Repres­enting Enthalpy Change

4 ways to represent ΔH:
1. Thermo­che­mical equations with energy terms
CH4 + 2 O2 CO2 + 2 H2O + 890.8 kJ
2. Thermo­che­mical equations with ΔH terms
CH4 + 2 O2 CO2 + 2 H2O ΔH = -890.8 kJ
3. Molar enthalpies (ΔH­x)
ΔHcomb = -890.8 kJ/mol
4. Potential energy (Ep) diagrams

Hess' Law

Enthalpy change (ΔH) is determined by initial and final condit­ions of a system; it is inde­pen­dent of the pathway
The total ΔH of a multi-step reaction is the sum of the ΔH of its indi­vidual steps
Hess's Law formula:
ΔHreaction = Σ ΔHste­ps
This formula can be used in cases where the overall reaction is not feasible to be done in a calori­meter (i.e. reaction is too slow/too fast/too violent)
Rules:
1. If a reaction is flip­ped, flip the ΔH value's sign
2. If a reaction is mult­ipl­ied, multiply the ΔH value

Standard Enthalpy of Formation (ΔH°f)

The standa­rdized ΔH when 1 mol of a substance is formed (syn­the­siz­ed) directly from its elements to its standard state at SATP
The elements themselves have a ΔH°f of 0 (elements cannot be synthe­siz­ed)

Bond Energies (D) and Bond Enthalpy

Bond Energies
Stability of a molecule is related to the strength of its covalent bonds
The stre­ngth is determined by the energy required to break that bond
Bond Enthal­py:
ΔH for breaking a particular bond in 1 mol of a gaseous substa­nce
Always positive because energy is always required to break bonds
Used for pred­icting reaction types before the reaction is performed (not entirely accura­te)
Formula for predicting reaction type using D and bond H:
ΔH = Σ (nDbonds broken) - Σ (nDbonds formed)

Reaction Rates

The speed at which a reaction occurs
Can be fast (10-15s) or slow (years)
Measured by the change in the amount of reac­tants consumed or products formed at a given time interv­al(s)
Can be measured by volu­me, mass, colo­ur, pH, and elec­trical conduc­tiv­ity
Often expressed as a positive value for conven­ience, regardless of what is being measured
Average rate of reacti­on: rate of a chemical reaction between two points in time (one time interval); calculated from the slope of the secant of the time interval on a conc­ent­rat­ion­-time graph
Average rate of reaction formul­as:
How fast a reactant disapp­ears
- Δ[A]/Δt
How fast a product appears
Δ[B]­/Δt
Δ[A], Δ[B], Δt = [A]2 - [A]1, [B]2 - [B]1, t2 - t1
Units
mol/L⋅s
Instantaneous rate of reacti­on: rate of a chemical reaction at a single point int time; calculated from the slope of the tangent of the time position on a conc­ent­rat­ion­-time graph
 

Collision Theory

States that chemical reactions can only occur if the reactants have the right kinetic energy (speed) and orie­nta­tion to break reactant bonds and form product bonds
Effective collis­ion: a collision that has suff­icient energy and correct orient­ation of colliding particles to start a reaction
Ineff­ective collis­ion: a collision where the particles rebound, unch­anged in nature
Activ­ation energy (Ea): the minimum energy required for reactants to have for a collision to be effective
Activated comple­x/t­ran­sition state: unstable arrang­ement of atoms containing part­ially formed and part­ially broken bonds; maximum Ep point in the reaction
Rate of a reaction depends on the freq­uency of collis­ions and the fraction of those collis­ions that are effective.
Rate = freq­uency of collisions x fraction of collis­ions that are effe­ctive

Increasing Reaction Rates

5 factors that can increase a reaction rate: chemical nature of reactants, conc­ent­rat­ion, surface area, temp­era­ture, and cata­lysts
Chemical nature of reacta­nts
For any reactant, the activation energy required depends on the bond type (single vs double vs triple), the bond strength (D value), the number of bonds, and the size and shape of the molecu­le(s)
Concentration of reacta­nts
Concentration = amount of substance per unit volume (mol/L); applies only to solu­tions
[reactant] = collisions = rate
Rate α [react­ant] - as the conc­ent­ration increa­ses, the rate increa­ses, and vice versa
Surface area
Surface area = total area of all the surfaces of a solid figure
SA = collisions = rate
Rate α SA - as the surface area increa­­ses, the rate increa­­ses, and vice versa
Temperature of system
T = collisions + fraction of effective collisions = rate
Rate α T - as the temp­erature increa­­ses, the rate increa­­ses, and vice versa
Catalyst
Catalyst = substance that incr­eases the rate of a reaction without itself being consumed in the reaction; provide an alte­rnate pathway for the reaction with a lower Ea
Ea = fraction of effective collisions = rate
Rate α 1Ea - as the cata­lyzed activation energy decrea­ses, the rate increa­­ses, and vice versa

Rate Law

Math­ema­tical relati­ons­hip between the reaction rate and the conc­ent­rat­ion of reactants; needs expe­rim­ental data
Form­ula: Rate = k[A][B­][C]c
[A]/[B­]/[C] = concen­tration of reac­tants (only reac­tants are relevant); k = rate constant
Orders of Reaction
Order of reaction: the exponent used to describe the relati­onship between the [ i ] of a reactant and the rate of reacti­on; tells us how quickly the rate will increase when [conc] increases
Zero order
Rate = k[A]0; slope is flat; rate is not affected by [A]
First order
Rate = k[A]1; slope is an incr­easing straight line; rate α [A]
Second order
Rate = k[A]2; slope is an incr­easing curve; rate α [A]2
Total order of reaction = the sum of the exponents in the rate law equation
The only accurate data for concen­tration and rate is the initial rate, because as soon as the reaction starts, products are formed and the reverse reaction starts, making any rate measured after t = 0 affected by the products.

Reaction Mechanisms

Chemical reactions usually occur as a sequ­ence of elem­entary steps that, when added, result in the overall reaction
Mechanism is dependent on the slowest elementary step - the rate­-de­ter­mining step
Eleme­ntary step = a single molecular event in the reaction mechanism
3 criteria for a proposed reaction mechan­ism:
The elementary steps must add up to the overall reaction
The elementary steps must be physically reason­able - there should not be more than 2 reactants
The rate-d­ete­rmining step must be consistent with the rate law equation

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