Cheatography

# Gr. 12 Energy Changes and Rates of Reaction Cheat Sheet by nescafeabusive32

### Introd­uction

 Thermo­che­mistry: the study of the energy changes that accompany physical or chemical changes in matter Types of energy: E``p`` (potential energy) the energy of an object due to its positi­on/­com­pos­ition E``k`` (kinetic energy) the energy of an object due to its motion Thermal energy (E``th``): the total quantity of E``k`` and E``p`` in a substance; depends on how fast the particles are moving: more energy = more speed = more E``th`` Heat: the transfer of E``th`` from a warm object to a cool object Temper­ature: measure of the average E``k`` 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 ≠ E
``th``
! A cup of water at 90°C has a higher temper­ature than a bathtub of water at 40°C, but the water has more E
``th``
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 ΔE``th`` in a chemical or physical change Calori­meter: device used to measure ΔE``th`` 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 = T``final`` - T``initial`` (°C) Q = total amount of E``th`` 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:

Q
``system``
+ Q
``surrou­ndings``
= 0

Q
``system``
= - Q
``surrou­ndings``

### Enthalpy Change (ΔH)

 Enthalpy (H): the total amount of E``th`` 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 = |Q``system``| ΔH = ±|Q``surrou­ndings``| 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 Q``surrou­ndings`` = Σ Q``substances``

### Molar Enthalpy Change (ΔHx)

 Molar enthalpy change (ΔH``x``): 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 = nΔH``x``

### Repres­enting Enthalpy Change

 4 ways to represent ΔH: 1. Thermo­che­mical equations with energy terms CH``4`` + 2 O``2``  CO``2`` + 2 H``2``O + 890.8 kJ 2. Thermo­che­mical equations with ΔH terms CH``4`` + 2 O``2``  CO``2`` + 2 H``2``O ΔH = -890.8 kJ 3. Molar enthalpies (ΔH``x``) ΔH``comb`` = -890.8 kJ/mol 4. Potential energy (E``p``) diagrams See an example here

### 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: ΔH``reaction`` = Σ ΔH``steps`` 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 (nD``bonds broken``) - Σ (nD``bonds 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``, t``2`` - t``1`` 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 (E``a``): 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 E``p`` 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 E``a`` E``a`` = fraction of effective collisions = rate Rate α ``1``/``Ea`` - 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]a[B]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

Thank you for this!

Great job!

It's on point thank you