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Chemistry

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Alanna Persaud

on 7 February 2014

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Transcript of Chemistry

by: Alanna, Anuj, Jasspreet,
Jason, Aayushi, Sakshi

Energy Changes & Rates of Reactions
specific heat capacity
:

amount of energy needed to raise a gram of a substance 1 °C.
5.3- Hess' Law
5.2: Determining Enthalpy
of Reaction by Experiment
6.2
Rates of Chemical Reaction
Expressing Reaction Rates
Average and Instantaneous Rates of Reactions
Reaction Rates in Terms of Products and Reactants
Methods for Measuring Reaction Rates
Factors that affect Reaction Rates
6.1
* Change in the amount of reactants or products over time is called the
Reaction rate
* It is measure in mols/s
* In case of gaseous species or in solution, reaction rate is expressed in terms of concentration
* The rate expressed in terms of the concentration of the reactants is the one at which the concentration of the reactant(s) is decreasing
* The rate expressed in terms of the concentration of the products is the one at which the concentration of the product(s) is increasing
5.1 Energy and Change
Thermodynamics
What is Thermodynamics?
Thermodynamics is the study of energy and energy transfer or energy change
Change in energy can take place almost anywhere such as chemical reactions, physical changes and nuclear reactions
In what ways do us humans harness energy resources?
Fire
Electricity
Nuclear Energy,
E
T
C
The law of conservation of energy states that the amount of energy in the universe remains constant meaning it cannot be created or destroyed
The form of the energy changes.
EUniverse = ESystem + ESurroundings = 0
Law of Conservation of Energy
* Average rate of reaction is the average change in the concentration of a reactant or product per unit time over a given time interval
* The average rate of reaction is represented by the
slope of the secant
drawn between two points on a curve on a concentration-time graph.
* Instantaneous rate of reaction is the rate of the reaction at a particular time
* The
slope of the tangent
drawn at a particular point on the curve of a concentration-time graph represents the instantaneous rate of reaction
* A greater slope of tangent represents a faster reaction rate
* Reaction rates can be expressed in terms of the rate of disappearance of the reactants or in terms of the rate of production of the products
* Various unknown quantities can be determined by using stoichiometric relationship between the products and the reactants
The Rate Law:
Reactant Concentration
and Rate:

1) The rate law equation:
The reaction rates can be measured by monitoring the following:
* mass, pH and conductivity
* pressure
* color
*volume
Rate = k [A]^m [B]^n
K
=
Rate constant
(depends on temp)
A & B
=
Reactant Formulas
m & n
=
Stoichiometric coefficients

(obtained by experiment & do NOT
depend on temp)
• Increase in temperature=Increase in the rate of reaction

• Increase in the concentrations of the reactants=Increase in the rate of reaction

• Addition of a catalyst
• Increase in the surface area=Increase in the rate of reaction

• Type of reactants
2)Reaction orders:
Values of exponents (m&n)
define the order of reaction.
-If any given reactant has an exponent of 1,
the reaction is called
first order
in that reactant.
-If it has an exponent of 2,
the reaction is called a
second order
in that reactant.
-The
overall reaction order
is the sum of the exponents (m+n).
Example:
Rate = k[A]^1 [B]^2
> In the above rate law equation, the reaction is
first order in A
,

second order in B
and
third order (1+2) overall.
In order for a reaction to occur it means that reacting particles with collide with one another (atoms, molecules, ions)
3) Determining Rate Law
Values from experimental data:
The effect of concentration on reactant rates
Rate of the reaction will increase if there are more collisions per unit time as collisions are necessary for a reaction to occur. More reactant particles in a given volume ( greater concentration) will increase the number of collisions between the particles per second.
2ClO2 (aq) + 2OH (aq) -> ClO3 (aq) +H2O (l)
Example:
ACTIVATION ENERGY
the energy of the collision
 The reactants must collide with sufficient energy to break the bonds in the reactants and begin to form the bonds in the products
 In most reactions, only a fraction of the collisions have enough energy for the reaction to occur.
 Collision Energy is dependent on the kinetic energy of the system.
 Temperature is a measure of the average kinetic energy of the particles.
Steps:
In order for a reaction to occur it means that reacting particles will collide with one another (atoms, molecules, ions)
6.3 Collisions Theory
Rate of the reaction will increase if there are more collisions per unit time as collisions are necessary for a reaction to occur. More reactant particles in a given volume ( greater concentration) will increase the number of collisions between the particles per second.
The effect of concentration on reactant rates
Increasing the surface area of solid-phase reactant speeds up a reaction. With greater surface area, more collisions can occur.

For example- campfires started with paper and twigs have a higher concentration rather than with logs
Collisions theory and surface area
Not every collision between reactants results in a reaction.
-In order for a reaction to occur a collision must be effective
The criteria to be effective
-Correct orientation of reactants
-Sufficient collision energy
Beyond Collision Theory:
Reacting particle must collide with the proper orientation relative to one another or correct collision geometry
ORIENTATION OF REACTANTS
the energy of the collision
-The reactants must collide with sufficient energy to break the bonds in the reactants and begin to form the bonds in the products
In most reactions, only a fraction of the collisions have enough energy for the reaction to occur.
-Collision Energy is dependent on the kinetic energy of the system.
-Temperature is a measure of the average kinetic energy of the particles
ACTIVATION ENERGY
-The dotted line represent the activation energy.
-The shade part of the graph indicates the collisions with energy
that is equal to or greater than the activation energy.
MAXWELL –BOLZTMAN DISTRIBUTION
The Effect of Temperature
at both temperatures a relatively small
fraction of collisions have sufficient kinetic
energy –the activation energy–
to result in a reaction.
As the temperature of a sample increases,
the fraction of collisions with sufficient energy to cause a reaction increases significantly.
For many reactions, the rate
roughly doubles for every 10oC rise in temperature.
-used to explain what happens when molecules do collide in a reaction.
-it examines the change or transition from reactants to products
-kinetic energy of Reactant ->spotential energy as reactants collide (law of conservation of energy)
Transition State Theory
the potential energy of the a reaction versus the progress of the reaction

y-axis=potential energy,
x-axis= the progress of reaction over time.
Potential Energy Diagram
Exothermic rxn
the hill represents the activation energy barrier
slow reaction: high activation energy barrier, i.e. few reactants have sufficient kinetic energy for a successful reaction
fast reaction: low activation energy barrier
the reactants have a lower energy level than the products
the overall difference in potential energy between reactants and products is the enthalpy change
Earev = Eafwd -H
The top of the activation energy barrier on a potential energy diagram represents the transition state or change over point of the reaction.
The chemical species that exist at the transition state is referred to as the activated complex (neither reactant or product-has partial bonds and unstable)
Tracing a Reaction with a Potential Energy Diagram
Carbon monoxide reacts with nitrogen dioxide, carbon dioxide and nitric oxide are formed
Endothermic Reaction
Elementary ->series of steps that make up a reaction
Each of these steps are called elements of reaction.
This involves a single molecular element
2 NO(g) + 02-> 2NO2 (g)
step 1 NO (g)+ O2-> NO3
step 2 NO3(g)+ NO (g)-> 2NO2(g)
When you add the two element reactions, you now get the overall reaction
Elementary rxns can be:
Unimolecular-> When one molecule or ion reacts
CL2 ---2CL (g)
Bimolecular-> 2 particles collide and react
Termolecular-> Involve 3 particles colliding. Termolecular are rare because it is unlikely to have 3 particles react all at once.
Rate determining step-> The much slower elementary reaction, defines the overall rate.
Example: A + B -> C + D
assumes the elementary steps for this reaction are:
Step 1: Slow A + A -> C + E
Step 2: Fast E + B -> A + D
Rate law : The rate r, will always be proportional to the product of the initial concentrations of the reactants, where these concentrations are raised to some exponential values. This can be expressed as ra= K[ X]^m (Y)^n
Catalyst: Substance that increased the rate of a chemical reaction
How catalyst work?
This only works by lowering the activation energy of a reaction.
Homogenous catalyst: Catalyst and reactance’s are in the same phase in cobalt ions as a catalyst
ex: (q) Tart rate ions and hydrogen peroxide
Heterogenious- Both reactant and catalyst in different phases.
Example: O2 + I-> H2O in the presence of a solid platinum.
1. Find two experiments in which [ClO2] remains constant and [OH] changes. Compare the rates and concentrations to solve for n.
2. Find two experiments in which [OH] remains constant and [ClO2] changes. Compare rates and concentrations to find m.
3. Use data and your calculated values for m and n to solve for k, using the following equation
:
Rate= k[ClO2]^ m [OH]^ n
Question:

Determine the rate law equation and the value of k
You can use the specific heat capacity of a substance to calculate the amount of energy that is needed to heat a given mass a certain number of degrees
You can also use the specific heat capacity to determine the amount of heat that is released when the temperature of a given mass decreases
Higher specific heat capacity means more energy is required to raise or lower the temperature
heat capacity ( c )
:
amount of energy required to raise the temperature of an object 1 °C
Equation for heat capacity is
Q = heat (j)
m = mass (g)
c = specific heat capacity (J/g °C)
T = T final-T initial (°C or K)
Measuring Heat Capacity in a Lab
Calorimeter
: closed system used to measure enthalpy changes in chemical and physical reactions. Works by insulating a system from its surroundings.
Temperature of the system is measured and from there you can determine the amount of heat absorbed or released from the reaction.
Heat:
-can be defined as the amount of kinetic energy, Q
-there IS a difference between heat and cold
-cold is a LACK of heat

Temperature:
-the measure of the average amount of kinetic energy within a substance or system, T
Enthalpy:
the total energy of a system at a constant pressure
H= KE + PE

Enthalpy Change:
the difference between the enthalpy of the reactants and the enthalpy of the products
Hrxn = Hproducts - Hreactants
Enthalpy
Enthalpy Change in Chemical Reactions
water is often used as a controlled surrounding.
c= 4.184 (J/g ● °C)
Endothermic Reaction:
More energy is absorbed during the process of breaking bonds between reactants
Energy if absorbed from the surrounding area because of this reaction
Exothermic Reaction:
More energy is released in process of forming bonds between reactants
Energy is released into the surrounding area as a result of this reaction
enthalpy diagrams are used to show the difference in energy between reactants and products.
ex:
Enthalpy Diagrams
Q reaction = -Q insulated system
Coffee cup calorimeters are well suited for aqueous solutions.
Assume the solution has the density and specific heat capacity as water.
The water in the solution provides or absorbs the energy that is released or absorbed by the chemical reaction
Water
Density: 1.00g/mL
Specific Heat Capacity: 4.184 J/g C)
**Table in textbook on pg. 282**
Exothermic Reactions:
H2(g) + 1/2O2(g) H2O(l) + 285.8kJ

H2(g) + 1/2O2(g) H2O(l) H°rxn= -285.8kJ


Endothermic Reactions:
117.3kJ + MgCO3(s) MgO(s) + CO2(g)

MgCO3(s) MgO(s) + CO2(g) H°rxn = 117.3kJ
Thermochemical Equations
Nuclear Fusion:
2 or more smaller nuclei fuse to form larger nucleus (i.e. occurs on the Sun)

Nuclear Fission:
Heavy nucleus splits into lighter nuclei (i.e. Power plants)
Nuclear Reactions
Thermodynamics is the study of energy and the change in energy
Heat is the amount of kinetic energy in a substance
Temperature is the average amount of kinetic energy in a substance
Enthalpy is the total energy within the bonds of a substance at a constant pressure
Enthalpy CHANGE is the difference between the enthalpy of the reactants and the enthalpy of the product
Endothermic reactions absorb heat while Exothermic reactions release heat
Nuclear reactions create a lot more energy than chemical reactions because of the way nuclear bonds work
Conclusion
Chapters 5&6
Ex. The specific heat capacity of aluminum is 0.900 J/g °C. How much energy is needed to raise the temperature of an 850g block of aluminum from 22,8°C to 94.6°C?
Q= ?
m= 850g
c= 0.900 J/g°C
T= 94.6-22.8
= 71.8°C
Q= (850g)(0.900J/g°C)(71.8°C)
Q= 54 927 J
TIME FOR SOME FUN! :)
It's JEOPARDY TIME!!!!!
What is Hess' Law of heat summation?
Hess' law of heat summation is the law that pertains to enthalpy changes of reactants and products in a reaction.

Hess' Law primarily revolves around these 2 statements
1) The value of H for any reaction can be written as the sum of the H's for each individual steps that make up the process
2) Enthalpy change in a reaction is independent of the pathway or intermediate steps in the process.
Combining Chemical Equations Algebriacally
When solving a system of equations to attain one fully balanced chemical equation, there can be an infinite amount of steps involved.
the guidelines for which to go about solving these types of systems are as follows:
1) Look for the target equation
2) Rearrange the given set of equations to attain the target equation
3) Reversing an equation also means that you must change the sign of the H by multiplying it by -1
4) If multiplying or dividing a chemical reaction, each coefficient needs to be multiplied or divided by the same integer throughout even the H of that equation
Standard Enthalpies of formation
Standard Molar Enthalpies of all elements in their natural state are always Zero
What are Formation reactions?
In a formation rxn a substance is formed from elements in their natural state and the enthalpy change is called Molar enthalpy of Formation
the Formula for to find the H of these reaction is :
See on board example
See on board example
H
= E(n H products)-E(n H reactants)
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