Transcript H reactants

Reaction Energy and Kinetics
Equilibrium
Chapter 17 and 18
I. Energy
A. Definitions
1. Energy is the ability to do work or
produce heat.
2. Potential energy- energy due to the
composition or position of an object.
3. Kinetic energy – energy of motion
4. Law of conservation of energy – in
any chemical reaction energy can be
converted from one form to another,
but it is not created or destroyed.
5. Chemical potential energy – energy
stored in a substance because of its
composition.
B. Heat
1. Definition – energy that is in the
process of flowing from a warmer
object to a cooler object.
2. Symbol – q
3. Measuring heat
a. calorie – amount of heat required to
raise the temperature of 1 gram of
pure water by 1 degree Celsius
b. Joule – SI unit of heat and energy
1 J = 0.2390 calories
1 cal = 4.184 J
C. Specific Heat
1. Definition – the specific heat of any
substance is the amount of heat
required to raise the temperature of
one gram of that substance by 1
degree Celsius.
2. Specific Heat Table
Substance
Water(l)
Aluminum
Carbon
Copper
Iron
Gold
Specific Heat J/gK
4.18
0.897
0.709
0.385
0.449
0.129
Which substance would be the best conductor of
heat?
Which substance would be the best insulator of
heat?
3. Calculating Heat Evolved & Absorbed
a. The heat absorbed or evolved in a rxn
depends on the specific heat, mass of
substance, and amount of which the
temp changes.
b. Equation
q = c x m x ∆T
q = heat absorbed or evolved J
c = specific heat of substance J/(g ·°C)
m = mass of the sample in grams
∆T= change in temp °C
(Tfinal – Tinitial)
Problem
1. In the construction of bridges and
skyscrapers, gaps must be left between
adjoining steel beams to allow for the
expansion and contraction of the metal
due to heating and cooling. The
temperature of a sample of iron with a
mass of 10.0 g changed from 50.4C to
25C with the release of 114 J heat. What
is the specific heat of iron?
2. Determine the specific heat of a material if
a 35 g sample absorbed 48J as it was
heated from 293K to 313K.
3. If 980 kJ of energy are added to 6.2L of
water at 291K, what will the final
temperature of the water be?
II. Heat in Chemical Reactions
A. Measuring Heat
1. Calorimeter – insulated device used for
measuring the amount of heat absorbed or
released during a chemical rxn.
a. A known mass of water is placed in the
calorimeter.
b. The water absorbs the energy released
from the reacting system or provide the
energy absorbed by the system. (energy
flows from hot to cold)
2. Determining specific heat using calorimeter
Example:
Suppose you put 125 g of water into a
foam cup calorimeter and find that its
temperature is 25.6 C. Then, you heat a 50
g sample of metal to a temperature of 115C
and put the metal into the water. The final
temperature of the water is 29.3C. What is
the specific heat of the metal?
2. The temperature of a sample of water
increases from 20C to 46.6C as it absorbs
5650 J of heat. What is the mass of the
sample?
III. Thermochemistry
A. Definitions
1. Thermochemistry – study of heat
changes that accompany chemical rxns
and phase changes.
2. System – specific part of the universe
that contains the reaction or process you
wish to study
3. Surroundings – everything in the
universe other than the system
4. Universe – system plus surroundings
B. Enthalpy
1. Enthalpy, H, – heat content of a system
at constant pressure
2. ∆Hrxn(change in heat of system)=
heat absorbed or released in a
chemical rxn
∆ Hrxn = Hproducts – Hreactants
3. Negative ∆ H – Exothermic Rxn
4Fe (s) + 3O2(g)  2Fe2O3(s) + 1625 kJ
or
4Fe (s) + 3O2(g)  2Fe2O3(s) ∆H =-1625 kJ
Both equations indicate 1625kJ released after
reaction.
Therefore, (-) sign means products have 1625 kJ
less energy than reactants (lost to surroundings)
Energy Diagram:
4. Positive ∆ H- Endothermic Rxn
27kJ + NH4NO3(s)  NH4+1(aq)+ NO3-1(aq)
or
NH4NO3(s)  NH4+1(aq)+ NO3-1(aq) ∆H =27
kJ
Both equations indicate 27 kJ absorbed:
endothermic rxn: ∆H (+) sign; gain of energy
by products - ∆Hreactants < ∆Hproducts
Energy Diagram:
5. Calculating ∆H°rxn
a. ∆H°rxn = ∆H°products - ∆H°reactants
Equation must balanced and you
must multiply by the number of moles
of products and reactants.
b. Example: Using the standard heats of
formation given, calculate the ∆H°rxn for:
CH4(g) + 2O2  CO2(g) + 2H2O(g)
Substance
CH4 (g)
CO2(g)
H2O(g)
∆H°rxn (kJ/mol)
-74.81
-393.5
-241.8
IV. Thermochemical Equations
A. Writing Thermochemical Equations
1. Thermochemical Equation – balanced
rxn that includes the states of all
reactants and products and the energy
change
2. The nature of the rxn or process is
written as a subscript.
3. A zero is written as a superscript if the
reaction is carried out at standard
conditions (1 atm and 298K).
4. Enthalpy(heat) of combustion- ∆H°comb –
enthalpy change for the complete
burning of 1 mole of a substance.
B. Changes of State
1. Definitions
a. Molar enthalpy (heat) of vaporization∆Hvap- Heat required to vaporize one
mole of the a liquid.
b. Molar enthalpy(heat) of fusion – ∆Hfus
heat required to melt one mole of a
solid
2. Thermochemical Equation for Changes
of State
a. The vaporization of water and melting
of ice equations:
H2O(l)  H2O(g) ∆Hvap = 40.7 kJ
H2O(s)  H2O (l) ∆Hfus = 6.01 kJ
b. The reverse processes have the same
numerical values but opposite signs.
∆Hfus = -∆Hsolid
∆Hvap = -∆Hcond
c. Table of Standard Enthalpies
H2O
∆Hvap
(kJ/mol)
40.7
∆Hfus
(kJ/mol)
6.01
Ethanol
C2H5OH
38.6
4.94
Methanol
CH3OH
35.2
3.22
Ammonia
NH3
23.3
5.66
Substance
Formula
Water
3. Calculating Energy in a Reaction
Problem 1:
A bomb calorimeter is useful for measuring the
energy released in combustion reactions. The
reaction is carried out in a constant volume
bomb with a high pressure of oxygen. How
much heat is evolved when 54.0g
glucose(C6H12O6) is burned according to this
rxn?
C6H12O6 (g)+ 6O2 (g) 6CO2(g) + 6H2O(l)
∆Hcomb = -2808 kJ
Problem 2
Calculate the heat required to melt 25.7g
of solid methanol at its melting point.
Problem 3
What mass of methane must be burned in
order to liberate 12880 kJ of heat?
Hcomb = -891 kJ/mol
V. Shifting Equilibrium
A. Definitions
1. Equilibrium – a dynamic condition in
which two opposing changes occur
at equal rates in a closed system.
2. Le Chatelier’s Principle – states that
if a system is subjected to a stress,
the equilibrium is shifted in the
direction that tends to relieve the
stress.
B. Le Chatelier’s Principle
1. Provides a means to predict the
influence of stress factors on
equilibrium systems.
2. In a reversible chemical reaction: the
rxn can move forward or backwards.
A+B=C+D
C. Stress Factors and Equilibrium
1. Changes in Pressure
a. Increase in pressure – shift to reduce
# of gas molecules Note: pressure
doesn’t affect solids or liquids
Example:
b. Decrease in pressure – shift to
increase # gas molecules.
Example:
2. Changes in Concentration A + B  C+D
a. Increase in concentration of reactants
– shift to producing more product (to
use up reactant)
Example:
b. Decrease in concentration of reactants
– shift to reverse reaction to make
more of reactant
Example
3. Changes in Temperature A + B C+D
a. Increase in temp. – equil shifts to use
up heat (energy)
Example:
b. Decrease in temp – equil shifts to form
more energy
Example:
C. Equilibrium Constant
1. Definition
a. Equilibrium Expression, Keq - numerical
value of ratio of product concentrations to
reactant concentrations.
Keq = [C]c[D]d
[A]a[B]b
aA + bB  cC + dD
*Don’t include solids and liquids in
expression
2. Examples - Write, Keq , equilibrium constants
a. N2 (g) + 3H2  2NH3(g)
b. 2NaHCO3(s)  Na2CO3(s) +CO2(g) +
H2O(g)
3. Calculating Keq – when concentrations are
given.
Example: Calculate the Keq for part a
above
[NH3] = .933 mol/L
[N2] = .533 mol/L
[H2] = 1.600 mol/L
a. The Keq depends on the temperature
of reactants and products.
b. Keq is constant at a given temperature.
c. A large value of Keq means that at
equilibrium, the product is present in a
larger amt than the reactants.(forward
rxn favored.)
d. A small value of Keq means that at
equilibrium, the product is present in
smaller amounts than the reactants.
(reverse rxn favored)
e. When the Keq is 1, the forward or
reverse rxn is not favored.