Download Energy, work, heat and chemical reactions

Energy,
work, heat
and
chemical
reactions
What is work (in chemical terms)?
work -- occurs when an object is moved against an applied force
Work = Force x Distance
d
Fgravity
Ffriction
Moving an object horizontally against
the force of friction
d
d
Lifting an object vertically
against the force of gravity
What is work (in chemical terms)?
Ffriction = 0
Moving an object horizontally across a
frictionless surface
Work = 0
Work and energy
work -- occurs when an object is moved against an applied force
work -- occurs when an object is moved against an applied force
Work = Force x Distance
energy -- the capacity of matter to perform work
Felectrostatic
d
Fspring
Compressing a spring
+
d
+
The greater the amount of energy that a substance or object has, the
greater the amount of work that it can perform
+
The SI unit for energy is the joule (J)
Moving a charged particle
against an electric field
Another commonly used unit for energy is the calorie (cal)
1 cal = 4.184 J
Kinetic energy and potential energy
Work and energy
work -- occurs when an object is moved against an applied force
kinetic energy -- energy possessed by matter due to its motion
energy -- the capacity of matter to perform work
KE = ( 1 / 2 ) m v2
Types of energy
more mass
higher speed
greater kinetic energy
kinetic energy -- energy possessed by matter due to its motion
baseball at rest
potential energy -- stored energy that is possessed by matter (i.e.,
stored energy that has the potential to be released later)
KE = 0
baseball in flight
KE > 0
-- gravitational
-- elastic (e.g., compressed springs)
-- chemical (energy stored in the chemical bonds between atoms in a
compound)
heat -- internal energy of a substance due to the motion of its atoms
(or molecules)
Kinetic energy and potential energy
Other examples of potential energy
potential energy -- stored energy that is possessed by matter (i.e.,
stored energy that has the potential to be released later)
Example: gravitational potential energy -- the energy contained by an object
due to its elevation
Elastic potential energy -- for example, the energy stored in a
compressed spring
100 ft
PE = mgh
more mass
greater height
greater potential energy
20 ft
PE = 0
10 ft
Ouch!
#@$#*!!
x x
PE > 0
Heat
Other examples of potential energy
heat -- internal energy of a substance due to the motion of its
atoms (or molecules)
Chemical potential energy -- the energy stored in chemical bonds
H
• heat is nothing more than atoms (and molecules) in motion
H
H
propane
H
• the faster an atom/molecule moves around, the more heat it
carries
C
C
H
C
H
H
H
adenosine triphosphate
(ATP)
water
Energy and chemical changes (reactions)
Energy and chemical changes (reactions)
In all chemical changes, matter either releases or absorbs energy
In all chemical changes, matter either releases or absorbs energy
Example of energy release: Combustion of methane
Example of energy absorption: Photosynthesis
CH4 +
methane
2 O2
CO2 + 2 H2O +
oxygen
carbon
dioxide
+
energy
water
2 H2
hydrogen
electricity
CO2 + 2 H2O +
carbon
dioxide
water
heat, light
Example of energy absorption: Electrolysis of water
2 H 2O
energy
+
O2
oxygen
water
energy
CH2O
sunlight
organic
compounds
+
O2
oxygen
Example of energy release: Respiration
CH2O
organic
compounds
+
O2
oxygen
CO2 + 2 H2O +
energy
carbon
dioxide
metabolic
energy
water
Heat
Energy and chemical changes (reactions)
heat -- internal energy of a substance due to the motion of its
atoms (or molecules)
• heat is nothing more than atoms (and molecules) in motion
• the faster an atom/molecule moves around, the more heat it
carries
In all chemical changes, matter either releases or absorbs energy
Example of energy release: Combustion of methane
CH4 +
2 O2
methane
CO2 + 2 H2O +
carbon
dioxide
oxygen
water
heat, light
Example of energy absorption: Electrolysis of water
2 H 2O
+
energy
2 H2
water
hydrogen
Energy and chemical changes (reactions)
In all chemical changes, matter either releases or absorbs energy
water
O2
oxygen
energy
CH2O
sunlight
organic
compounds
Bond energy (chemical potential energy)
Energy is released when a bond is formed
58 kcal / mol
Example: Formation of Cl2 molecule
Example of energy absorption: Photosynthesis
carbon
dioxide
+
electricity
water
CO2 + 2 H2O +
energy
+
O2
oxygen
Cl
Cl
Cl
Cl
Energy must be supplied to break a bond
Example of energy release: Respiration
Example: Breaking Cl-Cl bond
CH2O
organic
compounds
+
O2
oxygen
CO2 + 2 H2O +
energy
carbon
dioxide
metabolic
energy
water
58 kcal / mol
Cl
Cl
This is called the bond dissociation energy
Cl
Cl
Bond dissociation energy is a
measure of bond strength
Conservation of energy
Law of conservation of energy
Stronger bonds require more energy to break
Energy can be neither created nor destroyed
-- but it can be changed from one form to another
58 kcal / mol
Cl
Cl
Cl
Cl
For chemical changes:
total energy before reaction = total energy after reaction
226 kcal / mol
N
Reactions that release energy:
N
N
N
Total bond energy
of reactants
=
Total bond energy
of products
+ released energy
Chemical potential energy
Conservation of energy
Heat
Heat in chemical reactions
Reactions are classified as either exothermic or endothermic
Law of conservation of energy
Energy can be neither created nor destroyed
exothermic reaction -- heat (energy) is released by the reaction
-- but it can be changed from one form to another
CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(l) + 890 kJ
For chemical changes:
total energy before reaction = total energy after reaction
Reactions that absorb energy:
Total bond energy
of reactants
+
Bond energy of reactants > Bond energy of products
endothermic reaction -- heat (energy) is absorbed by the reaction
absorbed energy
=
Total bond energy
of products
2 H2O(l) + 286 kJ
2 H2(g) + O2(g)
Bond energy of reactants < Bond energy of products
Chemical potential energy
Heat
Chemical potential energy
Heat of reaction
Energy and chemical changes (reactions)
heat of reaction ( enthalpy change: !H ) -- the quantity
of heat (energy) produced or absorbed during a reaction
Example of exothermic reaction: Combustion of methane
CH4
!Hreaction =
Hproducts –
–
Energy of bonds
broken in reactants
Hreactants
Exothermic reactions: Heat of reaction (!H) is negative
2 O2
CO2 + 2 H2O + 890 kJ
CH4 + 2 O2
Chemical potential energy
Energy of bonds
formed in products
heat of
=
reaction
+
reactants
Energy
(released)
!H = – 890 kJ
CO2 + 2 H2O
products
Endothermic reactions: Heat of reaction (!H) is positive
Bond energy of reactants > Bond energy of products
Energy and chemical changes (reactions)
+
286 kJ
2 H2
+
exothermic reaction: heat (energy) is released by the reaction
O2
-- heat of reaction is negative
Chemical potential energy
2 H2 + O2
products
Energy
(absorbed)
Heat in chemical reactions
Reactions are classified as either exothermic or endothermic
Example of endothermic reaction: Electrolysis of water
2 H 2O
CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(l) + 890 kJ
CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(l)
!H = – 890 kJ
!H = 286 kJ
endothermic reaction: heat (energy) is absorbed by the reaction
-- heat of reaction is positive
2 H 2O
reactants
Bond energy of reactants < Bond energy of products
!H is negative
!H is positive
2 H2O(l) + 286 kJ
2 H2(g) + O2(g)
2 H2O(l)
2 H2(g) + O2(g)
!H = 286 kJ
Homework assignment
Chapter 6 Problems:
7.25, 7.26, 7.27, 7.28, 7.29