Transcript File
4.a Thermochemistry
Intro
Thermochemistry
study of transfers
of energy (E) as
heat (q) occurring
during chemical
and physical
changes
Energy
Energy
– capacity to do some kind of
work
Energy is always involved in a chemical
or physical change
Law
of Conservation of Energy –
energy cannot be created or destroyed
Energy is transferred in chemical or
physical changes
Energy Cont’d
Energy
Changes
Exothermic – process that releases heat
into the environment
• Typically feels warm
Endothermic – process that absorbs heat
from the environment (uses energy)
• Typically feels cold
Kinetic
Energy (KE) – energy of an
object that is due to the object’s
motion
Temperature
Temperature (T):
average KE of the
particles in a
sample
As KE increases, so
does T (direct
relationship)
Temp Scales
Farenheit
Celsius
Kelvin
Kelvin (K) Temp Scale
Kelvin
– developed so that we could
have a system with no negative values
Starts at -273.15 ○C = absolute zero = 0 K
Add 273 to temp value for ○ C
Example: 10 ○C + 273 = 283 K
Heat
Heat
- (q) energy transferred between
samples because of their difference in T
moves spontaneously from matter with
higher T to matter with lower T until
thermal equilibrium (= temp) is reached
heat can’t be measured directly so T is
used to track transfer of heat
Energy transferred is measured in
Joules (J)
Heat Transfer
Heat
measured
using a calorimeter (reaction
container that is surrounded by water)
the
energy given off during a reaction
is equal to the energy absorbed by the
water
qabsorbed qreleased
Two Types of Calorimeters
Specific Heat
the
amount of energy transferred
during a temperature change depends
on 3 factors:
type of material
mass of material
size of T change
Specific
heat: (c) the amount of energy
required to raise the T of one gram of
substance by one °C or K
Specific Heat –Math
problems
measured under constant pressure cp
q
cP
OR q c P m ΔT
m ΔT
cp = specific heat at constant pressure
(J/g•K or J/ °C)
q = heat (J or KJ)
m = mass (g
Δ T = change in temperature (Tfinal – Tinitial )
Example
A 4.0 g sample of glass was heated from 274 K
to 314 K and was found to have absorbed 32
J of energy.
Find the specific heat of the glass sample.
q
32 J
J
cP
0.20
m ΔT (4.0g)(314 274K)
gK
Example
How much energy will the same glass
sample gain when heated from 314 K to 344
K?
q cP m ΔT
J
(0.20
)(4.0g)(344 314 K) 24 J
gK
Example
If 200. g of water at 20°C absorbs 41,840 J of
heat, what will its final T be? The specific heat
of water is 4.184 J/g°C.
q cP m ΔT cP m (Tfinal Tinitial)
J
41,840J (4.184 ) (200.g) (Tfinal 20 C)
gC
Example
41,840J
Tfinal 20 C
J
(4.184 ) (200.g)
gC
41,840J
Tfinal
20 C 70 C
J
(4.184 ) (200.g)
gC