Thermochemistry Energy absorbed or released as heat in a chemical or physical change is measured in a calorimeter In some calorimeters known quantities of reactants are submersed in water and combusted The energy given off is noted by the temperature change in water ID: 159119
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Slide1
the study of transfers of energy as heat that accompany chemical reactions and physical changes.
ThermochemistrySlide2
Energy absorbed or released as heat in a chemical or physical change is measured in a
calorimeter
.In some calorimeters, known quantities of reactants are submersed in water and combusted. The energy given off is noted by the temperature change in water.
Heat and TemperatureSlide3
Heat and TemperatureSlide4
Temperature – a measure of the average kinetic energy of the particles in a sample of matter.
The greater the kinetic energy, the greater the temperature.
Joules – the SI unit of heat as well as all other forms of energy
Heat and TemperatureSlide5
Heat – the energy transferred between samples of matter because of differences in their temperature
Energy transferred as heat always moves spontaneously from matter at a higher temperature to matter at a lower temperature.
Heat and TemperatureSlide6
Specific heat – is the amount of energy required to raise the temperature of one gram of substance by one Celsius degree or one Kelvin.
Joules per gram per Kelvin or joules per gram per Celsius
degreeQ = (m)(C
p
)(ΔT)
C
p
= specific heat at a given pressure ; q = energy lost or gained; m = mass of sample; ΔT = the change in temperature
Heat Capacity and Specific HeatSlide7
EXAMPLE: A 4.0 g sample of glass was heated from 274 K to 314 K, a temperature increase of 40 K, and was found to have absorbed 32J of energy as heat.
What is the specific heat of this type of glass?
Heat Capacity and Specific HeatSlide8
Given: m = 4.0 g q = 32 J ΔT = 40 K C
p
= ??? Q = (m)(Cp)(ΔT)
32 J = (4.0 g)(C
p
)(40 K)
32 J = 160 (C
p
)
0.20 J/
gK
= C
p
Heat Capacity and Specific HeatSlide9
How much energy will the same glass sample gain when it is heated from 314 K to 344 K?
Heat Capacity and Specific HeatSlide10
(
0.20 J/
gk)(4.0g)(71K – 41K)(0.8)(30) = 24 J
Heat Capacity and Specific HeatSlide11
Determine the specific heat of a material if a 35 g sample absorbed 48 J as it was heated from 293K to 313K
.
If 980 kJ of energy are added to 6.2 L of water at 291 K, what will the final temperature of water be?
Now You Try….Slide12
Heat of ReactionHeat of reaction – the quantity of energy released or absorbed as heat during a chemical reaction.
Heat of ReactionSlide13
2 H2
(g) + O
2 (g) 2 H2
O (g)
In the above equation 2 mol of hydrogen gas is ignited to consume 1 mol of oxygen gas and form 2 mol of water.
This is an explosive reaction and 483.6 kJ are produced.
We would re-write this as a
thermochemical
equation.
2 H
2
(g) + O
2
(g)
2 H
2
O (g) + 483.6 kJ
Heat of ReactionSlide14
Doubling the reaction would likewise produce double the heat released.
4 H
2 (g) + 2 O
2
(g)
4 H
2
O (g) + 967.2 kJ
Fractional co-
efficients
are sometimes used in
thermochemical
equations
.
H
2
(g) + ½ O
2
(g)
H
2
O (g) + 241.8 kJ
Heat of ReactionSlide15
The physical states of reactants and products must always be included in thermochemical
equations because they influence the overall amount of energy exchanged.
(s) = Solid (aq
) =
aqueous
(g
) = gas
(
l) = liquid
Heat of ReactionSlide16
The energy absorbed or released as heat during a chemical reaction at constant pressure is represented by ΔH.
“H” is the symbol for a quantity called enthalpy.
Only changes in enthalpy can be measuredEnthalpy change – the amount of energy absorbed or lost by a system as heat during a process at constant pressure.
ΔH =
H
products
-
H
reactants
Heat of ReactionSlide17
Thermochemical equations are usually written by designating the enthalpy change, rather than the energy as a reactant or product
.
2 H2
(g) + O
2
(g)
2 H
2
O (g) ΔH = -483.6
kJ/mol
Note how enthalpy change is a negative number. This means energy is evolved, or given off, during the reaction.
Heat of ReactionSlide18
The opposite would look like the following:
2 H
2O (g)
2 H
2
(g) + O
2
(g) ΔH = + 483.6 kJ/mol
Heat of ReactionSlide19
An
exothermic
reaction gives off energy, and therefore has a negative enthalpy change.
Heat of ReactionSlide20
An
endothermic
reaction receives energy, and therefore has a positive enthalpy change.Heat of ReactionSlide21