Unit Twelve
Introduction to Thermochemistry
Thermochemistry:
• Thermochemistry is defined as the transfer of energy as heat that
accompanies chemical reactions and physical changes.
• In chemistry, thermochemistry helps us to understand the flow of
heat energy that occurs in a physical or chemical process.
• Note to self: we did this already when we looked at the heating curve
of water!
• The ice had to absorb energy in order to melt and then to evaporate.
Useful facts about heat energy (enthalpy):
• Heat energy is called enthalpy and the symbol for change in enthalpy is H
.
• Heat is measured according to temperature .
• Temperature defines kinetic energy so basically, this describes how fast the
molecules are moving.
• Heat transfer is always from hot to cold . Heat transfer will continue until
the temperature is the same (equilibrium).
• When studying heat transfer, it is important to recognize the direction of
heat flow.
Heat Capacity:
• Heat capacity- is the measure of the materials storage capacity for
thermal energy (heat). Heat capacity is measured as the energy required
to raise the temperature of a given material by 1°C.
• Low heat capacity- takes less energy to heat up
• High heat capacity- takes more energy to heat up and can store more thermal
energy.
Heat capacity & the ocean
• The high heat capacity of water (4.18J/g ·°C) is very important to our
weather and environment. Because oceans are so large they can
store a lot of energy. They help to moderate temperature changes.
The law of conservation of energy states that:
• Energy cannot be gained nor lost during any chemical or physical
process, but it can be transferred . That means when something feels
cold or warm, energy is simply being transferred.
Understanding Energy Flow & System-Surrounding
• System = the material or process that is being studied
• Surrounding = anything outside the system that can give/absorb heat.
Endothermic
exothermic
For an endothermic process the system absorbs energy
from the surround.
Draw and arrow to describe this flow of energy.
For an exothermic process, the system releases energy
to the surrounding.
Draw and arrow to describe this flow of energy.
Identify the system and surroundings.
Hand warmer
Ice pack
Burning match
Melting ice cube
NaOH (aq)
NH4Cl (aq)
MgO + HCl
Graphing activity and heat energy (enthalpy) :
____ MgO + ____ HCl  ____ MgCl2 + ____ H2O
Title: _______________
TIme
(min)
(x-axis)
Temperature
(°C)
(y- axis)
0
20
1
24
2
27
3
29
4
29
°C
Time (min.)
Exothermic reactions:
Describing heat transfer for an exothermic reaction:
During an exothermic process energy is _____________ .
(absorbed or released)
The reactants start at a _____________ (high/low) energy.
The products formed are at a _________ (high/low) energy
state.
The reaction will feel __________________ (warm/cold)
Endothermic reactions:
During an endothermic process energy is a _____________
. (absorbed or released)
The reactants start at a _____________ (high/low) energy.
The products formed are at a _________ (high/low) energy
state.
The reaction will feel __________________ (warm/cold)
Calculations and specific heat capacity:
• Reminder: heat capacity defines a materials ability to store thermal
energy.
• Specific heat capacity (s) = the amount of energy (J) required to raise 1
g of a substance 1°C.
• Useful equation: q = m·s·T
q = energy in Joules (J)
m = mass in grams (g)
s = specific heat
T = change in temperature (final – initial)
Specific heat capacity problems:
• Equation:
q = m·s·T
• How much heat does it take to raise the temperature of a piece of
copper that weighs 10.0 g from 15oC to 46oC? The specific heat of
copper is .334 J/goC.
• m = 10.0g
• s = .334 J/g°C
• T= 46°C-15°C = 31°C
Answer: 103.5 J
Specific heat problems:
• How much heat does it take to raise the temperature of 25.0g of
water from 23 oC to 100oC? The specific heat of water is 4.18 J/g·oC.
• m = 25.0g
• s = 4.18 J/g°C
• T= 100°C- 23°C = 77°C
Answer: 8,047J
Specific heat problem:
• What is the enthalpy of solution in J/g if 4.35 g of NaOH is dissolved into 110 g of water. The
temperature of the water changes from 23oC to 27oC. The specific heat of water is 4.18 J/g·oC
Solve for q :
• m = 4.35g + 110g = 114.35g
• s = 4.18 J/gC
• T = 27C-23C = 4C
answer : q = 1906 J
Enthalpy of solution
J/g NaOH
answer : 438J/g NaOH
Is this exothermic or endothermic? Justify your answer.
Answer: exothermic the water (surrounding) absorbed energy from the NaOH (system)