Download Chemistry: Dr. Wilhelm & Ms. Greendyk

Name: ______________________________
Mods: ___________
Unit Twelve:
Introduction to Thermochemistry
Chemistry: Dr. Wilhelm & Ms. Greendyk
Thermochemistry is defined as the transfer of energy as _______________ that accompany
chemical reactions and physical changes.

In chemistry, __________________ helps us to understand the flow of ________ energy
that occurs in a ______________ or __________ process.
Note to self: we did this already when we looked at the heating curve of water!
The ice had to _______________ energy in order to melt and then to evaporate.
Useful facts about heat energy (enthalpy):

Heat energy is call ___________ and the symbol for change in enthalpy is ___________ .
____________ is measured according to ____________ .

Temperature defines ____________________ , how fast the molecules are moving.

________________ is always from ___ to ______
the temperature is the ___________ (equilibrium).

When studying heat transfer, it is important to recognize the _____________ of heat flow.

______________________- 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.
. Heat transfer will continue until
o Low heat capacity- takes less energy to heat up
o High heat capacity- takes more energy to heat up and can store more thermal
energy.
 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 __________ nor __________during any chemical or physical process, but it can be
____________________ . That means when something feels cold or warm, energy is simply being
transferred.
Understanding Energy Flow & System-Surrounding
System =
Surrounding =
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.
Examples:
Hand warmer
Ice pack
Burning match
Melting ice cube
NaOH (aq)
NH4Cl (aq)
MgO + H2O
System
Surrounding
Hot or cold
Endothermic -Exothermic Reactions:
A chemical reaction occurs when bonds break and new bonds form. Breaking bonds between
atoms or ions requires energy and forming bonds between atoms or ions releases energy.
In an EXOTHERMIC REACTION (ek soh THUR mik), the energy released as
the products formed forms is greater that the energy required to break the
bonds of the reactants. The energy is usually released as _________.
For example, some stoves use natural gas. When natural gas burns, it
releases heat. This heat is used to cook your food. Similarly, the reaction
between oxygen and other fuels that produce fire, such as wood, coal, oil, or
the wax of the candle shown in the figure, release energy in the form of light
and heat.
In an ENDOTHERMIC
REACTION (en doh THUR mik), more
energy is required to break the bonds of the
reactants than is released by the formation of
the products. The energy can be absorbed
from nearby matter. When energy is
absorbed, it causes the surroundings to
become cooler. In the figure, baking soda
undergoes an endothermic reaction when it is
mixed with vinegar. The reaction absorbs
heat from its surroundings, so the reaction
feels cold. Not all endothermic reactions
results in a temperature decrease. Many
endothermic reactions occur only when heat is constantly added, as when you fry an egg.
Heat must be applied throughout the entire process in order for the reactions that cook the egg
to continue.
How might each of the reactions feel if you were to put your finger near it?
Candle in the cupcake - ___________________________________________
Baking soda & Vinegar - ___________________________________________
Graphing activity and heat energy (enthalpy) :
Magnesium oxide (MgO) and hydrochloric acid (HCl) react to form Magnesium chloride and water in the
following unbalanced reaction. Use coefficients to balance this reaction:
____ MgO + ____ HCl  MgCl2 + ____ H2O
The temperature for this reaction was recorded every minute and is presented below.
TIme (min)
(x-axis)
Temperature (°C)
(y- axis)
0
20
1
24
2
27
3
29
4
29
Plot the data presented the table. Be sure to include a title and label your axis.
Title: _____________________________________
1. During the four minutes the temperature ________________ (increased/decreased)
2. Calculate the change in temperature ( T) for this reaction: T = Tfinal – Tinitial .
3. Is the reaction endothermic (absorbs heat) or exothermic (gives off heat)? Explain how you
determined this.
Energy Diagrams and Chemical reactions:
1. Describing heat transfer for an exothermic reaction:
a.
During an exothermic process energy is _____________ . (absorbed or released)
b.
The reactants start at a _____________ (high/low) energy.
c.
The products formed are at a _________ (high/low) energy state.
d.
The reaction will feel __________________ (warm/cold)
e.
The following energy diagram for an exothermic reaction is shown below.
-
f.
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)
g.
Label the following energy diagram for an endothermic reaction.
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)
1. 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=
s=
T=
2. 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=
s=
T=
1. 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.
a. Solve for q :
m=
s=
T
b. Enthalpy of solution = J/g NaOH
c. Is this exothermic or endothermic? Justify your answer.