Ch 24 - Thermochemistry
Example:
Example:
 H=
 H=
The Heat of reaction of foods and fuels is measured accurately using a bomb
_______________.
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Ch 24 - Thermochemistry
Determination of the heat of reaction of Hydrochloric acid with sodium hydroxide
In this experiment a known number of moles of hydrochloric acid is mixed with an equal number of moles of sodium hydroxide in an insulated container. The rise in temperature is measured and this is used to calculate the heat of reaction.
HCl + NaOH NaCl + H20
This is a _____________ reaction: the acid and base react together to make ______ and water.
Since concentrated solutions of acid and base can be ______________ safety googles must be worn. Dilute solutions may not result in a big temp change so concentrated solutions must be used.
1.
Use a graduated cylinder to measure out 50cm 3 of a 1M solution of
_________________________ into a ____________________ cup.
2.
Use a graduated cylinder to measure out 50cm3 of a 1M solution of
_________________________ into a different ___________________ cup.
3.
The __________ temperature of each solution is measured separately and then the ____________ temperature is calculated.
4.
Add one solution to another in a polystyrene cup and measure the highest
___________ increase
*Polystyrene is a good heat insulator ( has __________ heat ___________) and will reduce heat loss from the container
**Add the solutions together quickly - Be careful to avoid splashing which would result in
_____ of reactants.
***Place a lid on the cup to reduce heat loss through the gap.
****Stir the mixture continuously
5.
Calculate the heat of reaction using the following formula:
Heat change = mc ∆ T
Increase the accuracy of the experiment by:
1.
Using a ______________ thermometer which is very sensitive to temp chages
2.
Using _________________ to measure the volumes of the solutions instead of graduated cylinders .
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Ch 24 - Thermochemistry
Example 1 : Calculate the heat of reaction (using the formula ΔH = mcΔT) for the reaction between and nitric acid sodium hydroxide from the following experimental results:
HNO
3
+ NaOH NaNO
3
+ H
2
O
Volume of nitric acid = 200 cm 3 of 1.0 M
Volume of sodium hydroxide = 200 cm 3 of 1.0 M temperature rise = 6.9 o C
Specific heat capacity of the mixture = 4060 J Kg —1o C —1
Answer:
1. What is the heat change in the reaction?
Heat change = mc ∆ T
2. How many moles of nitric acid were reacted?:
3.
What is the heat of reaction?
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Ch 24 - Thermochemistry
Breaking chemical bonds requires energy ( an _____________________ process) activation energy (E a or E act
) - the energy input into a reaction to allow chemical bonds to be broken.
Making chemical bonds releases energy ( an ____________________ process)
***So overall the ∆H for a reaction will depend on the amount of energy needed to break the bonds and also the amount of energy released when new bonds are made.
Example for the combustion of methane:
Energy taken in to break bonds:
C-H ( 412 X 4) = 1648 C=O ( 743 X 2) = 1486
Energy released as new bonds are made:
O=O 496 X2 = 992 O-H 464 X 4 = 1852
Total = 2640Kj/mol Total 3338Kj/mol
Overall ∆H = 2640 – 3338 = -698Kj/mol ( the reaction is ________________ )
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Ch 24 - Thermochemistry
Heats of combustion is measured using a ___________ calorimeter.
1.
A known _________ of substance is placed inside the bomb.
2.
The bomb is filled with ___________.
3.
The bomb is placed into a known amount of water.
4.
The substance is ignited _________________and the temperature rise is measured and usedfor further calculations.
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Ch 24 - Thermochemistry
Example 1:
The combustion of liquid benzene is described by the following equation
2C
6
H
6
+ 15O
2
12CO
2
+ 6H
2
0
Given that the heats of formation of carbon dioxide gas, liquid water and liquid benzene are -394,-
286 and 49kJmol
-1
respectively, calculate the heat of combustion of liquid benzene
Answer:
Use ∆H r =
∑ ∆H f products
-
∑ ∆H f reactants
∑ ∆H f
products 12 ( -394) + 6 (-286) =
∑ ∆H f
reactants 2 ( 49) + =
∆H r =
∑ ∆H f products
-
∑ ∆H f reactants
∆H r =
-6444 – ( 98) = - heat of reaction for the given equation ( for 2 moles of benzene)
Heat of combustion is heat change when 1 mole of a substance completely reacts in excess oxygen so heat of combustion of benzene is:
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Ch 24 - Thermochemistry
Example 2
The combustion of methane is described by the following balanced equation
CH
4
+ 2O
2
CO
2
+ 2H
2
0 ∆H = -890.4kJmol-1
Given that the heats of formation of carbon dioxide gas, liquid water are -394,-286 kJmol -1 respectively, calculate the heat of formation of methane
Answer:
∆H r
= -890.4kJmol
-1
∑ ∆H f
products
= r =
f
-
f
∑ ∆H f
reactants
=
Let x = heat of formation of methane
 Sub in the correct values into the formula: ∆H r =
∑ ∆H f
products
-
∑ ∆H f
reactants

Heat of formation of methane is
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Ch 24 - Thermochemistry
Check your learning of EXOTHERMIC AND ENDOTHERMIC REACTIONS
Green = I can do this already
Orange = I am not sure of this – must revise it
Red = I cant do this yet – must learn it
Can you do the following?
Define Exothermic reaction and know what the sign of
∆H is.
Define Endothermic reaction and know what the sign of
∆H is.
Know what Combustion of alkanes and other hydrocarbons makes heat energy, carbon dioxide and water
Write out and balance an equation for any simple combustion reaction of a hydrocarbon
Define Heat of reaction (general term).
Mandatory experiment 5.1
To measure the heat of reaction of hydrochloric acid and sodium hydroxide. CALCULATIONS!!
HL - Define Bond energy (concept only – no calculations except for the illustrative example indicated).
Heat of combustion. Bomb calorimeter as an instrument for accurately measuring heats of combustion. Heats of combustion of different fuels.
Use of the bomb calorimeter in determining calorific values of foods. Kilogram calorific values definition and uses (calculations not required).
HL - Heat of formation definition
HL - Law of conservation of energy definition
HL -
Hess’s law definition
HL - Simple calculations of heat of reaction, using heats of formation of reactants and products
HL - Simple calculations of heat of formation, using other heats of formation and one heat of reaction. (Other kinds of heat of reaction calculation not required.)
Green Orange Red
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