Additivity of Heats of Reaction: Hess' Law The Russian chemist and doctor, Germain Hess helped to formulate some of the early ideas of thermodynamics. One of the ideas he formulated deals with the heats of reactions in mechanisms. As smaller reactions are added and intermediates removed, the overall enthalpy (heat of reaction) is the sum of the smaller reactions added. In this experiment, you will use a Styrofoam-cup calorimeter to conserve energy so that the heat released by three reactions can be measured accurately. Two of the reactions add up to provide a net reaction. By measuring the heats of reaction accurately, we will see if Hess' ideas are accurate. BA pg L * C Ind + Pathung Pathway 3 # 1 2 Reaction NaOH(s) → Nat(aq) + OH(aq) →H₂O(1)+Na+(aq) + Cl(aq) NaOH(s) + H+(aq) ) + Cl(aq) Na+(aq) + OH(aq) + H+(aq)) + Cl(aq) → H2O(l) + Na+ (aq) + Cl(aq) OBJECTIVES In this experiment, you will • Combine equations for two reactions to obtain the equation for a third reaction. • Use a calorimeter to allow for the temperature change in each of three reactions to be accurately measured. • Calculate the heat of reaction, AH, for the three reactions. • Use the results to confirm Hess's law. PRE-LAB In the table found in the introduction above, cross out the intermediates (chemical species that are found on both sides of the reaction arrows) for reactions 1 and 2 to verify that the two reactions add up to give reaction 3. (1 pt) pg

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Additivity of Heats of Reaction:
Hess' Law
The Russian chemist and doctor, Germain Hess helped to formulate some of the early
ideas of thermodynamics. One of the ideas he formulated deals with the heats of
reactions in mechanisms. As smaller reactions are added and intermediates removed,
the overall enthalpy (heat of reaction) is the sum of the smaller reactions added. In
this experiment, you will use a Styrofoam-cup calorimeter to conserve energy so that
the heat released by three reactions can be measured accurately. Two of the reactions
add up to provide a net reaction. By measuring the heats of reaction accurately, we
will see if Hess' ideas are accurate.
BA
pg L
*
C
Ind
+ Pathung
Pathway
3
#
1
2
Reaction
NaOH(s)
→ Nat(aq) + OH(aq)
→H₂O(1)+Na+(aq) + Cl(aq)
NaOH(s) + H+(aq) ) + Cl(aq)
Na+(aq) + OH(aq) + H+(aq)) + Cl(aq) → H2O(l) + Na+ (aq) + Cl(aq)
OBJECTIVES
In this experiment, you will
• Combine equations for two reactions to obtain the equation for a third reaction.
• Use a calorimeter to allow for the temperature change in each of three reactions
to be accurately measured.
• Calculate the heat of reaction, AH, for the three reactions.
• Use the results to confirm Hess's law.
PRE-LAB
In the table found in the introduction above, cross out the intermediates (chemical
species that are found on both sides of the reaction arrows) for reactions 1 and 2 to
verify that the two reactions add up to give reaction 3. (1 pt)
pg
Transcribed Image Text:Additivity of Heats of Reaction: Hess' Law The Russian chemist and doctor, Germain Hess helped to formulate some of the early ideas of thermodynamics. One of the ideas he formulated deals with the heats of reactions in mechanisms. As smaller reactions are added and intermediates removed, the overall enthalpy (heat of reaction) is the sum of the smaller reactions added. In this experiment, you will use a Styrofoam-cup calorimeter to conserve energy so that the heat released by three reactions can be measured accurately. Two of the reactions add up to provide a net reaction. By measuring the heats of reaction accurately, we will see if Hess' ideas are accurate. BA pg L * C Ind + Pathung Pathway 3 # 1 2 Reaction NaOH(s) → Nat(aq) + OH(aq) →H₂O(1)+Na+(aq) + Cl(aq) NaOH(s) + H+(aq) ) + Cl(aq) Na+(aq) + OH(aq) + H+(aq)) + Cl(aq) → H2O(l) + Na+ (aq) + Cl(aq) OBJECTIVES In this experiment, you will • Combine equations for two reactions to obtain the equation for a third reaction. • Use a calorimeter to allow for the temperature change in each of three reactions to be accurately measured. • Calculate the heat of reaction, AH, for the three reactions. • Use the results to confirm Hess's law. PRE-LAB In the table found in the introduction above, cross out the intermediates (chemical species that are found on both sides of the reaction arrows) for reactions 1 and 2 to verify that the two reactions add up to give reaction 3. (1 pt) pg
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