Lab # 5
Experimental Application of Hess's Law
Experiment 2: The heat of formation of solid Calcium
Hydroxide
Diana Bakos
Stein van der Plas
Liam Beck
SCH4U 01
November, 2014
Introduction
The first law of thermodynamics states that during a chemical reaction energy is conserved. Hess's law
follows this application because if a reaction were to proceed in a series of steps the enthalpy change
for the overall reaction would equal the sum of all the enthalpies for the individual steps. This is due to
the fact that enthalpy is a state function it does not does not depend on the pathway taken to convert
reactants into products. Furthermore, Hess's law states that one can add up two or more separate
equations to arrive at a target chemical equation. This can also help determine the enthalpy of the
target equation because all the separate enthalpies of the individual reactions would equal that of the
target equation. This is the reason as to why chemists view Hess's law as one of the most important
concepts in chemistry. It can be used to find the enthalpy values for reactions that may be difficult to
perform directly or too dangerous perform directly. Therefore, Hess's law can be used to determine the
overall energy required for a chemical reaction, without even carrying out said experiment.
In the following experiment, applications of Hess's law were used to determine the change of
enthalpy for the formation of solid Calcium Hydroxide Ca(s) + O2(g) + H2(g) → Ca(OH) by
adding two separate equations. A coffee cup calorimeter was used to calculate the enthalpy of the
first reactions. The first reaction conducted was that of Ca(s) + 2 H2O(l) -----> Ca(OH)2(s) +
H2(g) . The enthalpy of the other reaction needed H2(g) + ½O2(g) → H2O(l) was researched.
Objectives
Please refer to page 1of 2
Materials
Please refer to page 1of 2
Procedure
Please refer to page 2of 3
Observations: Temperature Measurements for the Enthalpy of Combustion of Magnesium ribbon
Time (seconds)
Experiment 2: Equation 5
Ca(s) + 2 H2O(l) -----> Ca(OH)2(s) + H2(g)
0s
22.5 oC
30
40.3 oC
60
43.1 oC
90
43.0 oC
120
42.8 oC
150
42.6 oC
180
42.4 oC
210
42.2 oC
240
42.1 oC
270
300
41.9 oC
41.8 oC
Temperature of Ca(s) + 2 H2O(l)
43.5
43
Temperature (deg C)
42.5
42
41.5
Temperature
41
40.5
40
30
60
90
120
150
180
210
240
270
Time (s)
Data Analysis- Experiment 2: Part 3:
Equation 7: Ca(s) + 2 H2O(l) -----> Ca(OH)2(s) + H2(g)
Data Collected
Volume of 1.0 M HCl solution used
Mass of magnesium
100mL
0.099g
300
Initial temperature of H2O solution
before mixing
Final temperature of Ca(OH)2(s)
solution
Calculations
22.5 oC
ΔT
=Tfinal -Tinitial
=42.4 oC -22.5 oC
=20.9oC
Q= mc ΔT
Energy produced in reaction
42.8 oC
= (100g)(4.184J/g oC) (20.3oC)
=-8494J
Moles of Calcium used
ΔH
0.99gCa×
=
=
1𝑚𝑜𝑙 𝐶𝑎
40.08𝑔𝐶𝑎
= 0.025molCa
qrxn
n
8494J
0.025mol
= -340kJ/mol-1
eqn (6) 2H2(g) + O2(g) -------> 2H2O(l) ΔH2= -572 kJ/mol-1 ×2
eqn (7) Ca(s) + 2 H2O(l) -----> Ca(OH)2(s) + H2(g) ΔH2 = 3.4x102kJ/mol-1
______________________________________________________________________________
eqn (5) Ca(s) + O2(g) + H2(g) ----> Ca(OH)2(s)
ΔH5 = ΔH6 + ΔH7
= 2(-572 kJ/mol-1) + (-3.4x102kJ/mol-1)
= -1484 kJ/mol-1
% 𝑒𝑟𝑟𝑜𝑟 =
(−804kJ · mol − 1) − (−987 kJ · mol − 1)
(−987 kJ · mol − 1)
% error = 18.54%
Discussion
Through the following experiment the molar enthalpy of the formation of calcium
hydroxide was calculated to be -804 kJ/mol-1. The following result was calculated by using a
coffee cup calorimeter to determine the enthalpy of Ca(s) + 2 H2O(l) → Ca(OH)2(s) + H2(g),
which was +3.4x102kJ/mol-1. The enthalpy of the last reaction needed was researched. According
to Chemwiki H2(g) + ½O2(g) → H2O(l) has the molar enthalpy of -572 kJ/mol-1. In order to
calculate the enthalpy change of the equation for the formation of calcium hydroxide the separate
enthalpies were manipulated using Hess's law. It was observed that the reaction was exothermic
because the final temperature was greater than the initial temperature. However, the following
experiment was not accurate because the value calculated holds a 18.54% error compared to the
theoretical value. The error could have been a result of heat being lost to the surroundings
instead of being absorbed into the system, the assumptions made in the calculations and faulty
time readings. Heat was lost to the surrounding because the coffee-cup calorimeter was a closed
system. To maximize the efficiency of the calorimeter within the lab one could include an
insulator. Preferably one, that could decelerate the effect of heat flow by conduction of
convection. Another, source of error was the assumptions needed to be made to complete the lab
calculations. The experiment called for one to assume that the solution had the same specific heat
capacity as water. This error cannot truly be improved, but one could conduct the experiment
several times to calculate several heats of formation of calcium hydroxide and then arrive at an
average. This would probably help with the accuracy of the experiment. The last source of error
that will be discussed is faulty time reading. During the experiment, there may have been times
when the reactants did not fully mix, which meant that the reaction did not reach its full
potential. Hence, the wrong temperature could have been recorded during the different time
intervals. To insure that that the solution fully reacted the experiment could be recorded over a
longer time interval.
Source of error
The errors that occurred in this lab could have been a result of heat being lost, the
assumptions made in the calculations and faulty time readings. Due to the coffee-cup calorimeter
being a closed system, heat was exchanged between the system and the surroundings.
Consequently, the temperature of the air surrounding the system increased. Since, heat is always
transferred from an object at higher temperature to the object at lower temperature until thermal
equilibrium is reached the warm air, which is less dense than cold air, was forcibly replaced by
the cold air. This process turned into a continues cycle. To maximize the efficiency of the
calorimeter within the lab one could include an insulator. Preferably one, that could decelerate
the effect of heat flow by conduction of convection. For example, cotton is an excellent insulator
because the thousands of tiny air spaces between the fibers of the cotton would slow down the
rate of transmission of energy by trapping it. Another, source of error was the assumptions
needed to be made to complete the lab calculations. The experiment called for one to assume that
the solution had the same specific heat capacity as water. Chemists usually assume that the
specific heat capacity of a dilute solution is very close to the specific heat capacity of pure water.
Hence, the specific heat capacity of HCl may not be 4.184 J K-1 g-1, but is somewhat precise to
what the true value may be. This error cannot truly be improved, but one could conduct the
experiment several times to calculate several heats of combustion for magnesium and then arrive
at an average. This would probably help with the accuracy of the experiment. The last source of
error that will be discussed is faulty time reading. During the experiment, there may have been
times when the reactants did not fully mix, which meant that the reaction did not reach its full
potential. Hence, when the waiting for the solution to reach its maximum level, the thermometer
may have read that the temperature was cooling or at a constant and that time could have been
recorded. To improve this error, the experiment could be recorded over a longer time interval
because that would insure that the solution had fully reacted. The solution may not have fully
reacted at the beginning.
Conclusion
In conclusion, the molar enthalpy of the combustion of solid Calcium Hydroxide was
-1484 kJ/mol-1The following result was calculated by using a coffee cup calorimeter to
determine the enthalpy of Ca(s) + 2 H2O(l) → Ca(OH)2(s) + H2(g), which was
+3.4x102kJ/mol-1. The enthalpy of the last reaction needed was researched. According to
Chemwiki H2(g) + ½O2(g) → H2O(l) has the molar enthalpy of -572 kJ/mol-1. A series of
steps were followed to do the calculation, which applied Hess's law. This law states that
enthalpy is a state function and that it is possible to arrive at a target equation ( Ca(s) +
O2(g) + H2(g) ----> Ca(OH)2(s)) by adding up two or more separate equations because the
sum of all the enthalpies for the individual steps would equal the overall reaction. Overall,
the reaction only had a 18.54% error, which suggests that the experiment was inaccurate.
Work Cited