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5 page worksheet.
Heat Relationships in Physical and Chemical Processes
Activity B.5
Date: Section #: Instructor:
Name:
Collective Analysis
The energy change, ΔHdissolution, that occurs while dissolving an ionic compound in water can be
exothermic or endothermic. Qualitatively illustrate the expected relationship between mass of
compound and temperature change of solution in a Styrofoam cup calorimeter filled with water.
Use different lines (solid, dashed) to represent the different ΔHdissolution (exothermic or
endothermic)
Mass of compound (g)
Te
m
pe
ra
tu
re
c
ha
ng
e
(C
)
How would the graph above change if the volume of water in the calorimeter was increased?
Explain.
1
In a simplified two-step process, dissolving an ionic solid consists first of breaking the ionic
bonds that hold the solid together (lattice energy, ΔHlattice) followed by hydration of the resultant
ions by water molecules (hydration energy, ΔHhydration), where the dissolution energy, ΔHdissolution
= ΔHlattice + ΔHhydration.
Use the data in the group file to describe the periodic relationship between the heat of dissolution
(i.e., the energy change upon dissolving, ΔHdissolution) for M-Cl compounds (M=Li, Na, K).
Consider the two following models that can be used to explain the periodic trend in ΔHdissolution.
MODEL #1: The periodic trend in ΔHdissolution is determined by ΔHlattice.
MODEL #2: The periodic trend in ΔHdissolution is determined ΔHhydration.
Which model predicts the correct trend in ΔHdissolution for M-Cl compounds (M=Li, Na, K)?
Explain. [Hint: the strength of an ionic bond is proportional to the force of attraction between the ions, F = q1q2/r2
where q1 and q2 are the charges on each ion, and r is the distance between the ions]
2
Use data from the group file to plot the mass of magnesium and temperature change for heat of
reaction experiment using magnesium and hydrochloric acid. Use the x-axis for the mass of
sample (m), and the y-axis for the temperature change (ΔT). Use different markers to label the
data collected using the different calorimeters (Styrofoam cup and glass beaker).
44
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1
Draw a best-fit line through the data and determine the algebraic equation of the line for each
calorimeter. Show your work.
How would the above graph change if each sample of magnesium was reacted
3
.0 M
hydrochloric acid (instead of the 2.0 M acid used during the lab)? Explain.
3
Use data in the group file to determine the average specific heat capacity, c, of copper. Use the
equation for the total heat flow, qmetal + qwater = 0 (show your work for one sample, provide
values for four other samples). Compare your experimental values to the published value for
copper.
Mass of copper
(g)
Specific heat capacity
(J/gºC)
Use data in the group file to determine the average specific heat capacity, c, of each unknown
metal using the equation for the total heat flow, qmetal + qwater = 0 (show your work for one
sample, provide values for four other samples). Identify each metal from the following list (Al, c
= 0.897 J/gºC; Ni, c = 0.54 J/gºC; Zn, c = 0.388 J/gºC; Sn, c= 0.228 J/gºC; Pb, c = 0.129 J/gºC).
Mass of Metal A
(g)
Specific heat capacity
(J/gºC)
Mass of Metal B
(g)
Specific heat capacity
(J/gºC)
4
Expand Your Thinking
Consider the process of dissolving ionic compound which has a negative ΔHdissolution
(exothermic). Draw a particulate level model to illustrate the two steps of this process: breaking
ionic bonds in the solid and hydrating the ions with water molecules. Illustrate the relative
magnitude of energy flow during these two steps using arrows (longer arrows = higher energy).
Carefully consider the direction of energy flow and specifically identify the source (where the
energy comes from) and the sink (where the energy goes to) during each step.
Step 1: breaking ionic bonds
Energy source:
Energy sink:
Explain your reasoning:
Explain your reasoning:
Step 2: hydrating ions with H2O
Energy source:
Energy sink:
How would the above models change if the process of dissolving the ionic compound was
endothermic (positive ΔHdissolution)? Explain.
5
Lab Report
