Periodic Trends in Reactivity
Introduction:
An arsonist set fire to DVC! Police quickly arrived at the scene, and created a
perimeter to detain all suspicious looking people who were fleeing the school. They
found six suspects. The fire department appeared shortly after, and attempted to put
out the flames with a fire hose. To their surprise, the water did not put out the fire.
Instead, it caused an explosion. As a crime scene investigator, you will use forensic
science to determine which suspect is guilty, and which chemical was used to start
the fire.
Crucial Background Knowledge :
The periodic table is structured such that elements with similar properties are
aligned within the same vertical group. This leads to smoothly varying trends in
properties such as ionization energy and atomic radius as you move down a specific
group, or as you move horizontally along a given row. The reactivities of the elements
also follow well-defined trends. The elements in a group (vertical column) have
similar properties because they have valence electrons in similar configurations. By
observing an element’s physical characteristics, as well as it’s reactivity in the lab,
you can determine which group of the periodic table it most likely belongs to. In this
lab, you will compare the properties of elements found on the 6 suspects to the
properties of the element that ignited the DVC fire to determine who is guilty.
P u r p o s e : To explore the reactivity trends of metals in groups and periods of the
periodic table.
P r e - l a b R e q u i r e m e n t s : 1.
1. APA Title Page
2. Typed introduction paragraph:
 Follow template on project 2 brief (same as murder lab)
 Background information should include facts that you already know about: characteristics of metals vs nonmetals -reactivity of various groups on the
periodic table. (which groups are most and least reactive? What types of
elements do metals and nonmetals react with? Do they react with water?
Bases? Acids?)
 Hypothesis: Write as an “If______, then _______” sentence structure. Think
about your prediction. If something reacted STRONGLY to the water from the
fire hose, which group is it probably in?
Periodic Trends in Reactivity
 Sample hypothesis sentence frames: “If the six elements in question are reacted
with water, then the element in group ________ will react most vigorously.” “If
the six elements in question are reacted with water, then the
(metals/nonmetals/metalloids) will have the weakest reaction.” This means
that they could not have exploded upon contact with water from the fire hose,
and that their owners are innocent. “If the six elements are reacted with
water, ____________ will have the strongest reaction, but ______________ will
not react at all.
3. Methods paragraph (typed)
 Do your best! This is a rough draft that you will be able to edit later. It is useful
to write it ahead of time to get a jump start on the write up, and to familiarize
yourself with the procedure you will be performing.
 Write it by reading the procedures (for all 3 parts) in this lab and summarizing
the key points.
 When you edit later, be sure to include how many g of each metal you tested,
what glassware/equipment you used (with size in mL), and how many trials
you performed
Periodic Trends in Reactivity
Procedures:
Part 1: Trends in Physical Characteristics
1. Obtain samples of the 6 elements that were confiscated from the suspects.
2. Observe each element, and record qualitative data below. Describe physical
characteristics such as color, texture, shape, and malleability. DO NOT touch any
elements with your hands! They could be dangerous! Instead, use forceps or a metal
spatula.
Data Part 1:
Table 1: Physical Characteristics of Suspicious Elements
Element
UNKNOWN
substance #1
Observations: Physical Characteristics
Magnesium
Calcium
UNKNOWN
substance #2
UNKNOWN
Substance #3
Tin
PART 1 ANALYSIS:
1. Look at the periodic table. Are magnesium, calcium, and tin classified as metals,
nonmetals, or metalloids? Do the physical characteristics that you just observed
match the ones that we learned in class? Explain why, using evidence from table 1
to support your claim.
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Periodic Trends in Reactivity
2. Based on your observations of the 3 unknown substances, which ones do you think
are metals? Nonmetals? Metalloids? Explain, using evidence from table 1 to support
your claim.
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Part 2: Reactivity in Water
Procedures:
1. Your instructor will do a demonstration of unknown substance 1’s reaction with
water. Record observations in the table below.
2. Place a small sample of magnesium in a test tube and cover with water.
Record observations. * Make sure you use a new, clean test tube for each
chemical to avoid cross contamination and dangerous reactions.
3. Place a small sample of calcium in a test tube with several milliliters of
water. Record observations.
4. Place a small sample of unknown substance #2 in a test tube and cover with
water. Look closely and record observations.
5. Place a small sample of tin in a test tube and cover with water. Look closely
and record observations.
6. Place a small sample of unknown substance #3 in a test tube and cover with
water. Look closely and record observations.
Periodic Trends in Reactivity
Data Part 2
Table 2: Reactivity in Water
Metal
UNKNOWN
substance #1
Obs ervat i on s: Rea ct ivi t y i n wat er
Magnesium
Calcium
UNKNOWN
substance #2
Tin
UNKNOWN
Substance #3
ANALYSIS PART 2:
1. What might be a reason for the difference in the way magnesium and calcium
react in water? (Hint: Look at their locations on the periodic table. Lithium,
sodium, and potassium are also progressively more reactive).
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2. Why do you think that tin was unreactive in water? (hint: think about what group it
is in on the periodic table)
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Periodic Trends in Reactivity
3. Explosions are chemical changes, because they produce gas and release heat.
Hence, the fiery DVC explosion was caused by an element chemically reacting with
water. Based on your observations from Table 2, which elements could have caused
the explosion (if a large amount was used)? Which suspects might be guilty? Which
suspects are most likely innocent? Explain, using evidence from table 2 to support
your claim.
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Part 3 - Reactivity in HCl
Although 4 of the subjects are probably innocent, you decide that it’s a good idea to
determine what unknown elements that they are carrying before releasing them
from custody.
1.
Obtain a small sample of magnesium, unknown substance #2, tin, and
unknown substance #3. You may reuse the Mg from Part 1. Carefully dump out
the water from the test tube containing the magnesium but keep the magnesium
in the test tube.
2.
Place each sample in a separate tube.
3.
Add a small amount of dilute HCl to each test tube, just enough to cover
the sample. Record your observations.
Part 2 – Reactivity in HCl
Metal
Magnesium
UNKNOWN
substance #2
Tin
UNKNOWN
Substance #3
O b s e r v a t i o n s : R e a c t i o n i n HCl
Periodic Trends in Reactivity
ANALYSIS PART 3
1.
Put the 4 elements from part 2 in order, from least to greatest
reactivity.
2. Using your data from parts 1,2, and 3, try to determine what the 3 unknown
elements are. Explain your guesses, with evidence.

(example: Unknown substance #1 is probably a metal/nonmetal/metalloid,
because it had a shiny, silver appearance (table 1). Furthermore, unknown
substance 1 is probably in group ____ on the periodic table, because it
reacted very strongly/weakly with water (table 2). Unknown substance 1
could be (element) because it was more reactive than the known alkaline
earth metals. (element) shows this relative reactivity because it is located
to the left of the alkaline earth metals on the periodic table.)
Unknown substance 1:
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Unknown substance 2:
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Unknown substance 3:
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Periodic Trends in Reactivity
3.
Reactivity INCREASES/DECREASES as you move down a group, and
INCREASES/ DECREASES as you move across a period. (circle the correct
trend).
4.
Although unknown substance 1 seems to be the most reactive in water,
Calcium reacted strongly as well. If you were able to collect a small amount of
metal from the crime scene, what additional tests could you perform to see if it
were calcium or unknown substance 1?
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5.
What are some pros for using reactivity to identify unknown
substances? What are some cons?
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6. A) Based on your complete lab results, which suspect do you think is guilty? Which
element do you think the suspect used to burn down DVC?
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B) Would you feel comfortable sending a suspect to jail based on your lab results? Why
or why not?_____________________________________________________________________
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Periodic Trends in Reactivity
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Periodic Trends in Reactivity