Student Exploration: Reaction Energy
Vocabulary: calorimeter, chemical bond, endothermic, enthalpy, exothermic, Hess’s law
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
1. Two magnets are stuck together. What might you have to do to get them to separate?
Pull them apart
2. Sup...
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Student Exploration: Reaction Energy
Vocabulary: calorimeter, chemical bond, endothermic, enthalpy, exothermic, Hess’s law
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
1. Two magnets are stuck together. What might you have to do to get them to separate?
Pull them apart
2. Suppose you held two magnets a short distance apart, then let go. What would happen?
They would connect
3. Think about the magnets in terms of energy. In which case do you increase the potential
energy of the magnets? In which case do you increase the kinetic energy of the magnets?
Gizmo Warm-up
Just like magnets, atoms of different elements are
attracted together to form chemical bonds. Breaking
these bonds requires energy. When a new bond forms,
energy is released and temperatures rise. In the Reaction
Energy Gizmo™, you will explore how the energy of
chemical bonding relates to temperature changes that
occur during chemical reactions.
To begin, check that Reaction 1 and Forward are selected. In this reaction, hydrogen (H2) and
oxygen (O2) react to form water (H2O). The reaction takes place inside a device called a
calorimeter. Inside the calorimeter, a small chamber holds the reactants. The rest of the
calorimeter is filled with water.
1. Click Play ( ). What happens? When I clicked play, all of the separate H2 and O2
molecules formed to create H2O.
2. How does the temperature change? The temperature is increased while the molecules are
reacting to form water, and the increase in temperature slows once the molecules are all
bonded to create the H2O molecules. The temperature raises from 21.0 Degrees Celsius to
26.0 Degrees Celsius while the molecules were bonding.
, Activity A: Get the Gizmo ready:
Energy of Check that Reaction 1 and Forward are selected.
chemical bonds Select the INVESTIGATION tab.
Introduction: The heat energy stored in a chemical system is called the enthalpy (H) of the
system. When atoms are joined by a chemical bond, energy must be added to pull them apart.
This increases the enthalpy of the system. When a chemical bond forms, energy is released as
shared electrons move into lower-energy orbitals. This causes the enthalpy to decrease.
Question: How can you predict how much energy is released in a chemical reaction?
1. Predict: In the warm-up activity, you observed how the reaction inside the chamber affected
the temperature of the surrounding water. Based on what happens to the surrounding water,
do you think heat energy (enthalpy) is absorbed in the reaction or released? Explain.
I think, based on what happened to the surrounding water, that enthalpy was released
during the reaction and is therefore increased because energy was released as the shared
electrons moved into lower-energy orbitals.
2. Observe: In the Gizmo, the energy required to break a chemical bond is modeled by placing
a molecule into a set of mechanical claws. Place one of the hydrogen (H2) molecules
between the claws, and press Break bond.
A. What happens? Energy is absorbed to break apart the H2 bond.
B. Look under the Energy absorbed column of the table. How much energy was
required to break this bond? 436 kJ/mol was required to break the bond.
Note: The energy is given here in units of kilojoules per mole (kJ/mol). This is the
energy, in kilojoules, required to break all of the H–H bonds in one mole of H2 gas.
C. Remove the hydrogen atoms from the claws and then break apart the other H–H
molecule. What is the total energy absorbed so far? The total energy absorbed so far
is 872 kJ/mol.
3. Measure: Notice that the oxygen atoms are connected by a double covalent bond. This is
because the oxygen atoms share two pairs of electrons. Place the oxygen molecule in the
claws and press Break bond.
A. How much energy is required to break the first O–O bond? 349 kJ/mol
is required to break the first O2 bond.
B. Press Break bond. How much energy is needed to break both bonds? 495 kJ/mol is
required to break both bonds.
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