Download Ch6-Energy in Chemical Reactions-Chemical Reactions

Louisiana Tech University, Chemistry 100. POGIL(Process Oriented
Guided Inquiry Learning) Exercise on Chapter 6. Energy and
Chemical Reactions: Enthalpy and Hesse’s Law Calculations
Why?
In chemistry, the mole is the standard measurement of amount. When substances
react according to chemical equations, they do so in simple ratios of moles. However,
balances give readings in grams. Balances DO NOT give readings in moles. So the
problem is that, when we compare amounts of one substance to another using mole, we
must convert to moles from grams which are actually measured. It is also important that
we understand the mole as the basis for all calculations in chemistry and have conceptual
understanding of how Mole is related to Avogadro’s number.
Learning Objectives
The students should be able to understand the concepts and have working knowledge in
the following:
6.7 Where does the Energy come from?
1.Understand the origin of the enthalpy change for a chemical reaction in terms of bond
enthalpies.
6.8 Measuring Enthalpy Changes: Calorimetry
1.Describe how calorimeters can measure the quantity of thermal energy transferred
during a reaction.
6.9 Hess's Law
1.Apply Hess's law to find the enthalpy change for a reaction.
6.10 Standard Enthalpy of Formation
1.Use standard molar enthalpies of formation to calculate the thermal energy transfer
when a reaction takes place.
6.11 Chemical Fuels for Home and Industry
1.Be able to define and give examples of some chemical fuels and to evaluate their
abilities to provide heating.
6.12 Foods: Fuels for Our Bodies
1.Describe the main components of food and evaluate their contributions to caloric
intake.
Success Criteria
Understand the differences between physical and chemical change, ability classify
matter as matter as homogenous/heterogeneous mixtures & pure substances,
separate mixtures into elements and compounds, name elements and give symbols.
Resources
Chemistry: The Molecular Science 1st Edition, John W. Moore, Conrad L. Stanitski and
Peter C. Jurs.
Prerequisites
High school chemistry: Definition of matter. Scientific method, Dalton’s Atomic theory,
Components of atoms: nucleus (proton and neutrons) and electrons. Atoms, molecules
and ions.
New Concepts
The mole (6.022 x 1023 particles)-Chemists Dozen
Chemists measure chemical in grams as the amount in the reaction. Therefore, we
need a conversion factor to convert grams to atoms or molecules. Mole is the connection
or the conversion factor between atoms and grams. Mole is just a large number 6.022 x
1023 for counting atoms like dozen -12 for counting to make counting of eggs easier.
Since atoms are so small, we need large number of them to make it physically observable
and able to weigh in gram quantities.
Gram Mole and the Avogadro's Number
The gram mole is the grams of any chemical substance using the value atomic mass
obtained from the periodic table. E.g. for carbon gram mole is 12.01 grams of carbon
since its atomic mass is 12.01 amu in the periodic table. if you take atomic mass in grams
the number of atoms is simply 6.022 x 1023 atoms or particles. In other words gram
atomic weight or gram molecular weight contains 6.022 x 1023 atoms, molecules or
particles. This number is called Avogadro's number or mole of particles.
Mole
Type of particle
23
6.022 x 10
atoms
molecules
ions
Gram mole
gram atomic weight gram molecular weight gram ion weight
12.01 g of He
12.01 g of He
--------18.02 g of H2O
----18.02 g of H2O
----x.xx g of NO3
--------x.xx g of NO3How many moles of iron (Fe) are present in 180.1g of elemental iron?
Atomic mass (A.M) Fe = 55.85 g/mol This is a problem to convert grams to moles.
Conversion factor is 55.85 g Fe = 1 mol
180.1 g Fe
x
1 mol
= 3.225 mol Fe
55.85 g Fe
What is the mass of 7.5 x 105 atoms of Cu in grams?
This problem requires conversion of atoms to grams. 1 mol = 63.55g Cu = gram atomic
weight 1 mol = 6.022 x 1023 atoms 6.022 x 1023 Cu atoms = 63.55 grams Cu Therefore,
7.5 x 105 Cu atom x
63.55 g Cu
=7.9 x 10-17g Cu
6.022 x 1023 Cu atoms
An atom weighs 7.47 x 10-23 g. What is the name of the element this atom belongs to?
First convert g to amu and look up in the periodic table and find out the element.
Conversion factor: 1g = 6.022 x 1023 amu.
7.47 x 10-23 g x 6.022 x 1023 amu
= 44.98 amu
1g
In the periodic table, element with an atomic mass closer to the value calculated is Sc
(Scandium). The element is Sc.
Key Questions (relatively simple to answer using the Focus Information)
1) What is a
a) Mole
b) gram-mole?
2) What is the mass in grams of a Cu atom?
3) An atom weighs 4.66 x 10-23 g. What is the name of the element this atom belongs to?
4) What is the molecular mass (M.M.) of benzene (C6H6) and how many grams are there
in 1 more of benzene?
5) Find the formula weight (F.W.) of NaCl.
6) Determine the number of moles of CO2 (M.M. 44.01 ) in 454 grams.
7) Determine the mass in grams of 3.60 mol of H2SO4 (M.M 98.08).
8) Calculate the grams present in 3.40 x 10-5 moles 0.100 moles of Na2CO3 (F.W.
105.99).
9) Determine the number of molecules of H2O in 3 moles H2O.
10)How many atoms are present in 17.0 mol of water?
Exercises (Applying concepts)
Of course if we used some other mass unit for the mole such as "pound mole", the
"number" would be different than 6.022 x 1023.
11)Given 5 moles of Sulfuric Acid having a formula of H2SO4 answer the
following questions:
a) How many grams of Sulfuric Acid would this be?
b) How many molecules of Sulfuric Acid would that be?
c) How many Hydrogen atoms would be in this amount of Sulfuric Acid?
d) How many Oxygen atoms would there be in this much Sulfuric Acid?
12) Given the chemical reaction: N2 + 3 H2 ---> 2 NH3,
a) Write all more conversion factors in the chemical equation:
b) Determine the moles of H2 that will react with the 4 moles of N2
c) Determine the grams of H2 that will react with the 4 moles of N2