Chemical ReactionsChemical Reactions

What is a chemical reaction? A chemical equation uses symbols to

represent what happens at the atomic level.

What happens when you boil water? What happens during the electrolysis of water?

Chemical equations follow the Law of Conservation of Mass◦ Conservation of atoms, not conservation of

molecules (molecular structures can change)

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ReactionEquation

Adjust coefficients, not subscripts, to balance an equation

Coefficients are whole-number integers in the lowest ratio

Reactants Products

H2 + O2 H2O

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Coefficients vs. Subscripts

Draw structures for the following substances:

H2, Br2, HBr, CO, CO2, Cl2, AlCl3, N2, H2, NH3

Use your drawings to balance the equations below. Add more substances as needed by adding coefficients.

H2 (g) + Br2 (l) HBr (g) CO (g) + O2 (g) CO2 (g) Al (s) + Cl2 (g) AlCl3 (g) N2 (g) + H2 (g) NH3 (g)

Write correct formulas for reactants and products.

Begin balancing with the most complex formula.

Balance polyatomic ions as a single unit. Check each reactant and product to verify

the coefficients. Mg(NO3)2 + Cr2(SO4)3 MgSO4 + Cr(NO3)3

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Balance the following equations:◦ Mg + O2 MgO ◦ H2O2 H2O + O2

◦ NaN3 Na + N2 ◦ CaCO3 CaO + CO2

◦ Mg3(PO4)2 + NaOH Mg(OH)2 + Na3PO4

◦ KClO3 KCl + O2

◦ Na + H2O NaOH + H2

◦ AgNO3 + MgCl2 AgCl + Mg(NO3)2

◦ CH3OH + O2 CO2 + H2O

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Na + H2O

Gummy Bear

Burn methanol

Alkali metals in H2O

Balanced equations:◦ 2 Mg + O2 2 MgO

◦ 2 H2O2 2 H2O + O2

◦ 2 NaN3 2 Na + 3 N2

◦ CaCO3 CaO + CO2

◦ Mg3(PO4)2 + 6 NaOH 3 Mg(OH)2 + 2 Na3PO4

◦ 2 KClO3 2 KCl + 3 O2

◦ 2 Na + 2 H2O 2 NaOH + H2

◦ 2 AgNO3 + MgCl2 2 AgCl + Mg(NO3)2

◦ 2 CH3OH + 3 O2 2 CO2 + 4 H2O88

Balance the following equations: __Fe2(SO4)3 + __K3PO4 __K2SO4 + __FePO4

__Hg(NO3)2 + __ KI __HgI2 + __ KNO3

__Li2O(s) + __ H2O __LiOH

__HBr + __Ba(OH)2 __BaBr2 + __H2O

__K2PtCl4 + __NH3 __Pt(NH3)2Cl2 + __KCl

Section 2.6

In lab, we want to predict amount of product we can expect from a reaction

Have to use stoichiometry: quantitative study of reactants and products in a reaction (mole to mole ratio)

Use the mole to relate one substance in a reaction to another (equation coefficients, see Table 3.1) ◦ 2H2 (g) + O2 (g) 2H2O (g)

We use coefficients in a balanced equation to determine the mole ratio in which substances need to react.

MgCl2(aq) +2AgNO3(aq) 2AgCl(s) + Mg(NO3)2(aq)

◦ 1 molecule of MgCl2 reacts with 2 molecules of AgNO3 to produce 2 molecules of AgCl and 1 molecule of Mg(NO3)2

◦ Molecules are not practical! Scale up to moles!

Use molar mass:◦ Mass ÷ molar mass = moles

Concentration: amount of solute present in a given amount of solution

Molarity: moles of solute in 1 L of solution◦ moles solute / liters solution; M = mol / L

If I have 1.50 mL of a 0.100 M solution of NaCl, how many moles are in the sample?◦ M * V = moles

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Mole

Mass

Particles

Use coefficients

from equatio

n

1 mole = 6.022 x

1023 particles

(atoms or m

olecules

)

molar mass

Courtesy: www.unit5.org/christjs/Stoichiometry/Mole%20Island%20Diagram.ppt

Molarity

volume

Mole

Mole to mole conversions MgCl2(aq) +2AgNO3(aq) 2AgCl(s) + Mg(NO3)2(aq)

1.00 mol xs ?

How many moles of solid product do you predict to form?

Cannot compare grams of one to grams of another!!

1515

Predict products (with phases) and balance each equation.

Calculate the moles of precipitate (ppt) that can be made in the following reactions.

NaCl (aq) + Hg(NO3)2 (aq) ◦ 0.500 mol NaCl

FeCl3 (aq) + NaOH (aq) ◦ 2.35 mol NaOH

Li2SO4 (aq) + AgCH3COO (aq) ◦ 0.1098 mol AgCH3COO

We need to be able to determine amounts of chemicals to use so they react in the exact ratio needed.

For example, we need 2 molecules of H2 and 1 molecule of O2 to make 2 molecules of H2O.

Do you want to count individual atoms and molecules?

Size of atoms

Who wants to count:

Peppers?

Grains of rice?

WaterMolecules?

We can measure the mass of a pure sample but how do we know how many atoms or molecules are in that sample?

Mole: amount of a substance that contains a specific number of particles.

A mole (mol) is a quantity that tells us two things: ◦ the number of particles in a substance ◦ the mass of that substance (containing a

specific number of particles)

1 12C atom weighs 12 amu, 1 mole of 12C atoms weighs 12 grams.

1 dozen = 12 objects; 1 mole = 6.022 x 1023 objects. This is Avogadro’s number.

The moral: 1 mole of carbon-12 weighs 12 grams and contains 6.022 x 1023 atoms.

How big IS Avogadro’s number?◦An Avogadro's number of standard soft

drink cans would cover the surface of the earth to a depth of over 200 miles.

◦If you had Avogadro's number of unpopped popcorn kernels, and spread them across the US, the country would be covered in popcorn to a depth of over 9 miles.

◦If we were able to count atoms at the rate of 10 million per second, it would take about 2 billion years to count the atoms in one mole.

What do these substances have in common?

Atoms measured in amu, moles measured in grams

amu and g are the same value, different units◦ 1 atom 12C = 12 amu; 1 mol 12C atoms = 12 g◦ 1 atom 4He = 4 amu; 1 mol 4He atoms = 4 g

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1 mole contains 6.0221415 x 1023 particles ◦ Particles can be atoms, ions, molecules, or

formula units Determine what type of particle

Avogadro’s number will describe for the following:◦ Kr◦ C6H12O6

◦ MgCO3

◦ SF6

◦ CaBr2

◦ P

Atomic, molecular, and formula masses are not practical in lab. We can scale up to calculate the mass of a mole of substances.

Molar mass is the same value as atomic mass, just with different units. Molar mass (g/mol: (mass per 1 mole of substance)

What is molar mass of C6H6? What is molar mass of Mg(OH)2? Molar masses have 6.022 x 1023 particles

in the substance.

Al CO2

Cl2 SO2 CaCl2 H2SO4

AlCl3

Mg3(PO4)2

Na2CO3

Conversions:◦ If we know the mass of a substance, how can we

calculate number of moles? mass moles (use molar mass)

◦ If we know how many moles of a substance, how can we calculate number of particles? moles number of particles (use Avogadro’s

number)

particles 10 x 6.02

mol 123g)(in massmolar

mol 1

Mole

Mass

Particles

1 mole = 6.022 x

1023 particles

(atoms or m

olecules

)

molar mass

Courtesy: www.unit5.org/christjs/Stoichiometry/Mole%20Island%20Diagram.ppt

1. How many moles are in a 2.67 g sample of Ag?2. How many grams are in a 0.45 mol sample of

Mg3(PO4)2?3. How many atoms are in a 16.3 g sample of S?4. How many formula units are in 0.81 g of MgCl2?5. How many ions are in 0.81 g of MgCl2? (Hint:

How many ions are in 1 formula unit of MgCl2?)6. How many moles of ions are in 0.81 g of MgCl2?7. How many C atoms are in 3.64 g of C6H12O6?8. How many O atoms are in 0.0765 g of CO2

9. Answers: 0.0248 mol Ag, 1.2 x 102 g Mg3(PO4)2, 3.06 x 1023 S atoms, 5.12 x 1021 f. un., 1.54 x 1022 ions, 0.026 mol ions, 7.30 x 1022 C atoms, 2.09 x 1021 O atoms

Calculate the number of atoms that are in a 6.00 g sample of carbon.

Calculate the number of moles in 12.04 x 1023 atoms of uranium.

Calculate the mass of 12.04 x 1023 atoms of uranium.

If you have 1 g samples of NaCl, CO2, and H2O, which sample will contain the fewest particles?

Answers: 4.34 x 1025 C atoms, 2 mole Uranium, 476.1 g Uranium, NaCl (fewest moles because largest MM)

How many atoms are in 12.987 g of iron?

How many H atoms are in a 3.4215 gram sample of H2O?

Atom/molecule/mole ratios in C6H12O6