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Chemistry 108 lecture notes Chapter 6: Reactions

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Chapter 6 Lecture Notes: Reactions

Chapter 6 Educational Goals 1. Define the term "chemical reaction". 2. Given the reactants and products in a chemical reaction, the student will be

able to write and balance chemical equations. 2. Identify oxidation, reduction, combustion, hydrogenation reactions. 3. Identify hydrolysis, hydration, and dehydration reactions of organic compounds. 4. Use stoichiometric calculations to determine the theoretical yield, and percent yield of a reaction. 5. Describe the difference in energy changes (∆G) for spontaneous and nonspontaneous reactions, and list the factors that affect the rate of a chemical reaction.

Reactions involve changes in matter resulting in new______________________. In chemical reactions, the covalent and ionic bonds that hold elements and compounds together are ________________ and/ or ____________________ bonds are formed. Evidence of Chemical Reactions 1) The __________________ changes 2) A _________________ is formed. 3) A __________________ is formed. 4) ________________ is given off or the appearance of_____________________.


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5) ___________________is emitted. 6) A new __________________ is emitted. 7) The _______________________ changes. 8) A permanent, new __________________ is formed. Reactions and Energy Changes Energy can be ________________ in a chemical reaction.

Energy can be_______________ in a chemical reaction. Chemical Equations Chemical changes are represented using_______________________.

(g) = __________________ (l) = __________________ (s) = __________________ (aq) = _________________ (dissolve in water)


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Things to remember when writing equations: Burning in air means reacting with ______________. Identify the ____________________of each chemical

• Metals are solids, except for Hg which is liquid Identify elements that come as _________________________ molecules

-The following elements come as diatomic elements: _______ ________ ________ ________ ________ ________ ________ Balancing Chemical Equations In a balanced chemical equation, the same _______________ of each atom appears on each side. Matter is neither created or destroyed in a chemical reaction

Method for Balancing

1) Make a table and count all atoms on each side of the equation

• If H2 or O2 is present, list them last. • a polyatomic ion may be counted as one “element” if it does not change in the

reaction.

2) Balance an element in the table by adding coefficients to the equation (start with the first element on the list)

3) Recount each atom and Repeat step 2 for all atoms until balanced. Example: ________H2(g) + ________O2(g) → _________ H2O(g)


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Example: ______N2 + ______O2 → _______N2O

Reactants Atom Products ← N →

← O →

Example:

______N2 + ______O2 → _______NO Reactants Atom Products

← N →

← O → Example:

_____Mg(s) + ______O2(g) → _______MgO(s) Reactants Atom Products

← Mg →

← O → Example:

• a polyatomic ion may be counted as one “element” if it does not change in the reaction

_____Al + ____FeSO4 → _____Al2(SO4)3 + ______Fe

← (SO4) → ← Al → ← Fe →


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Balance the following reaction for the combustion of propane: _C3H8(g) + ___O2(g) → ___CO2(g) + __H2O(g)

Avoiding Common Errors When Balancing Chemical Equations


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Stoichiometry (Pronounced: STOY-KEE-AHM-EH-TREE) Stoichiometry deals with calculations about the _________________ of reactants and products involved in a chemical reaction.

Stoichiometry allows a chemist or scientist to know how much of an element or reactant to use and how much product is expected to come out of the reaction.

A __________________ ____________________allows us to predict which reactant (if any) will run out first and how much product can be expected to form. Before we do calculations with chemicals, let’s do something we are all familiar with: Food!!! Suppose you want to make as many cheese sandwiches as possible for lunch. If you have 20 slices of bread, how many slices of cheese do you need?

2 bread slices + 1 cheese slice → 1 sandwich

Let’s do the same calculation for a chemical reaction now! Carbon monoxide reacts with oxygen to produce carbon dioxide:

2CO + O2 → 2CO2 How many CO2 molecules are produced from 2 CO molecules? ________ How many O2 molecules are needed to produce 2 CO2 molecules? ________ How many CO molecules are needed to react with 1 O2 molecule?________ How many moles of CO2 are produced from 2 moles of CO? __________ How many moles of O2 are needed to produce 2 moles of CO2? __________ How many moles of CO are needed to react with 1 mole of O2? __________

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Beginning with 6.0 moles of CO, how many moles of CO2 will be made if all of the CO is used up?

2CO + O2 → 2CO2

How many moles of O2 will be needed to react with 6.0 moles of CO?

How many moles of CO are needed to produce 7.50 moles of CO2?

Assume that you have an unlimited supply of O2.

• Since we can not measure the number of moles on a balance in the lab, we are usually given the number of grams of a reactant or product and need to determine the number of grams of other reactants or products in a reaction.

These calculations only work with__________________, not with mass (grams)! We will see why that is using our cheese sandwich example. If you have 20 grams of bread, how many grams of cheese do you need?....need more information!!!! Since the coefficients in an equation give us the ratio of __________________________, we must convert grams to moles in our calculations.

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The re-breather units used by some firefighters convert exhaled carbon dioxide (CO2) into oxygen gas (O2). How many grams of oxygen (O2) can be produced from 1.40 grams of potassium superoxide (KO2)?

4KO2 (s) + 2CO2(g) → 2K2CO3(s) + 3O2(g) How many grams of oxygen (O2) can be produced from 1.40 grams of potassium superoxide (KO2)?

How many grams of (CO2) will react with 1.40 grams of potassium superoxide (KO2)? 4KO2 (s) + 2CO2(g) → 2K2CO3(s) + 3O2(g)

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How many grams of (KO2) are needed to produce 5.70 g of O2

4KO2 (s) + 2CO2(g) → 2K2CO3(s) + 3O2(g)

You try one: How many grams of silver will completely react with 10.0 g of sulfur?

2 Ag(s) + S(s) → Ag2S(s)

In this course, we will always be given the amount of one reactant and we will assume an excess supply of all other reactants. If you are curious as to how to do stoichiometric calculations with a limited amount of all reactants, read the section in the text about limiting reagents. (email me for worksheets)

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Percent Yield • The amount of product obtained from a reaction is called the actual yield.

• To indicate how well the actual yield agrees with the theoretical (calculated) yield,

chemists report the______________ _________________:

x 100% Example: A drug company runs a large scale reaction to prepare the pain reliever acetaminophen. The theoretical yield of the reaction is 42 kg of acetaminophen. If 33 kg of acetaminophen are obtained, what is the percent yield of the reaction? General Types of Reactions Read the textbook (Chapter 6, Section 6, General Types of Reactions) and take notes on the 4 types of reactions: 1. __________________________________________ 2. _________________________________________ 3. _________________________________________ 4. _________________________________________


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Chemical Reactions

• In this chapter we will look at 5 classes of chemical reactions:

1) Oxidation-Reduction • Combustion • Standard Oxidation-Reduction

2) Hydrogenation 3) Hydrolysis 4) Hydration 5)Dehydration

• You will need to be able to identify the type of reaction and predict the product(s) Combustion Reactions

• Combustion reactions may occur when organic molecules react with oxygen gas (O2).

• This is also called burning!!! In order to burn something you need the 3 things: 1) A Fuel (hydrocarbon) 2) Oxygen to burn it with 3) Something to ignite the reaction (spark)

Example: Combustion of a hydrocarbon:

CxHy + O2(g) à CO2(g) + H2O(g)

• Products in combustion are ______________ _________________and________.

(although incomplete burning does cause some by-products like carbon monoxide)

• Combustion is a type of _____________________________________reactions.


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Oxidation and Reduction Oxidation-Reduction Reactions All reactions that involve a transfer of one or more ______________________are called oxidation-reduction reactions.

• Also called redox reactions. We say that the substance that loses electrons in the reaction is ___________________ and the substance that gains electrons in the reaction is_______________________.

• Use OIL RIG to remember Example: Example: Sodium Reacting with Chlorine Gas (Cl2)

2Na(s) + Cl2 (g) → 2NaCl(s)

• In this process, neutral sodium atoms (charge = 0) lose an electron (are oxidized) to form Na+ and each of the neutral chlorine atoms in Cl2 gain an electron (are reduced) to form Cl-.

• Since Cl2 removes electrons from Na, Cl2 is called the oxidizing agent. • Because Na gives electrons to Cl2, Na is the reducing agent.

Oxidation and Reduction of Organic Molecules Method for determining oxidation or reduction in organic compounds: An atom in a molecule is oxidized if it:

• gains oxygen (is attached to more oxygen atoms in the product than in the reactant) OR • loses hydrogen (is attached to fewer hydrogen atoms in the product than in the reactant)

An atom in a molecule is reduced if it: • loses oxygen (is attached to fewer oxygen atoms) OR • gains hydrogen (is attached to more hydrogen atoms)

Example: Combustion of methane: CH4 + 2O2 à CO2 + 2H2O Is carbon oxidized or is it reduced during this reaction?

• When CH4 is converted to CO2, carbon is ______________– it loses hydrogen and gains oxygen.

What is reduced? To where did the electrons that carbon lost go?

• O2 is ____________________– each oxygen in O2“lost” an oxygen.


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How is carbon oxidized when this happens?

How is oxygen reduced when this happens?

Reactions of Organic Molecules 1) Hydrogenation: Reduction of Alkenes

• Alkenes and other unsaturated hydrocarbons undergo a reduction reaction called catalytic hydrogenation, in which hydrogen gas (H2) in the presence of platinum (Pt), catalyst, acts as a reducing agent.

• During hydrogenation, the carbon atoms in an alkene are ________________ because they gain hydrogen atoms.


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You try it: Draw and name the product formed when the alkenes react with H2 and Pt. 1-butene cis-2-butene trans-2-pentene 2) Reactions Involving Water Water is a reactant or product in a number of reactions important to organic and biochemistry. In this section we will take a look at three of them –

a) Hydrolysis

b) Hydration

c) Dehydration


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2a) Hydrolysis of Esters

• In a hydrolysis reaction, water (hydro) is used to ______________ (lyse) a molecule. • Esters undergo hydrolysis – when treated with water in the presence of hydroxide ion

(OH-) they split to form a carboxylate ion and an alcohol.

Carboxylic acid vs. Carboxylate ion


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Hydrolysis Example: Hydrolysis is one of the factors that determine the length of time that some drugs remain active.

– The local anesthetics Procaine (also known as Novacain) is a short acting, and remains effective for little more than an hour. A particular enzyme present in the blood serum deactivates this anesthetic by catalyzing hydrolysis of the ester groups.

Predict the products formed by the hydrolysis of the ester shown below:

2b) Hydration of an Alkene In a hydration reaction, _______________________is added to a double bond.

Predict the products formed by the hydration of this alkene:

OH-

OH-


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2c) Dehydration of an alcohol Dehydration is the reverse of hydration.

• Water is ____________________from an alcohol to form a double bond (alkene).

Dehydration Example:

OH-

O


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Reactions Involving Water: Summary • Esters can be hydrolyzed. • Alkenes can be hydrated. • Alcohols can be dehydrated.

Practice Problems: Draw the product of each reaction.


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Free Energy and Reaction Rate Free Energy

• For the purposes of this course, you can think of free energy as the total energy: o Total energy = kinetic energy + potential energy

• If the free energy of the products (Gprod) is less than the free energy of the reactants (Greact), then a chemical reaction will occur.

• We live in a universe where matter tends to be at the lowest possible energy (or free energy).

Consider the combustion of propane:

CH3CH2CH3(g) + 5O2(g) → 3CO2(g) + 4H2O(g)

• ∆G has a value of -531 kcal/mol

• Because ∆G is ________________________, the reaction is spontaneous (happens). A spontaneous reaction has a negative ∆G and a nonspontaneous reaction has a positive ∆G. Reaction Energy Diagrams


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Reaction Rates: How Fast Does a Reaction Occur The rate of a reaction depends on the ________________________ energy:

The rate of a reaction depends on: _________________________________ .


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Reaction Rates: Catalysts A catalyst increases the rate of a reaction by _____________________ the ________________ ________________________.

Catalysis Example: Lactase (β-galactosidase)


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chapter 6 lecture notes: reactions - saddleback · pdf filechapter 6 lecture notes: reactions...

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