Chemical Reactions Occur in Predictable Ways

Chapter 6

What You Will Learn

▪ Classifying reactions as one of six different types

1. Synthesis (Combination) Reactions2. Decomposition Reactions3. Single Replacement Reactions4. Double Replacement Reactions5. Neutralization (Acid-Base) Reactions6. Combustion Reactions

What You Will Learn

▪ To predict the IDENTITY OF PRODUCTS of a chemical reaction

▪ To identify factors that affect the RATE of a chemical reaction

▪ To Define the RATE of a chemical reaction▪ To explain role of CATALYSTS in a chemical reaction

Chapter 6 Assignment

▪ Assignment to be handed in as 1 package on test day:– Workbook complete– Check your understanding Page 271 *odds– Check your understanding Page 281 *odds– Chapter Review – Checking Concepts 1-4

Chapter 6 Project

▪ Making graphic organizers for 2 main concepts▪ Page 282 – Prepare your own summary section *choose only 2▪ Page 583 – Graphic organizers *must have 2 different types – Venn diagram, Concept map, Spider Map, Flowchart, Cycle Chart etc…

*choose 2▪ Choose 2 of the following, 1. Six types of Chemical reactions2. Classifying and predicting Products of Reactions Based on the

Reactants Only3. Examples of Reactions Occurring at Different Rates4. Four Factors Affecting the Rates of Reactions

6.1 Types of Chemical Reactions

All known chemical reactions require energy to break the

chemical bonds in the reactants.

Energy is often released when new bonds form in the products.

DID YOU KNOW??…

LAB 6-1A Comparing Chemical Reactions

Purpose• Read the Lab• What is the purpose?#4 Observation Table• This is where you put your

observations!What did you find out?• #1-3 Full sentence

responses• Name & Date = Hand it in

▪ Lose marks for safety, clean up, focus, professionalism, and procedural issues.

▪ Full marks for being engaged, safe, respectful, and a best effort attempt.

▪ Due at end of class.

Steps to Writing Reactions

• Some steps for doing reactions1. Identify the type of reaction2. Predict the product(s) using the type of

reaction as a model3. Balance it

Don’t forget about the diatomic elements! (BrINClHOF) For example, Oxygen is O2 as an element.

1. Synthesis reactions

• Synthesis reactions occur when two substances (generally elements) combine and form a compound. (Sometimes these are called combination or addition reactions.)

reactant + reactant 1 product• Basically: A + B AB

• Example: 2H2 + O2 2H2O • Example: C + O2 CO2

Synthesis Reactions

2K(g) + Cl2 (g) 2KCl (s)

Potassium + Chlorine ?

Practice• Predict the products. Write and balance the

following synthesis reaction equations.• Sodium metal reacts with chlorine gas Na(s) + Cl2(g) • Solid Magnesium reacts with fluorine gas Mg(s) + F2(g) • Aluminum metal reacts with fluorine gas Al(s) + F2(g)

2. Decomposition Reactions

• Decomposition reactions occur when a compound breaks up into the elements or into a few simpler compounds

• 1 Reactant Product + Product • In general: AB A + B• Example: 2 H2O 2H2 + O2

• Example: 2 HgO 2Hg + O2

Decomposition Reactions

2HgO(s) 2Hg(l) + O2(g)

What is in this example?

Decomposition Exceptions FYI• Carbonates and chlorates are special case

decomposition reactions that do not go to the elements.

• Carbonates (CO32-) decompose to carbon dioxide and a

metal oxide• Example: CaCO3 CO2 + CaO

• Chlorates (ClO3-) decompose to oxygen gas and a metal

chloride• Example: 2 Al(ClO3)3 2 AlCl3 + 9 O2

Practice

• Predict the products. Then, write and balance the following decomposition reaction equations:

• Solid Lead (IV) oxide decomposes PbO2(s)

• Aluminum nitride decomposes AlN(s)

Practice

Identify the type of reaction for each of the following synthesis or decomposition reactions, and write the balanced equation:

N2(g) + O2(g) BaCO3(s) Co(s)+ S(s) NH3(g) + H2CO3(aq) NI3(s)

(make Co be +3)

Nitrogen monoxide

3. Single Replacement Reactions

• Single Replacement Reactions occur when one element replaces another in a compound.

• A metal can replace a metal (+) OR a nonmetal can replace a nonmetal (-).

• element + compound element + compound A + BC AC + B (if A is a metal) ORA + BC BA + C (if A is a nonmetal) (remember the cation always goes first!)

When H2O splits into ions, it splits intoH+ and OH- (not H+ and O-2 !!)

Single Replacement Reactions

2Li(s) + 2H2O(l) 2LiOH(aq) + H2 (g)

Lithium is a metal.

Single Replacement Reactions

• Write and balance the following single replacement reaction equation:

• Zinc metal reacts with aqueous hydrochloric acid Zn(s) + HCl(aq) ZnCl2 + H2(g)

Note: Zinc replaces the hydrogen ion in the reaction2

Single Replacement Reactions

• Sodium chloride solid reacts with fluorine gas NaCl(s) + F2(g) NaF(s) + Cl2(g)

Note that fluorine replaces chlorine in the compoundIs the equation balanced?

• Aluminum metal reacts with aqueous copper (II) nitrate Al(s)+ Cu(NO3)2(aq)

4. Double Replacement Reactions• Double Replacement Reactions occur when a metal

replaces a metal in a compound and a nonmetal replaces a nonmetal in a compound

• Compound + compound compound+ compound• AB + CD AD + CB

Double Replacement Reactions• Think about it like “foil”ing in algebra, first and last ions

go together + inside ions go together• Example: AgNO3(aq) + NaCl(s) AgCl(s) + NaNO3(aq)

• Another example:K2SO4(aq) + Ba(NO3)2(aq) KNO3(aq) + BaSO4(s) 2

Practice• Predict the products. Balance the equation1. HCl(aq) + AgNO3(aq) 2. CaCl2(aq) + Na3PO4(aq) 3. Pb(NO3)2(aq) + BaCl2(aq) 4. FeCl3(aq) + NaOH(aq) 5. H2SO4(aq) + NaOH(aq) 6. KOH(aq) + CuSO4(aq)

Neutralization (Acid-Base) Reactions

▪ When an acid and base are mixed they will neutralize each other.

▪ An acid and base react to form a salt and water.▪ Acid + Base Salt + Water▪ HX + MOH MX + H2O▪ X represents a negative ion - M represents a

positive ion

▪ Practice problems on page 263

5. Combustion Reactions• Combustion

reactions occur when a hydrocarbon reacts with oxygen gas.

• This is also called burning!!! In order to burn something you need the 3 things in the “fire triangle”:1) A Fuel (hydrocarbon)2) Oxygen to burn it with3) Something to ignite the reaction (spark)

Combustion Reactions• In general: CxHy + O2 CO2 + H2O

• Products in combustion are ALWAYS carbon dioxide and water. (although incomplete burning does cause some by-products like carbon monoxide)

• Combustion is used to heat homes and run automobiles (octane, as in gasoline, is C8H18)

Combustion Reactions

Edgar Allen Poe’s drooping eyes and mouth are potential signs of CO poisoning.

Combustion

• Example• C5H12 + O2 CO2 + H2O

• Write the products and balance the following combustion reaction:• C10H22 + O2

5 68

Mixed Practice

• State the type, predict the products, and balance the following reactions:

1. BaCl2 + H2SO4 2. C6H12 + O2 3. Zn + CuSO4 4. Cs + Br2 5. FeCO3

6.2 Factors Affecting the Rate of Chemical Reactions

1. Glow Sticks and Temperature2. HCl and Concentration3. Flour and Surface Area4. Potassium Iodide and Catalysts5. Diamonds and Pressure

Glow Sticks

A temperature demonstration

Single replacement reaction between Mg and HCl

A concentration demonstration

Flour Special Effects

A surface area demonstration

Elephant Toothpaste!A catalyst demonstration – adapted using yeast catalyst

How does the elephant toothpaste work?

▪ Foam is awesome! Each tiny foam bubble is filled with oxygen.▪ The yeast acted as a catalyst to remove the oxygen from the

hydrogen peroxide. Since it did this very fast, it created lots and lots of bubbles.

▪ The bottle got warm. The experiment created a reaction called an Exothermic Reaction - that means it not only created foam, it created energy in the form of heat!

▪ The foam produced is just water, soap, and oxygen so you can clean it up with a sponge and pour any extra liquid left in the bottle down the drain.

▪ This experiment is sometimes called "Elephant's Toothpaste" because it looks like toothpaste coming out of a tube, but don't get the foam in your mouth!

COLLISION MODEL

States that the rate of reaction is affected by the number of collisions of reactant molecules.

Molecules in a gas or a liquid are constantly colliding with each other.

ENDOTHERMIC:Lots of energy to get it startedEnergy absorbed overall

EXOTHERMIC:Needs a little energy to get startedEnergy released overall

5 FACTORS

There are four factors that affect the rate of reaction that we will discuss.

▪ TEMPERATURE▪ CONCENTRATION▪ SURFACE AREA▪ CATALYSTS▪ The 5th factor we will not discuss in this course.

PRESSURE

TEMPERATURE

Higher temperatures increase the rate of a chemical reaction because particles move faster at high temperatures and this creates more violent collisions more often.

Reaction Rate: Effect of TemperatureActivation Energy• Arrhenius: molecules must posses a minimum

amount of energy to react. • In order to form products, bonds must be broken

in the reactants.– Bond breakage requires energy.

• Activation energy, Ea , is the minimum energy required to initiate a chemical reaction.

Temperature and RateActivation Energy

Effect of temperature on rate

Increasing temperature increases the rate

Effect of temperature on rate

RATE

TEMPERATURE

Conclusion ?

Rate increases with increasing temperature

1 Time

CONCENTRATION

Higher concentration increase the rate of a chemical reaction because there are more particles to collide with.

Effect of concentration on rate

RATE

CONCENTRATION

Conclusion ?

Rate is directly proportional to concentration

1 Time

SURFACE AREA

A larger surface area increases the rate of a chemical reaction because there are more exposed particles in the compound.

CATALYSTS

A substance that increases a chemical reaction but does not get consumed by the reaction.

Therefore it is not considered a reactant or product.

Works by reducing the energy needed to break bonds, provides easier paths for reactions. *lowers the Ae!

QUESTION

Explain how you would use your knowledge of factors that affect the rate of reaction to cook a steak as quickly as possible.

▪ Temperature – increase to high▪ Surface Area – Cut steak into pieces (increase area)▪ Concentration – none▪ Cataylsts - none

QUESTION

What effect on the rate of reaction of a metal with 20 ml of dilute acid at room temperature would you expect if:A) the acid was cooled to 100C?B) 2 ml of concentrated acid was added?C) the metal was ground into powder before addition to the acid?

ANSWER

A) The reaction rate would decrease (slow down)B) the reaction rate would increase ( speed up)C) the reaction rate would increase ( speed up )

MENTOS GEYSER

Let’s go outside!

Iodine Clock

Probably the best rate of reaction demonstration ever!

Iodine clock explained

▪ It is called a clock reaction because you can change the amount if time it takes for the liquids to turn blue.

▪ The chemistry of the demonstration gets a bit complicated, but basically it is a battle of chemistry between the starch which is trying to turn the iodine blue, and the Vitamin C which is keeping it from turning blue. Eventually the Vitamin C loses and, bam! - you get instant blueness.

Iodine Clock *different reactants but essentially the same– Thanks Wikipedia!

▪ This reaction starts from a solution of hydrogen peroxide with sulfuric acid. To this is added a solution containing potassium iodide, sodium thiosulfate, and starch. There are two reactions occurring in the solution.

▪ In the first, slow reaction, iodine is produced:▪ H2O2 + 2I- + 2H+ → I2 + 2H2O▪ In the second, fast reaction, iodine is reconverted to 2 iodide ions by the thiosulfate:▪ 2S2O3

2- + I2 → S4O62- + 2I-

▪ After some time the solution always changes colour to a very dark blue, almost black.▪ When the solutions are mixed, the second reaction causes the triiodide ion to be consumed much

faster than it is generated, and only a small amount of triiodide is present in the dynamic equilibrium. Once the thiosulfate ion has been exhausted, this reaction stops and the blue colour caused by the triiodide – starch complex appears.

▪ Anything that accelerates the first reaction will shorten the time until the solution changes color. Decreasing the pH (increasing H+ concentration), or increasing the concentration of iodide or hydrogen peroxide will shorten the time. Adding more thiosulfate will have the opposite effect; it will take longer for the blue colour to appear.