Writing Simple Chemical Equations

Balancing and Classification of Reactions

Chemical Reactions• Balanced chemical equations represent

chemical reactions• A substance which undergoes a chemical

change has taken part in a chemical reaction• Remember the differences between chemical

and physical change?Chemical Change Physical Change

Burning Melting

Rusting Freezing

Signs of a Chemical ReactionIndicators of a probable

chemical reaction– These may be present in some

physical changes too

1. Color Changes2. Precipitation of a solid from a

solution3. Energy changes, heat or light

absorbed or released4. Odor Changes; like baked bread5. Gas release- sometimes occurs

in physical change too

Mini-Lab 6.1 Energy ChangeQuestion: How can we observe energy changes?Background Information: • All chemical reactions involve an energy

change. • Sometimes the change is so small that special

detection instruments are needed to observe it.

• Sometimes they can be observed easily. • Light, heat, sound, kinetic and potential are all

forms of energy.

Mini-Lab 6.1 Energy ChangeProcedure: 1. Place 25g of iron powder and 1g of NaCl

in a reseal able bag2. Add 30g of vermiculite to bag, seal bag

and shake3. Open bag and add 5ml of water, reseal

and shake gently4. Hold bag in your hands, note any

changes you observe

Mini-Lab 6.1 Energy Change

Analysis: (answer the following in your notebook1. Did you observe energy changes? If so,

describe them2. Did a chemical reaction take place? What

kind of reaction?3. Can you think of anytime in the real world,

where this reaction might be useful? Explain

Exothermic Reaction

The Reaction has as one of the products, energy, in the form of heat!

Endothermic ReactionsThese reactions use energy from the surroundings, in the form of heat, as one of the reactants; creating a cold product!

Chemical Equations1. Describe all changes

that take place2. Identify the

substances that react

a. Called Reactants3. Identify the

substances formeda. Called Products

• Reactants: Iron and Oxygen

• Product: Iron(III) Oxide Rust

The Description of Changes• More than what can be observed• Ex. “bubbles are formed”, does not tell what gas is

formed • A Chemical Equation is how we represent the

changes taking place in a reaction• Word equations are the simplest way to

describe reactants and products• An arrow, between reactants and products,

represents change• + signs are used to separate reactants and to

separate products

Word Equations• Example:

The combining of Vinegar (acetic acid) and Baking soda results in a vigorous reaction, forming a bubbly product

The word equation: Vinegar + Baking Soda Sodium acetate + water +

Carbon Dioxide

Chemical Equations• Shorter than word equations • more exact and specific• Replace the names of the substances with their

chemical formulasExample: (the previous reaction) HC2H3O2 + NaHCO3 NaC2H3O2 + H2O + CO2(Vinegar) Sodium Sodium Acetate water Carbon dioxideAcetic acid Hydrogen

Carbonate

• By examining the equation you can determine EXACTLY what elements are involved and how many atoms of each

Writing Basic Chemical Equations

1. Replace the names of the substances with their chemical formulas – (remember; use oxidation numbers to form stable octets

and write compound formulas)

2. Write the chemical equation with all reactants on one side, separated by a (+) sign

3. Draw an arrow pointing toward the products , also separated by a (+) sign. (recall diatomic elements)

4. Add symbols next to each compound, indicating physical state of matter (solid-s, liquid-l, gas-g)

Write Chemical Equations; Practice

1. Sodium metal + chlorine gas sodium chloride crystals

Answer: Na (s) + Cl2 (g) NaCl(s)

Reactants Product

Write Chemical Equations; Practice

2. Propane gas + oxygen carbon dioxide + water + energy

Answer: C3H8(g) + O2(g) CO2 +H2O(g) + energy

Write Chemical Equations; Practice

3. Zinc metal + hydrochloric acid zinc chloride solution + hydrogen

Answer: Zn(s) + HCl(aq) ZnCl2(aq) +H2(g)

Additional Practice

• Hand out #’s 26-30 - with partner (10 min)• Post answers!• Handout #’s 31-35 – with partner (8 min)• Post answers!

• Exit Ticket on your own!• Complete #36-50 for homework!

Balancing Chemical EquationsWhat is Balancing Equations?• Ensuring every atom in the reactants is present in the products in the same number and type!Why?

Law of Conservation of Matter: matter is neither created or destroyed, just transformed from one form to another

No creation, no destruction, just rearrangement!

Same Number and Type of Atoms

Balancing Chemical EquationsSteps of process:1. Write the chemical equation with all reactants on

one side, separated by a (+) sign 2. Draw an arrow pointing toward the products , also

separated by a (+) sign. (recall diatomic elements)3. Add symbols next to each compound, indicating

physical state of matter (solid-s, liquid-l, gas-g)4. Count the numbers of each type of element from

each side of the equation. 5. List the symbols and numbers of each.6. Change the coefficients as needed to ensure

conservation of matter. (numbers and types equal)

Example of Process• Chemical equation for aqueous magnesium chloride

plus silver nitrate solution making aqueous magnesium nitrate and solid silver chloride:

• Add symbols for physical state

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

• Count number and type of elements on each side

Matter is not conserved! Numbers are different!

Atoms in Reactants Atoms in Products

Mg- 1 Mg- 1Cl- 2 Cl- 1Ag- 1 Ag- 1N-1 N-2O- 3 O- 6

Practice: Count the number of Atoms

• Complete HO 1-10 (pairs) (20 min)

Last StepChange the coefficients as needed to ensure

conservation of matter. MgCl2(aq) + AgNO3(aq) Mg(NO3)2 (aq) + AgCl(s)Atoms in reactants Atoms in products Mg- 1 Mg- 1 Cl- 2 Cl- 1 -2 Ag- 1 -2 Ag- 1 2 N-1 - 2 N-2 O- 3 -6 O- 6 How can I change the coefficients to make then the same?1. A 2 added before the AgNO3(aq) in the reactants, will balance both N and

O2. Another 2 added before AgCl(s) in the products will balance both the Ag

and Cl

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

Predicting Oxidation Numbers• The total charge on the ion is known as the Oxidation Number of

the atomSome metals have the same oxidation number in all compounds• Group 1 elements , oxidation number = 1+• Group 2 elements , oxidation number = 2+• Aluminum, oxidation number = 3+

– Groups 3-12 Many have more than one oxidation # depending on the reaction

• Group 13 elements have 3 valence electrons, oxidation # =3+• Group 14 may have 2+ or 4+ oxidation number• Groups 15, 16 and 17 tend to gain electrons since they are

already ½ full – Their oxidation numbers are 3-, 2-, and 1- respectively

Write a Balanced Equation for Each of These Reactions

1. Sodium metal + chlorine gas sodium chloride crystals2. Propane gas + oxygen carbon dioxide + water + energy3. Zinc metal + hydrochloric acid zinc chloride solution +

hydrogenRemember: • use the oxidation numbers to construct octets and

determine formula compounds• Use your reference sheets for names of ionic substances

and covalent molecules. (check for diatomic molecules)

Answers

1. balance equations for ppt #1.docx2. balance equations for ppt #2.docx3. balance equations for ppt #3.docx

Answers

1Na (s) + Cl2 (g) NaCl(s)

Atoms reactants Atoms products

• Na – 1 Na – 1

• Cl – 2 Cl - 1• Add coefficient of 2 to NaCl to balance Cl• Add coefficient of 2 to Na to balance Na

2 Na (s) + Cl2 (g) 2 NaCl(s)

Answers

Diatomic molecule• 2. C3H8(g) + O2(g) CO2 +H2O(g) + energy• Propane from reference sheet• Atoms reactants Atoms products• C – 3 C – 1 3• H – 8 H – 2 8• O – 2 10 O – 3 6 10• Add coefficient of 4 to H2O to balance H

• Add coefficient of 3 to CO2 in product to balance C

• Add coefficient of 5 to O2 in reactants to balance O

• C3H8(g) +5 O2(g) 3 CO2 + 4 H2O(g) + energy• Now it is balanced! Matter was conserved!

Answers

• 3. Zn(s) + HCl(aq) ZnCl2(aq) +H2(g) • Diatomic molecule• Atoms reactants Atoms

products• Zn– 1 Zn– 1• H – 1 2 H – 2• Cl – 1 2 Cl - 2• Add coefficient of 2 to HCl to balance H and Cl• Zn(s) + 2 HCl(aq) ZnCl2(aq) +H2(g)• Now it is balanced! Matter was conserved!

Let’s Use the Internet to Help us Practice!

• Using a computer, Your Brain and your Newly Acquired Skills, practice balancing – Complete at least 10 equations from the beginner

level, then 10 at the intermediate level.– Then try 2 or three advanced ones!

• Track your progress and record the number of trials it took for each equation in your notebook!

• Complete an exit ticket, indicating how many tries it took for each equation

• Leave your exit ticket on the laptop cart when you have put your computer away properly!

More Balancing!• Access the Following Site:• http://education.jlab.org/elementbalancing/index.html

1. At least 10 Beginner equations and 10 Intermediate!Beginner

levelNumber of

trialsIntermediate

levelNumber of

Trials1 1

2 2

3 3

4 4

5 5

6 6

7 7

8 8

9 9

10 10

Advanced Level

Number of Trials

123

Exit Ticket1. Complete an exit ticket, indicating how many

tries it took for each equation.

2. Leave your exit ticket in the box AFTER you have put your computer away properly!

Name___________________ EXIT TICKET Total number of beginner equations you tried _____Total number of trials on beginner equations ______Total number of intermediate equations you tried ______Total number of trials on intermediate equations ______How many advanced equations did you try?How well do you understand this process? Not at all, A little, mostly, I got it!

Other Sites You Can Use To Practice

• http://chemistry.csudh.edu/lechelpcs/rxnbalancingcsn7.html (pretty good, hard but shows solutions after 3 tries)

• http://funbasedlearning.com/chemistry/chemBalancer

• http://www.sciencegeek.net/Chemistry/taters/EquationBalancing.htm (All at Once)

Types Of Reactions• There are 5 major types of chemical

reactions1. Synthesis2. Decomposition3. Single displacement4. Double displacement5. Combustion

• Reactions can be categorized by recognizing patterns which occur.

Synthesis• Pattern to recognize synthesis• Whenever two or more substances

combine to form a single product, the reaction is synthesis.

Decomposition• Pattern to recognize synthesis• Whenever a compound is broken down into

two or more simpler substances it is a decomposition reaction.

Single-Displacement Reaction• Pattern to recognize single displacement• Whenever one element takes the place of

another in a compound, it is a single displacement reaction.

Let’s Classify these reactions!

1. 2 Na + Cl2 2NaCl

2. PCl5 PCl3 + Cl2

3. 2Al + Fe2O3 2Fe + Al2O3

4. 2Ag2O 4Ag + O2

5. Cl2 + 2KBr 2KCl +Br2

6. CaO + SiO2 CaSiO3

Answers1. 2 Na + Cl2 2NaCl

2 substances combine so it is synthesis!2. PCl5 PCl3 + Cl2

A single compound is broken down into two simpler substances- it is decomposition!

3. 2Al + Fe2O3 2Fe + Al2O3

One element takes the place of another, it is single displacement!

Answers Continued

4. 2Ag2O 4Ag + O2

Decomposition5. Cl2 + 2KBr 2KCl +Br2

Single-displacement6. CaO + SiO2 CaSiO3

Synthesis

Double-Displacement Reactions• Pattern to recognize a double-displacement• Whenever the positive ions of two ionic

compounds are interchanged, it is a double displacement reaction.• At least one product must be a precipitate of

water!

Combustion Reaction• Pattern to recognize combustion• Whenever a substance combines rapidly

with OXYGEN to form one or more oxides, is a combustion reaction.

Let’s Apply what we have learned!

• Classify these reactions:1. PbCl2 + Li2SO4 PbSO4 + 2LiCl

2. CH4 + 202 CO2 + 2H2O

3. C6H1206 + 602 6CO2 + 6H2O

4. BaCl2 + H2SO4 2HCl +BaSO4

Answers1. PbCl2 + Li2SO4 PbSO4 + 2LiCl Interchanged

cations, so Double-displacement2. CH4 + 202 CO2 + 2H2O substance combines with

Oxygen and forms an oxide, so Combustion3. C6H1206 + 602 6CO2 + 6H2O Combustion

4. BaCl2 + H2SO4 2HCl +BaSO4 Double-displacement

Compare and Contrast Types Of ReactionsReaction Type General Equation

Synthesis Element/compound + Element/compound compoundExamples: 2 Na + Cl2 2NaCl CaO + SiO2 CaSiO3

Decomposition Compound two or more elements/compoundsExamples: PCl5 PCl3 + Cl2

2Ag2O 4Ag + O2

SingleDisplacement

Element a + compound bc element b + compound acExample: 2Al + Fe2O3 2Fe + Al2O3

Cl2 + 2KBr 2KCl +Br2

DoubleDisplacement

Compound ac + compound bd compound ad + compound bcExample: PbCl2 + Li2SO4 PbSO4 + 2LiCl BaCl2 + H2SO4 2HCl +BaSO4

Combustion Element/Compound + Oxygen OxideExample: CH4 + 202 CO2 + 2H2O C6H1206 + 602 6CO2 + 6H2O

Identify these Reactions as one of the five types studied

1. A + BX AX +B2. AB A+B3. AXZ AX + Z4. AB + CD AC + BD5. BC + D BD + C6. BD + oxygen B oxide + D + water7. AD + XY AY + DX8. + +

Answers1. Single displacement (AKA single replacement)2. Synthesis3. Decomposition4. Double Displacement (AKA Double replacement)5. Single Displacement (AKA Single replacement)6. Combustion7. Double Displacement (AKA Double replacement)8. Decomposition

Factors that Affect Direction Of Reactions• External factors modify the direction of

reactions• Many can change direction: Called Reversible– Like charging and draining a battery

• Adding or removing energy as heat can affect direction– Endothermic reactions; added heat pushes reaction

to the right– Exothermic reactions; added heat pushes the

reaction to the left

Reversibility of Reactions• Not all reactions are reversible– Like fuel burns, food is digested and paint hardens– New products are formed and at least one reactant is

used up• Some reactions automatically reverse to establish

equilibrium– No net (overall) change– Reactants and products change place, forming at

about the same rate– Reactants are never used up because they are always

being used then reformed from products– Ex. CaCo3 CaO + CO2

Initiating a Reaction• For a reaction to occur, particles must collide

with sufficient force to cause electrons to rearrange.

• The amount of energy needed to cause a reaction is called Activation Energy– Slow reactions have high activation energy

• To determine reaction rate, measure how quickly one reactant disappears or one product appears

Reaction Potential Energy DiagramShows the potential energy changes that occur as reactants become products. It has five distinct regions:

1. the potential energy of the reactants

2. the potential energy gain that must take place in order for old bonds to be stretched to the breaking point

3. the potential energy of the transition state

4. the potential energy released as new bonds form during a chemical change

5. the potential energy of the products.

Details

1. The flat region labeled "Reactants" shows the potential energy of the reacting particles relative to the products.• The actual potential energy of the

reactants is an unknown

2. Moving particles possess kinetic energy. When they collide, their kinetic energy is converted to potential energy.

3. The rising part of the graph represents the increase in potential energy that occurs when reactants collide.

4. The minimum gain in potential energy that results in the stretching of reactant bonds to the breaking point is called the activation energy (Ea). It can be determined by experiment.

Details

1. The top of the curve represents the point at which the bonds of the colliding particles are stretched to the breaking point.

2. The unstable group of atoms formed at this point are neither reactants nor products but something in between - a transitional structure called the activated complex. (#3)1. The potential energy of this structure is

very high because the bonds are stretched as far as possible.

2. This structure exists for the shortest amount of time imaginable.

3. In an instant, the particles either form new bonds to give new products or reform old bonds to give the original reactants.

Details 1. The falling part of the curve represents

the energy released when new bonds form between particles to make one or more products.

2. The potential energy difference between the reactants and the products is called the heat of reaction (ΔH).1. It represents the net energy change of

the reaction.3. If the potential energy of the products is

greater than that of the reactants, then the reaction is classified as endothermic.

4. If the potential energy of the products is less than that of the reactants, then the reaction is classified as exothermic.

Details

1. The second flat region represents the potential energy of the products. The actual potential energy of the products is also an unknown.

2. The difference in energy between reactant and products determines endothermic vs exothermic reactions1. absorbs energy vs releases energy

Important notes!

• Reactions with low activation energy are fast, while those with high activation energy are slow.

• The higher the activation energy, the slower the rate of the reaction.

• If the potential energy of the reactants is greater than that of the products, the reaction is exothermic - it results in the net release of potential energy as heat.

• If the potential energy of the reactants is less than that of the products, the reaction is endothermic - it results in the net gain of energy from some external source (e.g. the sun) which is then stored in the products.

Factors that Affect Rate Of Reactions1. Temperature; most reactions speed up

with higher temp2. Concentration; raising the concentration

of a reactant can speed up a reaction3. Catalysts; speeds up rate w/out being

changed by the reaction (enzymes in cells)

4. Inhibitor; slows down a reaction (preservatives)

Review Problems

• What are the correct coefficients of these reactions?1. KNO3 + H2CO3 K2CO3 + HNO3

2. SnO2 + H2 Sn + H2O

3. SeCl6 + O2 Se O2 + Cl2

4. N2 + H2 NH3

5. P4 + O2 P2O5

• Answer the following;1. What is activation energy?2. What happens if there is not enough activation energy?3. Why do you need a match to start a fire?4. What are the indicators that a reaction has taken place?5. What does each of the following mean, when in a chemical equation?

a) (g)b) (s)c) (l)d) (aq)

Answers to Review Problems1. 2, 1, 1, 22. 1, 2, 1, 23. 1, 1, 1, 34. 1, 3, 25. 1, 5, 2Answer the following;6. The amount of energy needed to start a reaction7. The reaction will not start8. It provides activation energy9. Color change, precipitation, energy change, odor or gas production10. What does each of the following mean

a) (g) gasb) (s) solidc) (l) liquidd) (aq) dissolved in water

Test Tomorrow!

• Study your notes and review Handout!• Practice answering questions, and writing and

balancing equations• Prepare by looking through your notes before

class!• Visit the wiki for additional help!• Organize your notebook, check your checklist!