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Chemical Equations& Reactions

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Learning Objectives

• Recognize observations indicating a chemical reaction (heat, light, color change, precipitate) has occurred.

• Write and balance chemical equations with symbols, formulas, identities and relative amounts of reactions and products.

• Recognize types of chemical equations (synthesis, decomposition, single and double replacement, etc.).

• Predict chemical reaction outcome using an activity series list.

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A Chemical Equation

• Represents with symbols and formulas, the identities and relative molecular or molar amounts of the reactants and products in a chemical reaction.

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Word Equation / Formula Equation

Methane + oxygen --> carbon dioxide + water

CH4(g) + 02(g) --> CO2(g) + H20(g)

Reactants Products

The above formula equation is not balanced.

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Chemical Reaction Indications

1. Production of energy as heat and/or light.2. Production of a gas.3. Formation of a precipitate – a solid produced

as a result of a chemical reaction in solution. A precipitate separates from the solution.

4. A color change.

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Chemical Equation Requirements

1. The equation must represent known facts.2. The formulas for the reactants and products

must be written correctly.3. The law of conservation of mass must be

satisfied. The number of atoms of each element must be the same on each side of the yield sign. After a formula is written correctly, coefficients can be placed in front of a formula to show conservation of mass.

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Diatomic MoleculesElement Symbol Molecular Formula Physical State at

Room Temp.Hydrogen H H2 Gas

Nitrogen N N2 Gas

Oxygen O O2 Gas

Fluorine F F2 Gas

Chlorine Cl Cl2 Gas

Bromine Br Br2 Liquid

Iodine I I2 Solid

When writing a chemical equation including any of the above elements, they are shown as diatomic molecules as in column 3 above.

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Coefficients

• When placed in front of a correctly written chemical formula, a coefficient multiplies the number of atoms of each element indicated in the formula

• 2O2 means 4 O

• 2H20 means 4 H and 2 O

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Practice

Write word and balanced chemical equations for: solid calcium reacts with solid sulfur to produce solid calcium sulfide. Include symbols for physical states.

Calcium(s) + sulfur(s) --> calcium sulfide(s)Ca(s) + S(s) --> CaS(s)Balanced: 1 Ca on each side 1 S on each side

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Write word and balanced chemical equation for:Hydrogen gas reacts with fluorine gas to produce hydrogen

fluoride gas.Hydrogen gas + fluoride gas --> hydrogen fluoride gasH2(g) + F2(g) --> HF(g)

H2(g) + F2(g) --> 2HF(g)

Solid aluminum metal reacts with aqueous zinc chloride to produce solid zinc metal and aqueous aluminum chloride.

Aluminum + Zinc Chloride --> Zinc + Aluminum ChlorideAl(s) + ZnCl2(aq) --> Zn(s) + AlCl3(aq)

Al(s) + 3 ZnCl2(aq) --> Zn(s) + 2 AlCl3(aq)

2 Al(s) + 3 ZnCl2(aq) -->3 Zn(s) + 2 AlCl3(aq)

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Translate these chemical equations into sentences:CS2(l) + 3O2(g) --> CO2(g) + 2SO2(g)

Liquid carbon disulfide reacts with oxygen gas to produce carbon dioxide gas and sulfur dioxide gas.

NaCl(aq) + AgNO3(aq) --> NaNO3(aq) + AgCl(s)

Aqueous solutions of sodium chloride and silver nitrate react to produce aqueous sodium nitrate and a precipitate of silver chloride.

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Write & Balance:Hydrazine, N2H4, reacts violently with oxygen to

produce gaseous nitrogen and water.N2H4(l) + O2(g) --> N2(g) + H2O(l)

Is it balanced?No. There are 4H on reactant side and 2H on

product side (H: 4/2) and O is 2/1.N2H4(l) + O2(g) --> N2(g) + 2H2O(l)

N: 2/2 H:4/4 O:2/2Now, it is balanced.

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Chemical Equation Indications

1. Coefficients indicate relative amounts of reactants and products (proportions, molecules, moles, grams, ratios).

H2(g) + Cl2(g) --> 2HCl(g)

1 molecule H2 : 1 molecule Cl2 : 2 molecules HCl

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2. Coefficients can be used to determine relative masses of reactants and products

H2(g) + Cl2(g) --> 2HCl(g)

1mol H2 x 2.02g H2 = 2.02 g H2

mol 1 mol Cl2 x 70.90 g Cl2 = 70.90 g Cl2

mol2 mol HCl x 36.46 g HCl = 72.92 g HCl mol

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• 3. The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reactions.

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Balancing Chemical Equations

1. Write equation.2. Write chemical formulae for each compound.3. Balance according to the Law of Conservation

of Mass by adjusting coefficients.4. Count atoms to be sure the equation is

balanced.

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Practice - Write word, formula, and balanced chemical equations for this reaction.

1. Magnesium and hydrochloric acid react to produce magnesium chloride and hydrogen.

Word Equation: Magnesium + hydrochloric acid --> magnesium chloride + hydrogen

Formula Equation: Mg(s) + HCl(aq) --> MgCl2(s) + H2(g)

Adjust coeffs: Mg(s) + 2HCl(aq) --> MgCl2(s) + H2(g)

Count atoms: Mg: 1/1 H:2/2 Cl:2/2

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Solid sodium combines with chlorine gas to produce solid sodium chloride.

Sodium(s) + chlorine(g) --> sodium chlorideNa(s) + Cl2(g) --> NaCl(s)

Balance: Na(s) + Cl2(g) --> 2NaCl(s)

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

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Types of Chemical Reactions1. Synthesis A + X --> AX2. Decomposition AX --> A + X3. Single-displacement A + BX --> AX + B4. Double-displacement AX + BY --> AY + BX5. Combustion – a substance combines with

oxygen releasing energy as light and heat.

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Activity Series – elements organized according to how they react

An element can replace any element placed below it BUT It cannot replace any element above it.

Zn can replace Cu but Au cannot replace Mg

Most active metals:Li react w/cold Co Do not react w/H20.

Rb H20 & acids Ni React w/acids, repla-

K replacing H2. Sn cing H2. React w/O2

Ba React w/O2 Pb forming oxides.

Sr forming oxides H2 React w/O2, forming

Ca Sb oxidesNa BiMg react w/steam CuAl (not cold H20) Hg

Mn and acids, repla-Ag Fairly unreactive,Zn cing H2. React Pt forming oxides only

Cr with O2 forming Au indirectly.

Fe oxidesCd

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Nonmetal activity series:

Most activeFClBrI

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Sample Problems – activity seriesZn(s) + H2O(l) --->

No reaction, water must be 100oC (steam) at least.Sn(s) + O2(g) -->

Yes, any metal more active than Ag will react w/O2 to form an oxide. (Sn is above Ag)

Cd(s) + Pb(NO3)2(aq) -->

Yes, Cd is above Pb. Products: Cd(NO3)2 + Pb

Cu(s) + HCl(aq) -->No, Cu is below H

50oC

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1. Synthesis Reaction

A + X --> AXSamples:Fe(s) + S(s) --> FeS(s)

2Mg(s) + O2(g) --> 2MgO(s)

H20 + SO3 --> H2SO4

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Synthesis with Oxides (see handout for more information!!)

CaO(s) + H2O(l) --> Ca(OH)2(s)

Pollution: SO2(g) + H2O(l) --> H2SO3(aq)

2H2SO3(aq) + H20(l) --> 2H2SO4(aq)

Oxides:CaO(s) +SO2(g) --> CaSO3(s)

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2. Decomposition Reaction

AX --> A + X

H20(l) --------> 2H2(g) + O2(g) (electrolysis)

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

electricity

D

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Decomposition of Metal OxideCaCO3 --->CaO + CO2

Decomp of Metal HydroxideCa(OH)2 ---> CaO + H2O

Decomp of Metal Chlorate2KClO3 ----> 2KCl + 3O2

Decomp of AcidsH2CO3 --> CO2 + H2O (occurs at room temp)

H2SO4 ---> SO3 + H2O

D

D

D

D

MnO2

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3. Single Displacement Reaction

A + BX --> AX + BOr Y + BX --> BY + X1. Fe + CuSO4 --> FeSO4 + Cu

2. Cu + 2AgN03 --> Cu(NO3)2 + 2Ag

3. CI2 + 2KI --> 2KCl + I2

How is 3. different from 1. or 2.?In 1.& 2. metals are being displaced.In 3. a halogen is being displaced.

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Hydrogen displaced by a metal:Mg + 2HCl --> H2 + MgCl2

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Double-Displacement Reaction

AX + BY --> AY + BXA,X,B, and Y in reactants are ions.AY and BX are ionic or molecular compounds.1. Formation of a Precipitate2KI(aq) + Pb(NO3)2(aq) --> PbI2(s) + 2KNO3(aq)

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Formation of a GasFeS(s) + 2HCl(aq) –> FeCl2(aq) + H2S(g)

Formation of WaterHCl(aq) + NaOH(aq) --> NaCl(aq) + H20(l)

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Combustion Reactions

2H2(g) + O2(g) --> 2H2O(g)

C3H8(g) + 5O2(g) --> 3CO2(g) + 4H20(g)

Other products are heat and light.

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Predicting Activity (use the activity series handout)

Zn(s) + H20(l) --> ?

No, not hot enough. Steam needed (100oC)Cr(s) + H2O(l) --> ?

Yes, Cr is above H on the chart.Pt(s) + O2(g) --> ?

No.

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Cd(s) + 2HBr(aq) -->Yes, Cd is above H Mg(s) + steam -->Yes

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Activity Series

Activity of metals: Activity of halogen nonmetals

Li <--Most active metal F2 <-- Most active nonmetal

Rb react w/cold Cl2

K H20 & acids Br2

Ba replacing H2. I2

Sr React w/O2

Ca forming oxides NaMg react w/steamAl (not cold H20)

Mn and acids, repla-Zn cing H2. React

Cr with O2 forming

Fe oxidesCdCo Do not react w/H20.

Ni React w/acids, repla-Sn cing H2. React w/O2

Pb forming oxides.

H2

SbBi React w/O2, forming

Cu oxidesHg

Ag Fairly unreactive,Pt forming oxides onlyAu indirectly.

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Solubility Chart