Ch. 8: Chemical Reactions and Chemical Quantities

Dr. Namphol Sinkaset

Chem 200: General Chemistry I

I. Chapter Outline

I. Introduction

II. Chemical Change

III. Writing and Balancing Equations

IV. Reaction Stoichiometry

V. Limiting Reactants and % Yield

VI. Three Types of Reactions

I. Reaction Chemistry

• The stereotypical chemist sits in a lab and runs reactions w/ glassware and chemicals.

• In reaction chemistry, we are concerned with two things.

1) What will happen if we mix this and that?

2) How much reactant do we need or how much product will form?

II. Chemical Change

• Chemical change occurs when atoms rearrange, such that the original compound becomes a different one.

• Contrast with physical change, in which the composition remains the same.

• These are closely related to chemical and physical properties.

II. Chemical/Physical Properties

• A chemical property is only displayed when a substance changes its composition. e.g. flammability – when a substance

exhibits flammability, it becomes one or more different substances

• A physical property is displayed without changing composition. e.g. smell or color

II. Chemical/Physical Change

III. Chemical Reactions

• Chemical changes occur via chemical reactions, of which there are several types.

• We represent chemical reactions with chemical equations.

• Chemical equations must be balanced. Why?

III. Balancing Equations

• Steps for balancing:1) Translate into formulas, if necessary. Reactants

on left, products on right.

2) Balance atoms one at a time. Start w/ most complicated compound or element that’s NOT O or H. Single elements are easy to balance last.

3) Can ONLY change coefficients, not subscripts!! Adjust to smallest whole numbers, if necessary.

4) Check work!

5) Specify states.

III. Balancing Practice

• e.g. Balance the following.a) Solid calcium carbonate reacts with nitric

acid to form carbon dioxide gas, liquid water, and aqueous calcium nitrate.

b) PCl3 + HF PF3 + HCl

c) C4H10(g) + O2(g) CO2(g) + H2O(g)

III. Last Thoughts on Balancing

• Coefficients distribute over all atoms in a formula.

• Never change a formula when trying to balance. e.g. MgO cannot be changed to MgO2 to balance O atoms.

• Never add other reactants/products to try to achieve balance.

• A balanced equation stays balanced when multiplied by a constant.

IV. Calculations w/ Equations

• A balanced equation allows calculations of amounts of reactants or products.

• If you know the # of moles of one substance in a balanced equation, you know the # of moles of any of the other substances.

• Numerical relationships between chemical amounts in a balanced chemical equation are called reaction stoichiometry.

IV. Mole Ratios• Many stoichiometric relationships exist

in any one balanced equation.• e.g. C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l)

1 mole C3H8 reacts w/ 5 moles O2

1 mole C3H8 produces 3 moles CO2

5 moles O2 leads to 4 moles H2O

• These can be converted to mole ratios.

1 mole C3H8

5 moles O2

1 mole C3H8

3 moles CO2

5 moles O2

4 moles H2O

IV. Mass-to-Mass Conversions

• How much CO2 produced from burning gasoline?

• 2 C8H18 + 25 O2 16 CO2 + 18 H2O

IV. Sample Problem

• How many grams of iron form when 135 g of aluminum reacts according to the reaction below?

Fe2O3(s) + 2Al(s) Al2O3(s) + 2Fe(l)

IV. Sample Problem

• How many kg of HNO3 forms, assuming adequate O2 and H2O, if 16 kg of NO2 reacts according to the equation below?

NO2(s) + O2(s) + H2O(l) HNO3(aq)

V. Limiting Reactants

• Consider the following reaction. 2CO(g) + O2(g) 2CO2(g)

• Submicroscopically, 2 molecules CO react with 1 molecule O2 to give 2 molecules CO2.

• How much CO2 forms if we have 2 molecules CO and 10 molecules of O2?

• We call CO the limiting reactant.

V. Sample Problem

• How many grams of Al2S3 can be formed according to the reaction below if 10.0 g Al is reacted with 15.0 g S8?

16Al(s) + 3S8(s) 8Al2S3(s)

V. Sample Problem

• What is the maximum amount of silicon dioxide that can be produced when 168 g of CaSiO3 reacts with 125 g of CO2 with adequate water according to the reaction below?

2CO2(g) + CaSiO3(s) + H2O(l) SiO2(s) + Ca(HCO3)2(aq)

V. Real-life Reactions• In reality, we cannot have complete

conversion to products.• Even if there was complete conversion,

difficult to actually collect all of the product.

V. Reaction Yields

• Stoichiometry gives us theoretical yield.• What is collected in lab is the actual yield.• The efficiency of a reaction is commonly

expressed as percent yield.

%100yield ltheoretica

yield actual yield %

V. Sample Problem

• Elemental iron can be obtained using the reaction below. If 167 g of Fe2O3 reacts with 85.8 g of CO to produce 72.3 g of Fe, what is the percent yield?

Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)

VI. Three Chemical Reactions

• We will now look briefly at 3 examples of chemical reactions: Combustion reactions Alkali metal reactions Halogen reactions

VI. Combustion Reactions

• A combustion reaction involves the reaction of a substance with O2 to form one or more oxygen-containing compounds. One of the products is often H2O.

Combustion reactions also emit heat.

VI. Typical Combustion Reactions

• In the combustion reactions you will commonly see, the reactant will only have C, H, and O. All of the C becomes CO2 and all of the H

becomes H2O.

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

If there are other elements in the reactant, they would form an oxide.

VI. Sample Problem

• Write a balanced reaction for the complete combustion of liquid C2H5SH.

VI. Alkali Metal Reactions

• Alkali metals have ns1 valence e- configs, so they are quite reactive.

• They react with nonmetals. 2M + X2 2MX

• They react with water. 2M(s) + 2H2O(l) 2M+

(aq) + 2OH-(aq) + H2(g)

VI. Na/Cl2 and Alkali Metals/H2O

VI. Halogen Reactions

• Halogens have ns2np5 outer e- configs, so they are quite reactive.

• React w/ metals to form metal halides. 2Fe(s) + 3Cl2(g) 2FeCl3(s)

• React w/ H2 to form hydrogen halides. H2(g) + I2(g) 2HI(g)

• React w/ each other to form interhalogens. Br2(l) + F2(g) 2BrF(g)