Unit A - ChemistryChapter A3 – Chemical Reactions

A3.1 - Important Examples of Chemical Change

Review: Chemical reactions Recall, in Chapter A1 we learned that all chemical reactions share the same two features:

1) EXOthermic reactions

chemical reactions that release energy, usually in the form of:

one common type of exothermic reactions is combustion =

ENDOthermic reactions chemical reactions that absorb energy the reaction will not but rather,

requires electricity or some kind of “activation” energy usually feel cold to the touch, like

Showing an energy change in a chemical reaction Sometimes, to illustrate whether a chemical reaction is ENDO or EXOthermic, the word

“energy” is included in the equation Endothermic reactions: energy is shown

e.g. photosynthesis - a plant’s cells are able to capture the sun’s light energy, and use it to convert into carbon dioxide + water + energy glucose + oxygen

Exothermic reactions: energy is shown e.g. in cellular respiration, a plant or animal’s cells burn the sugar to

glucose + oxygen carbon dioxide + water + energy notice that the two reactions are the reverse of one another

2) These new substances ( ) have different properties that the chemicals

that went into the reaction ( ) The reaction could involve:

the formation of a gas, as evidenced by the production of the formation of a solid, as evidenced by the solution becoming

, and forming Precipitates

Precipitates sometimes form when two aqueous solutions are mixed While both reactants were , one of the

products formed e.g. NaCl(aq) + AgNO3(aq) NaNO3(aq) + AgCl(s)

Review of states ELEMENTS:

their state is shown on the periodic table using a shading system:

COMPOUNDS: ionic compounds are , unless dissolved in water and highly

soluble, in which case, acids are molecular compounds can be , and are usually given to you

Conservation of Mass in 1789, French chemist Antoine Lavoisier made an important observation about chemical reactions

because his theory could be tested and no exception has been found to contradict it, his idea is accepted as a The Law of Conservation of Mass says that:

in a chemical reaction, matter cannot be

this means that,

Antoine Lavoisier knew very little about the structure of elements and compounds (recall, he was working PRIOR to Dalton, Thomson, Rutherford or Bohr’s model of the atom), he simply made a conclusion based on )

today, we are able to explain Lavoisier’s findings theoretically the total number of atoms of each element in a chemical reaction is equal before and after the

reaction a chemical reaction is not a destruction or production of atoms, but simply a

SOLID LIQUID GAS

A3.2 - Writing chemical equations

Chemical equations A symbolic representation of the process of chemical change Chemical equations can be written in word form or using chemical symbols, but all use the

following format: + +

It does not matter or what order the products are listed as long as the reactants are on the left and

Example: the combustion of methane The main gas in natural gas is methane, CH4(g) When the methane in your furnace burns in the presence of oxygen, it produces carbon dioxide

and water vapour The reaction can be depicted as follows:

Word equations word equations are the simplest form of writing a reaction equation rather than use formulas, word equations use the names of the compounds and elements

e.g. methane + oxygen carbon dioxide + water

Practice problems Read the following descriptions of reactions and create a word equation.

a) Solid magnesium metal reacts with hydrochloric acid to produce aqueous magnesium chloride and hydrogen gas.

b) An iron nail is placed in a solution of copper (II) chloride. As a result, small amounts of copper metal are produced in a solution of iron (II) chloride.

c) In a hydrogen fuel cell, hydrogen gas and oxygen gas react to produce liquid water.

Skeleton equations A skeleton equation is a chemical reaction equation written using chemical formulas It is not considered to be a finished equation because

It includes the both

e.g. CH4(g) + O2(g) CO2(g) + H2O(g)

Writing skeleton equations requires you to be able to write up to four correct chemical formulas Remember:

ionic compounds must acid formulas are balanced you must include states – look to the equation description to guide you

Practice problems: Write skeleton equations for the three reactions described above.

a)

b)

c)

Balancing equations Recall, the Law of Conservation of Mass says that the number of atoms of each element

This means, you may need more than one “batch” of a reactant to make all the products, or you

may make more than one batch of a product with the ingredients you have in the reactants Consider the reaction of the combustion of methane again

In order to use all the atoms in the methane, we need

This way, the total number of carbon atoms, oxygen atoms and hydrogen atoms stays consistent throughout

To illustrate this in a chemical equation, we use coefficients – large numbers written before the chemical CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

All chemical equations are balanced in this way – by adding coefficients in front of the chemical formula

The coefficient applies to the entire formula e.g. 2H2O means two water molecules,

To balance an equation:

write the skeleton equation,

once you have the skeleton equation written,

count the number of atoms of each element

start adding coefficients, re-tallying as you go until

Example: CH4(g) + O2(g) CO2(g) + H2O(g)

Elements Number of atoms in reactants

Number of atoms in products

C

H

O

Example: Aqueous iron (II) nitrate reacts with aqueous sodium phosphate. The products are aqueous

sodium nitrate and solid iron (II) phosphate.

Write the balanced chemical equation for this reaction. Step #1 – Write the skeleton equation, including states

Step #2 – Tally the number of atoms of each element *note – polyatomic ions can be kept as one unit, as long as they appear on both sides of the

equations

Element / ion # in reactants # in products

Fe

NO3

Na

PO4

Step #3 – Start adding coefficients

Practice problems: Balance the three reactions you wrote skeleton equations for:

Mg(s) + HCl(aq) MgCl2(aq) + H2(g)

Fe(s) + CuCl2(s) Cu(s) + FeCl2(s)

H2(g) + O2(g) H2O(l)

A3.3 - Five types of Chemical Reactions Chemical reactions

despite there being millions of different possible chemical reactions,

most reactions can be grouped into one of five categories: (also called )

Reaction #1: Formation (Synthesis) Two elements combine to form a compound

+ A + B AB

These reactions are exothermic - E.g. 2K(s) + Cl2(g) 2KCl(s)

How you recognize this reaction: It’s the only reaction where

What’s the challenge?

watch for (e.g. O2, H2. etc.) if the compound produced is an ionic compound,

if the compound includes a multivalent metal, unless otherwise specified, assume

(the one listed first on the periodic table)

Practice problems:

Magnesium oxide is a compound found in many cosmetics and lotions. It is produced by reacting magnesium metal with oxygen gas. a) Write the word equation for this reaction. b) Write the skeleton equation for this reaction. c) Balance the equation.

Write the skeleton equation and balanced equation for the formation of lithium oxide from its elements.

Write the skeleton equation and balanced equation for the formation of lead (IV) bromide from its elements.

Name the products in each of the following reactionsa) potassium + iodine b) magnesium + phosphorus c) cesium + chlorine

Complete each equation below (the products are ionic compounds)a) Na(s) + Br2(l) b) Mg(s) + F2(g) c) Al(s) + Cl2(g)

Reaction #2: Decomposition A compound breaks down into its composite elements

+ AB A+ B

These reactions are endothermic because the compound form is more stable than the elements,

e.g. electrolysis of water

H2O(l) O2(g) + H2(g)

How you recognize this reaction: It’s the only reaction where

What’s the challenge? watch for (e.g. O2, H2. etc.)

Practice problems: Pure aluminium is not typically found in nature, but can be produced by the decomposition of

aluminium chloride. Write the word equation for this reaction. Write the skeleton equation for this reaction. Balance the equation.

Write the balanced equations for these reactions:a) magnesium sulfide magnesium + sulfurb) potassium iodide potassium + iodine

Complete the following decomposition reactions and balance the equation:a) Al2O3(s) b) NiCl2(s) c) Na3P(s)

Reaction #3: Hydrocarbon combustion Hydrocarbons are molecular compounds containing hydrogen and carbon

e.g. octane C8H18(l), glucose C6H12O6(s), methane CH4(g)

“Combustion” means Like all fuels, hydrocarbons Because they produce , combustion reactions are always

Regardless of what hydrocarbon is burning, the products are carbon dioxide and water vapour hydrocarbon fuel + oxygen carbon dioxide + water CxHy + O2(g) CO2(g) + H2O(g)

How you recognize this reaction: The reactants are , or in the description of the reaction, it says

What’s the challenge? while these reactions always produce the same two products, they are

start by balancing the carbons, then the hydrogens, and leave the O2 coefficient to last

Practice problems: Complete and balance each equation:

a) CH4(g) + O2(g) b) C2H6(g) + O2(g) c) C3H8(g) + O2(g) d) C6H6(l) + O2(g)

Reaction #4: Single replacement An element reacts with a compound, and produces a new element and a new compound

+ + A + BC AC + B

If the reacting element is a metal, it will replace e.g. Na(s) + KCl(s) NaCl(s) + K(s)

If the reacting element is a non-metal, it will replace e.g. Br2(l) + MgCl2(s) MgBr2(s) + Br2(l)

How you recognize this reaction: The reactants are

What’s the challenge?

make sure you balance the charges a) if the new element is a non-metal, watch for b) if the reaction occurs in solution, you will need to check the solubility table for

Example: Copper is added to a solution of silver nitrate as shown on left: Cu(s) + AgNO3(aq) →

After reaction, two important observations are made: The solution has turned blue indicating the solid copper has dissolved

forming copper (II) ions which are blue. A crystal precipitate has formed on the outside of the copper wire, and is

silver in colour.

Practice problems: Write the products of this single replacement reaction, and balance the reaction.

While copper compounds found in drinking water are poisonous, pure copper is not. To remove copper compounds such as copper (II) chloride from water, aluminium reacts with the copper (II) chloride solution.

write the word equation for this reaction. write the skeleton equation. balance the equation.

Reaction #5: Double replacement Two compounds exchange ions to form two new compounds

compound+ compound compound + compound AB + CD AD + CB

“A” and “C” are both cations (+ ions) so they will always

“B” and “D” are both anions (- ions) so they will always

How you recognize this reaction: It’s the only reaction where

What’s the challenge? Because you will be dealing with two new ionic compounds, you will have to

You will also have to check

Example: When aqueous lead (II) nitrate and aqueous sodium iodide are mixed, a

bright yellow precipitate of solid lead (II) iodide forms, along with a second aqueous solution. a) Write the word equation.b) Write the skeleton equation, including states.c) Balance the equation.

Practice problems: Write the word, skeleton and balanced equations for the following reactions.

a) When aqueous copper (I) nitrate and aqueous potassium bromide are mixed, a precipitate of solid copper (J) bromide forms, along with a second product.

b) When aqueous aluminium chloride and aqueous sodium hydroxide are mixed, a precipitate and another aqueous solution form.

Summary of the five reactions:

Reaction type Format How to recognize:

Formation

Decomposition

Hydrocarbon combustion

Single replacement

Double replacement

Practice problems: Classify the reactions below as F, D, HC, SR or DR

a) C6H12(l) + 9O2(g) 6CO2(g) + 6H2O(g)b) CaCl2(aq) + Na2SO4(aq) CaSO4(s) + 2NaCl(aq)

c) 2Fe(s) + O2(g) 2FeO(s)d) Be(s) + 2LiBr(aq) BeBr2(aq) + 2Li(s)e) MnI4(aq) Mn(s) + 2I2(s)

Predicting the products of chemical reactions Once you become familiar with the five reaction types, you will be expected to classify reactions

without the products, AND predict the products of the reaction.a) step #1 – b) step #2 – use the reaction type to c) step #3 – write the formula for the product(s)d) step #4 – include the statese) step #5 –

Example: Classify each of the following reactions, and complete the reaction.

a) PbBr4(aq)

b) Ni(NO3)3(aq) + Na2SO3(aq)

c) C8H18(l) + O2(g)

d) Cu(s) + Au(ClO3)3(aq)

e) Zn(s) + S8(s)

A3.4 - The Mole

“Mass” as a method of counting Counting is useful when the number of things to be counted is relatively small. In some cases it

isn't. Substituting a mass amount for a counted amount is a common practice in chemistry because of

the impracticalities of

The mole In the case of atoms, the solution is not as simple because counting out enough atoms to make up a

kilogram is totally impractical, if not impossible. 

Atoms are so small that it takes an incredibly huge number of them to make up a visible amount. An iron spike for example might contain more than 6.02 x 1023 atoms of iron. Dealing with such large numbers of atoms on a consistent basis can get to be tiresome after a

while. That's where the mole comes in. A mole is a “group” of atoms We use groupings of objects all the time to make counting large amounts easier, for example:

“a pair of shoes” means “a dozen eggs” means “a flat of pop” means

“A mole” is similar – but it means a much larger number – 602 000 000 000 000 000 000 000 atoms

Where do we get this number? Avogadro was the chemist that first suggested this amount – we now refer to 6.02 x 1023 as

A mole is

This number was determined by dividing the mass of a single carbon-12 atom into a 12 g mass of carbon-12.

Since the mass of a single carbon-12 atom is tiny (1.99 x 10-23 g), the value of a mole comes out to be a huge number: 6.02 x 1023

How do we measure moles? Since there’s no measuring device to measure moles, we use the mass of a mole of atoms

Since each element has different sized atoms, the mass of one mole of carbon is different than one mole of sulfur, etc.

Atomic molar mass The mass of one mole of an element is its

it is measured in The atomic molar mass of each element is listed on the periodic table

carbon: sulfur:

Practice problem: What is the mass of the following elements?

How do we know the molar mass of compounds? The periodic table only lists the molar mass of elements, so how can you find the molar mass of

compounds? To find the molar mass of a compound, you have to:

write list the number of multiply this number by the

Example: Find the molar mass of sodium chloride Formula: NaCl 1 atom of sodium and 1 of chlorine MNaCl = (1 x MNa) + (1 x MCl)

= (1 x 22.99 g/mol) + (1 x 35.45 g/mol)= 58.44 g/mol

Example: Find the molar mass of water Formula: H2O 2 atoms of hydrogen and 1 of oxygen MH2O = (2 x MH) + (1 x MO)

= (2 x 1.01 g/mol) + (1 x 16.00 g/mol)= 18.02 g/mol

Practice problems: What is the molar mass of the following compounds?

methane (CH4)

calcium chloride

sucrose (C12H22O11)

Moles, mass and molar mass The relationship between these three quantities can be summed up in a formula:

n = m where:M

n = , measured in m = , measured in M = , measured in

Using the formula You can use the formula n = m/M to find the number of moles of a sample if you know:

use its chemical formula to calculate

Example: How many moles are in 100 g of carbon dioxide?

m = 100g MCO2 = (1 x 12.01) + (2 x 16.00) = 44.01 g/mol n = m = 100g = 2.27 mol

M 44.01g/mol

Practice problems: How many moles of silicon are in a 56.18-g sample?

How many moles of potassium fluoride are in a 25.0-g sample?

You can also use the formula to find the mass of a sample, if you know: allows you to find

Using the formula to find mass requires you to

m = nM

Example: What is the mass of 10.0 mol of water? n = 10.0 mol MH2O = 18.02 g/mol (calculated in a previous question) m = nM = (10.0 mol)(18.02 g/mol) = 180.2 g

Practice problems: What is the mass of 5.0 mol of NaOH?

What is the mass of 4.3 mol of ammonia, NH3?

Showing your work Calculation questions are usually worth several marks, because in science, you are required to

show the process you went through to solve the question. Typically, a mole question will be worth 3 marks:

1 mark for 1 mark for 1 mark for

The mole concept and the Law of Conservation of Mass Recall, the Law of Conservation of Mass says that the total mass of the reactants is equal to

In balancing equations, you applied this Law

the coefficient you add in front of a compound in a balanced equation is

e.g. In the reaction 4 Na(s) + O2(g) 2Na2O(s)

reacted with to produce

Practice problem: 4 Na(s) + O2(g) 2Na2O(s) Using the coefficients as the number of moles, calculate the mass of the reactants and the mass of

the products.