Understanding Chemical Reactions

Chapter 6

What is a chemical reaction?

A chemical reaction is a chemical change which forms new substances.

Examples:- The chemical reaction H2(g) + ½ O2(g) → H2O(l) describes the formation of water from its elements.- Butane burns in air to produce carbon dioxide and water: 2C4H10(g) + 13O2 (g) → 2CO2 (g) + 10H2O (g)

Chemical Reactions

Reactant: Starting substance in a reaction.Written on the left before the arrow.

Product: End substance in a reaction.Written on the right after the arrow.

Example: Al + Br2 → AlBr3

(arrow reads as “yields” or “produces”)

Key Terms

A word equation represents a chemical reaction.

All the reactants are written on the left of the yield

arrow, separated by a plus sign (if needed).

All the products are written on the right of the yield

arrow, separated by a plus sign (if needed).

Word Equations

Reactant 1 + Reactant 2 → Product 1 + Product 2

Example: Rust

iron + oxygen → iron (III) oxide

What are the reactants?

What are the products?

Try this…

propane + oxygen → carbon dioxide + water

a) List all the reactants in the reaction.

b) List all the products in the reaction.

c) What is the purpose of the arrow?

Example # 2

#3 Write word equations for the following reactions:

a) CaCl2 and Na2SO4 react to form CaSO4 and NaCl

b) BaCO3 reacts when heated to produce BaO and CO2

c) AgNO3 reacts with KCl to produce AgCl and KNO3

4. Write word equations to represent thefollowing chemical reactions:

a) Carbon dioxide and water are produced in a human cell during respiration. The reactants are sugar and oxygen.

b) B) Stalactites form in caves when calcium bicarbonate reacts to form calcium carbonate, water and carbon dioxide gas.

Can you figure out what is missing in the following chemical reactions?1. Aluminum resists corrosion (rust) because it reacts with a gas in the air to form a protective coating of aluminum oxide.Aluminum + ____________ → aluminum oxide

2. Zinc Metal, used as a coating in galvanized iron, also reacts with air to form a coating that resists further corrosion.zinc+ oxygen → ___________________

3. When aluminum foil is placed in a solution of copper (II) chloride, copper metal and another solution are formed:aluminum + copper (II) chloride → copper + ___________

Count the number of atoms of each type in the reactants and the products. In this case we can leave NO3

- as one group because it is a polyatomic ion and stays together.

Mg + 2HNO3 → H2 + Mg(NO3)2

Reactants Products

Mg: ______ Mg: _____H: _______ H: ______NO3

- : _______ NO3- : _______

Atom counting…

H2(g) + ½ O2(g) → H2O(l)Reactants Products

H: _______ H: _______

O : _______ O : _______

You try…2C4H10(g) + 13O2 (g) → 2CO2 (g) + 10H2O (g)Reactants ProductsC: ________ C: _______H: _______ H: _______O : _______ O : _______

A skeleton equation is a representation of a chemical reaction in which the formulas of the reactants and products are used instead of the names of the compounds.

Reactants are still separated from each other by a “+” and reactants and products are separated by an arrow

Example: CH4 +O2 → CO2 +H2O(Instead of : butane + oxygen → carbon dioxide + water)****Here is where the HOFBrINCl’s begin to come in to play!

Skeleton Equations

Change the word equations to skeleton equations:1) Iron reacts with hydrogen sulfate to form

iron (III) sulfate and hydrogen gas2) Potassium hydroxide reacts with hydrogen

phosphate to produce potassium phosphate and hydrogen gas

3) Ammonia and oxygen gas react to form nitrogen monoxide and water.

4) Boron trifluoride and lithium sulfite produce diboron trisulfite and lithium floride

Try these…

But wait...

There's a problem with our skeleton equations....

Can you spot it?

Example: CH4 + O2 → CO2 + H2O

The law of conservation of mass states that: in achemical reaction, the total mass of the reactants is always equal to the total mass of the products.

Experiments show that atoms in a chemical reactionare not changed themselves, and the number of atomshas to stay the same from before the reaction to after.

The Law of Conservation of Mass

How many atoms of each element are there in eachmolecule of:

a) H2SO4b) Al2(SO4)3c) CH3COOH

Refresher on Counting Atoms…

1. When a log burns in your fire place, why are isthe mass of the ashes less than the mass of the log?

Try these…

2. a) Count the number of atoms of each type ofreactant in the reaction below and record them in thetable below. Count the number of atoms of eachtype of product and record them below. b) What do your results say about conservation of mass?

# of atoms Reactants Products

Carbon

Oxygen

Hydrogen

CH4 + 2O2 → CO2 + 2H2O

3. A solid has a mass of 35g. When it is mixedwith a solution, a chemical reaction happens. Ifthe total mass of the products is 85g, what wasthe mass of the solution?

4. Solution A gas a mass of 60g. Solution B hasa mass of 40g. When they are mixed, a chemical reaction occurs in which a gas is produced. If the mass of the final mixture is 85g, what mass of gas was produced?

What's wrong with this reaction?Al +Br2 → AlBr3It does not follow the Law of Conservation of Mass.How do we fix that? By adding coefficients to increase the number of molecules.Remember: 3 Na2SO4

CoefficientSubscripts***Remember this rule above all else: YOU CANNOT CHANGE SUBSCRIPTS, only coefficients

Example: Iron reacts with oxygen to form magnetic iron oxide (Fe3O4)

Step 1: Write the word equation.

Iron + Oxygen → magnetic iron oxide

Step 2: Write the skeleton equation, replacing words for formulas.

Fe + O2 → Fe3O4

How to Balance

Step 3: Count the numbers of atoms of eachtype in reactants and products.

Fe + O2 → Fe3O4

Reactants: Products:Fe: 1 Fe: 3O: 2 O:4

Step 4: Multiply each of the formulas byappropriate coefficients to balance the numbersof atoms.

To balance the three irons on the right, multiply the ones on the left by 3 multiply the 2 oxygens on the left by two to balance with the 4 on the right.

3 Fe + 2 O2 → Fe3O4

Magnesium metal reacts with nitric acid (HNO3) to form hydrogen gas and magnesium nitrate.Step 1: Write the word equation for the reaction

Magnesium + nitric acid → hydrogen +magnesium nitrate

Step 2: Write the skeleton equation by replacing the formulas for the name.

Mg+ HNO3 → H2 + Mg(NO3)2

Try another:

Step 3: Count the number of atoms of eachtype in reactants and products.****(in this case we can count the NO3 as one atom because it stays together)****Mg+ HNO3 → H2 + Mg(NO3)2

Reactants: Products:Mg:1 Mg: 1H: 1 H: 2NO3: 1 NO3: 2

Place a 2 in front of the HNO3

Mg+ 2HNO3 → H2 + Mg(NO3)2

1. a) Why is the following equation not balanced?

N2 + H2 → NH3

b) What is wrong with the way the above equation was balanced here?

N2 + H3 → N2H3

You try…

2. Balance:a) Na + Cl2 → NaClb) K + O2 → K2Oc) H2 + O2 → H2Od) H2 + Cl2 → HCle) N2 + H2 → NH3f) CO + O2 → CO2g) Al + Br2 → AlBr3h) N2H4 + O2 → H2O + N2i) CH4 + O2 → CO2 + H2O

3.For each of the following, write the correct skeleton equation, and then balance it to form a chemical equation.

a) copper (II) oxide + hydrogen → copper + water

b) lead (II) nitrate + potassium iodide → lead (II) iodide + potassium nitrate

c) calcium + water → calcium hydroxide + hydrogen

d) lead (II) sulfide +oxygen → lead + sulfur dioxide

e) hydrogen sulfide → hydrogen +sulfur