Chemical Reactions

Questions

1. What is the difference between a chemical and physical change?

2. Give an example of a chemical change and a physical change.

3. How can you tell a chemical reaction has occurred?

Chemical vs. Physical

• ____________________- deals with energy and states of matter. – Ex: Crushing a can, melting an ice cube.

• _____________________- takes place on a molecular level and a new substance is produced.– Ex: Burning, cooking an egg.

Chemical Reactions

• Chemical Reaction – When two or more substances come together to form a new substance.

• Chemical Reactions are all around us!

• What are some examples of chemical reactions in your life?

How To Tell A Chemical Reaction Has Occurred

• New substance• Light• Heat• Color Change• Gas• Odour• Sound

Questions

4. What is a chemical equation? On what side of the equation are the products and reactants located?

5. Describe one chemical reaction that occurs in everyday life.

Reactions• Chemical Equation – Way of representing a chemical reaction.– Ex:

• _____________________ – substances being brought together to react (before reaction)

• _____________________ – substances formed (after reaction)

Reactants Products

• Reactants yield (produce) the products.– Ex: The reaction of Hydrogen and Oxygen yields Dihydrogen

Monoxide (water)

Reaction Symbols

• (l) = Liquid

• (g) = Gas

• (s) = Solid

• (aq) = Aqueous Solution (solvent is water)

• = Points in direction of reaction

Questions

6. What is a diatomic element? Give an example. What is one way of remembering which elements they are?

7. What is the Law of Conservation of Mass?

Diatomic Elements

• Diatomic Elements are those that occur naturally as two. (Di = 2)–Ex: Hydrogen occurs as H2 not as H.

• There are 7 diatomic elements:

How to Remember Diatomic Elements

• Use the acronym I Bring Clay For Our New Home:

– I = Iodine – Bring = Bromine – Clay = Chlorine – For = Fluorine – Our = Oxygen – New = Nitrogen – Home = Hydrogen

Diatomic Elements form a 7 on the periodic table plus Hydrogen in the corner!

Chemical Equations: In Words

• Write down the following chemical reaction in words:

2 NO + O2 2NO2

• _________________________________________________

_________________________________________________

__________________________________

Chemical Equations: Balancing

• Law of Conservation of Mass – The mass of the reactants

_______________________________________________.

• In other words, what goes in to the reaction, must come out! Nothing just disappears!

Ex: Formation of WaterH2 (g) + O2 (g) H2O (l)

• There are two hydrogens on both sides, but only two oxygens on the left and one on the right. So the equation needs to be balanced!

Chemical Equations: Balancing

• In order to balance an equation (have equal number of atoms on each side), you must place coefficients (numbers) in front of the atoms or molecules.

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

(you can use an inventory list to help you)

Balancing Equations• Rules:– Can’t add extra molecules or atoms into the

equation!– Never change molecular formulas!

• Tips:– If an element occurs as a free element, balance

this element last. – Can use fractions to help in balancing, but need to

clear the fractions by multiplying by a common multiplier.

Questions

8. What does it mean to be an aqueous solution?

9. What are the five different types of reactions?

10. What is a synthesis reaction?

11. What is a decomposition reaction?

12. What is a single displacement reaction?

13. What is a double displacement reaction?

14. What is a combustion reaction?

Chemical Equation from Words• When writing down an equation, make sure to

write down the states of each element and compound.

Ex: Aluminum reacts with Copper (II) Chloride to form Copper metal and Aluminum Chloride.

Write the balanced equation for this reaction.

Types of ReactionsYou need to be able to identify the type of reaction and

predict the product(s)

1. _____________________

2. _____________________

3. _____________________

4. _____________________

5. _____________________

Steps to Writing Reactions

1. Identify the type of reaction2. Predict the product(s) using the type of reaction as a model3. _____________________!!

Synthesis Reactions• Synthesis reactions occur when two substances

(generally elements) combine and form a compound.

Reactant + Reactant 1 Productor

• Examples:

Practice• Predict the products. Write and balance the

following synthesis reaction equations.• Sodium metal reacts with chlorine gas

• Solid Magnesium reacts with fluorine gas

• Aluminum metal reacts with fluorine gas

Decomposition Reactions• Decomposition reactions occur when a

compound breaks up into the elements or to a few simpler compounds

• 1 Reactant Product + Product or

• Examples:

Decomposition Exceptions• Carbonates and chlorates are special case

decomposition reactions that do not decompose into their elements.

• Carbonates (CO32-) decompose to carbon dioxide and a

metal oxide• Example:

• Chlorates (ClO3-) decompose to oxygen gas and a metal

chloride• Example:

Practice

• Predict the products. Then, write and balance the following decomposition reaction equations:

• Solid Lead (IV) oxide decomposes PbO2 (s)

• Aluminum nitride decomposes AlN (s) 2

Single Displacement Reactions

• Single Displacement Reactions occur when one element replaces another in a compound.

• A metal can replace a metal (+) or a nonmetal can replace a nonmetal (-).

• element + compound element + compound (if A is a metal)

(if A is a nonmetal)

*Remember the cation (+) always goes first in a compound*

Single Displacement Reactions

• Write and balance the following single displacement reaction equation:

• Zinc metal reacts with aqueous hydrochloric acid

Note: Zinc replaces the hydrogen ion in the reaction

Single Displacement Reactions• Sodium chloride solid reacts with fluorine gas

*Note that fluorine replaces chlorine in the compound*

• Aluminum metal reacts with aqueous copper (II) nitrate

Double Displacement Reactions• Double displacement Reactions occur when a

metal replaces a metal in a compound and a nonmetal replaces a nonmetal in a compound

• Compound + compound compound+ compoundor

• Examples:

Double Displacement PracticePredict the products and balance the equations.

1. HCl(aq) + AgNO3(aq)

2. CaCl2(aq) + Na3PO4(aq)

3. Pb(NO3)2(aq) + BaCl2(aq)

4. FeCl3(aq) + NaOH(aq)

5. H2SO4(aq) + NaOH(aq)

6. KOH(aq) + CuSO4(aq)

Combustion Reactions• Combustion reactions occur when a hydrocarbon

reacts with oxygen gas.

• Also called burning!• You need three things to burn something:

Combustion Reactions• In general:

• Products in combustion are always carbon dioxide (CO2) and water (H2O).

• Incomplete burning causes byproducts such as carbon monoxide.

• Combustion is used to heat homes and run automobiles (octane, as in gasoline, is C8H18)

Combustion Reactions

• Example:

• Write the products and balance the following combustion reaction:• C10H22 + O2

Questions

15. What is an exothermic reaction? Give an example.

16. What is an endothermic reaction? Give an example.

Endothermic vs. Exothermic

• Reactions can be classified as either Endothermic or Exothermic depending on whether it gives off energy or takes energy in.

• Energy can be in theform of light or heat.

• Energy Change is denoted by ∆H.

• ∆H = kJ/mol

Exothermic Reactions

• Exothermic Reactions are those that give off energy (light or heat).

• Reactants Products + Energy

• Ex: In the formation of water2H2 + O2 2H2O + 483.6 kJ

• ∆H = - 483.6 kJ/mol

• ∆H is negative in exothermic reactions because energy is given off or lost.

Exothermic Reactions

• Some examples are:

Endothermic Reactions• Endothermic Reactions are those that

absorb energy (heat and light).

• Ex: In the melting of ice6.00 kJ + H2O(s) H2O(l)

∆H =• ∆H is positive in endothermic reactions

because it is energy needed for the reaction to occur.

Endothermic Reactions

• Some examples:– Melting– Baking– Cooking– Ice Packs