Chemical

Reactions

Did a Chemical Reaction Take Place?

• There are several ways to tell if a chemical reaction has occurred…– Temperature change– Color change– Odor– Precipitate is formed (solid)– Gas is formed

• A substance that undergoes a reaction is called a reactant.– Reactants are written on the left of the arrow

• When reactants undergo a chemical change, each new substance formed is called a product.– Products are written on the right of the arrow

• Reactants Products

Writing Chemical Equations

• Reactants – starting chemical

• Products – substances that are formed

• + separates substances (usually read as “and”)

separates reactants from products (usually read as gives, produces, or yields)

Writing Chemical Equations

• The simplest way to represent a reaction is by using words to describe all the reactants and products, with an arrow placed between them to represent change.

• Reactants are placed to the left of the arrow, and products are placed to the right.

• Plus signs are used to separate reactants and also to separate products.

Word Equations

Vinegar Baking Soda

• Word equations describe reactants and products, but they are long and awkward and do not adequately identify the substances involved.

• Word equations can be converted into chemical equations by substituting chemical formulas for the names of compounds and elements.

Chemical Equations

• The equation for the reaction of vinegar and baking soda can be written using the chemical formulas of the reactants and products.

• By examining a chemical equation, you can determine exactly what elements make up the substances that react and form.

Chemical Equations

Chemical Reactions

• It may also be important to know the physical state of each reactant and product.

• Symbols in the parentheses are put after formulas to indicate the state of the substance.

• Solids, liquids, gases, and in water (aqueous) solutions are indicated by the symbols (s), (l), (g), and (aq).

Try this example…

• Write the skeleton equation for solid magnesium reacting with oxygen gas to give solid magnesium

• Mg (s) + O2 (g) MgO• You can see that you have to remember

how to write formulas and what elements are diatomic in order to write equations!

• The mass of the products is always the same as the mass of the reactants that react to form them.

• The law of conservation of mass summarizes these findings.

• Matter is neither created nor destroyed during a chemical reaction.

• What you start with has to equal what you end with

Balancing Chemical Equations

Balancing Chemical Equations

• For a chemical equation to accurately represent a reaction, the same number of each kind of atom must be on the left side of the arrow as are on the right side.

1. Write the skeleton equation for the reaction2. Count the number of atoms of each element of the

reactants3. Count the number of atoms of each element of the

reactants4. Add / Change the coefficients to make the numbers

of each element equal5. YOU CAN NEVER CHANGE A SUBSCRIPT!6. Write the coefficients in the lowest possible ratio7. Check your work

Steps for Balancing Chemical Equations

Examples

• Write the balanced chemical equation for the reaction between hydrogen and chlorine to give hydrochloric acid

• H2 + Cl2 HCl

H – 2 H - 1Cl – 2 Cl – 1

• H2 + Cl2 2HCl

H – 2 H - 2Cl – 2 Cl – 2

More examples

• Write and balance the reaction between sodium hydroxide and calcium bromide to give calcium hydroxide and sodium bromide

• 2 NaOH + CaBr2 Ca(OH)2 + 2NaBr

More examples

• Write and balance the reaction for potassium iodide reacting with lead (II) nitrate to form potassium nitrate and lead (II) iodide

• 2 KI + Pb(NO3)2 2 KNO3 + PbI2

Classification of Reactions

• There are 5 major classifications of reactions:– Synthesis (Combination)– Decomposition– Combustion– Single Replacement – Double Replacement (Metathesis)

Synthesis (Combination)

• Synthesis – when 2 or more substances react to produce 1 product

• Of the form: A + B AB• Examples:

– 2 Fe + 3 Cl2 2 FeCl3– 2 Na + Cl2 2NaCl– CaO + H2O Ca(OH)2

– Mg + O2 MgO

Decomposition

• Decomposition – when a single compound breaks down into 2 or more compounds

• Of the form: AB A + B • Note that this is the exact opposite of

synthesis• Examples:

– 2 Na3N 2 Na + 3 N2

– NH4NO3 N2O + 2H2O– 2H2O H2 + O2

Combustion

• Combustion – when O2 combines with a hydrocarbon to form CO2 and H2O

• Of the form: hydrocarbon + O2 CO2 + H2O

• For example:– 2CH4 + O2 CO2 + 2H2O

– 2CH3OH + 3O2 2CO2 + 4H2O

When Balancing Combustion Reactions

• Put a two in front of the hydrocarbon to start.

• Balance them in the order C, H, O

• Make sure your answer is in the lowest whole number ratio

Single Replacement

• Single Replacement – when the atoms of one element replace the atoms of another element in a compound

• Of the form : A + BX B + AX

• For example:– 2 Li + 2HOH 2 LiOH + H2

– Cu + 2AgNO3 2 Ag + Cu(NO3)2

Activity Series

Single Replacement Reactions

• In order for a single replacement reaction to occur, the element that is by itself must be strong enough to push the other element out of the way.

• The activity series show you the relative pushing power of the elements.

• In order for the reaction to occur, the lone element must be above the element in the compound

• Otherwise…NO REACTION NR

Single Replacement Reactions

• You need to know which chart you are to look at…metals or halogens.

• A metal can replace another metal

• A halogen can replace another halogen

Will these reactions occur?

• Will the following reaction occur? If so, complete and balance the reaction.

• Ag + Cu(NO3)2

• Ag + Cu(NO3)2 NR

• No reaction because Ag is below Cu in the Activity series of metals

Will these reactions occur?

• Will the following reaction occur? If so, complete and balance the reaction.

• Mg + AlCl3

• 3Mg + 2AlCl3 3MgCl2 + 2 Al

• The reaction WILL occur because Mg is above Al in the activity series

Will these reactions occur?

• Will the following reaction occur? If so, complete and balance the reaction.

• Br2 + MgCl2

• Br2 + MgCl2 NR

• This reaction will NOT occur because Br is not above Cl in the activity series of halogens

Double Replacement (Metathesis)

• Double Replacement – a reaction involving the exchange of ions between 2 compounds

• Of the form: AX + BY BX + AY

• Examples:– Pb(NO3)2 + 2KI PbI2 + 2KNO3

– Na2CO3 + 2AgNO3 2NaNO3 + Ag2CO3

Double Replacement (Metathesis)

• In order for a double replacement reaction to take place, one of 3 things must be formed:

• Precipitate (solid)

• Gas

• Weak electrolyte – usually water

Solubility Rules

Soluble or Insoluble?

• Classify the following as soluble or insoluble? (Hint: insoluble means precipitate)

• NaCO3

• Soluble• PbSO4

• Insoluble (precipitate)• Co(OH)2

• Insoluble (precipitate)• Ba(NO3)2

• Soluble• (NH4)3PO4

• soluble

Reactions with a gas

• Carbonates, bicarbonates, and sulfides react with acids to form a gas

• For example:

• HCl + Na2S

• 2HCl + Na2S H2S (g) + 2NaCl

• The sulfide or carbonate formed will result in the gas

Reactions with a gas

• HCl + NaHCO3 • HCl + NaHCO3 H2CO3 (g) + NaCl

• HCl + NaHCO3 H2O + CO2 (g) + NaCl

Will these reactions occur?

• Will the following reaction occur? If so, complete and balance the reaction.

• Na2CO3 + AgNO3

• Na2CO3 + AgNO3 Ag2CO3 + NaNO3

• Na2CO3 + AgNO3 Ag2CO3 (s) + NaNO3

• Na2CO3 + 2AgNO3 Ag2CO3 (s) + 2NaNO3

Will these reactions occur?

• Will the following reaction occur? If so, complete and balance the reaction.

• HCl + NaOH • HCl + NaOH NaCl + HOH

• HCl + NaOH NaCl + H2O

Will these reactions occur?

• Will the following reaction occur? If so, complete and balance the reaction.

• NaOH + K2SO4

• NaOH + K2SO4 Na2SO4 + KOH

• NaOH + K2SO4 Na2SO4 (aq) + KOH (aq)

• NaOH + K2SO4 NR