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Chemical
Reactions
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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
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• 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
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• 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
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• 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
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• 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
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• 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
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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).
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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!
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• 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
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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.
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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
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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
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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
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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
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Classification of Reactions
• There are 5 major classifications of reactions:– Synthesis (Combination)– Decomposition– Combustion– Single Replacement – Double Replacement (Metathesis)
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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
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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
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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
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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
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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
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Activity Series
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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
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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
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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
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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
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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
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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
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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
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Solubility Rules
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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
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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
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Reactions with a gas
• HCl + NaHCO3 • HCl + NaHCO3 H2CO3 (g) + NaCl
• HCl + NaHCO3 H2O + CO2 (g) + NaCl
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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
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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
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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