Ch. 8 Writing chemical reactions and looking for reaction patterns.
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Transcript of Ch. 8 Writing chemical reactions and looking for reaction patterns.
Ch. 8Writing chemical reactions
and looking for reaction patterns.
What is a Chemical Reaction?• Compounds break up and the
atoms get rearranged• The same atoms are there
before & after (conservation of mass)
• New substances are made• Evidence: gas released, heat
& energy change, precipitate forms (a solid, so solution gets cloudy)
• Overall, energy is conserved
How we show a reaction:
• WORDS: methane + oxygen burns to produce carbon dioxide and water
• SYMBOLS: CH4 + O2 CO2 + H2O
• But this isn’t balanced; it needs to have same number of atoms on each side, so we add coefficients to balance it.
Here it is, balanced:
• CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
• The (g) stands for “gas” • This means: 1 mole of methane
reacts with 2 moles of oxygen; making 1 mole of carbon dioxide and 2 moles of water.
Aluminum reacts with Bromine
How to Balance the Reaction
Three Steps to write and balance a reaction: • 1. Write down the correct formulas for
the reactants & the products• 2. Count up the types of atoms on the
left side and see if they match those on the right side
• 3. Add coefficients in front of the molecules until all atoms are balanced (both sides equal)
• Never Change Subscripts when adding coefficients!
A reminder about Diatomic Elements!Long live Mrs. HOFBrINCl !If it’s not one of these, then it is
shown a “monatomic” in a chemical reaction; for example, iron is just “Fe”
Five Types of Reactions to Recognize
Combustion (Burn!)
• Some Hydrocarbon burns (CxHy) in oxygen and produces carbon dioxide & water
• x and y represent the numbers of C and H atoms, and they vary a lot
• Example: Pentane, C5H12, burns. Write and balance the reaction.
• C5H12(g) + 8O2(g) 5CO2(g) + 6H2O(g)
Synthesis Reactions (Make!)
• Two elements react to make one product.
• Example: Magnesium burns in oxygen.• 2Mg(s) + O2(g) 2MgO(s) • Note: This is NOT a combustion
reaction. Why not?• Ans: Mg is not a hydrocarbon.
Decomposition Reactions (Break!)
• One reactant compound is heated and makes two or more products.
• Example: Water is broken down by electrolysis to make hydrogen and oxygen gas. Write and balance the reaction:
• 2H2O(l) 2H2(g) + O2(g)
Single Displacement (switch!)
• One element changes places with another element in a compound.
• compound + element compound + element
• Silver nitrate reacts with copper to yield copper (II) nitrate and silver.
• 2AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2Ag(s)
• Cu displaced Ag in AgNO3
• Which metals can displace others is seen in an “Activity Series” chart.
Double Displacement (switch!)
• Two compounds break up and switch partners.
• AB + CD AD + CB
• Calcium chloride and silver nitrate react to produce calcium nitrate and silver chloride.
• CaCl2(aq)+ 2AgNO3(aq) Ca(NO3)2(aq) + 2AgCl(s)
Acid + Base is a type of double displacement reaction.
• acid + base → water + salt
• Sulfuric acid, H2SO4 , and sodium hydroxide react. Write the reaction.
• H2SO4 + 2NaOH → 2HOH + Na2SO4
• Or:• H2SO4 + 2NaOH → 2H2O + Na2SO4
A helpful chart of the 5 reaction patterns:
A quick “Guide to the 5”If the reactant(s) are: Then it’s a ____ reaction And the products are
2 elements Synthesis One compound
1 compound heated Decomposition 2 or more chemicals
Element and a Compound Single Displacement An element and a compound
2 ionic Compounds Double Displacement 2 ionic compounds
A hydrocarbon burning Combustion CO2 and H2O
Learn these Patterns!
• These five patterns will help you write and balance almost all the types of reactions we come across in inorganic chemistry!