SNC2D Grade 10 Academic Science Chemistry – Synthesis and Decomposition Reactions

Thanks, Tony!

Chemistry: Chapter 5 Notes


Chapter 5.1—Synthesis and Decomposition Reactions


  • 6 clues that suggest a chemical change is occurring are:
    • formation of gas
    • formation of a precipitate—an insoluble solid formed in a chemical reaction
      • it’s a new product
    • change in colour
    • change in odour
    • change in temperature
    • production of light



  • the new substances/products that form during chemical changes will depend on the type of chemical reaction occurring
  • understanding the different types of chemical reactions will allow you to identify what products are most likely to form
    • chemists classify chemical reactions into different categories, and four of them include: synthesis, decomposition, single displacement, double displacement



  • a synthesis reaction is a chemical reaction in which two or more reactants combine to produce a new product
  • the general chemical equation for a synthesis equation is: A + B AB
  • in a synthesis reaction, 2 or more reactants combine to form one product
    • the reactants can be any combination of elements and compounds, but the product will always be a compound
  • when a space shuttle blasts off, the incredible power of its main thrusters is generated from a synthesis reaction; the reactants are liquid hydrogen and liquid oxygen, which combines to form water vapour
    • 2H2(l) + O2(l)  2H2O(g)
  • all diatomic molecules are gases at room temperature, except for bromine (liquid) and iodine (solid)
  • the Haber process is the synthesis reaction that occurs to form ammonia (an important component in fertilizers. making paper, extracting zinc and nickel from ores, in explosives, and in cleaning products); created by Fritz Haber
    • N2(g) + 3H2(g)  2NH3(g)
  • many atmospheric pollution problems are associated with synthesis reactions
    • nitrogen monoxide
      • N2(g) + O2(g)  2NO(g)
    • nitrogen monoxide can then undergo further synthesis to produce nitrogen dioxide
      • 2NO(g) + O2(g)  2NO2(g)
  • for the synthesis of binary ionic compounds composed of metals with one possible ion charge, you can use ion charges to determine the most likely product; the steps are:







Na(s) + Cl2(g)  ???

  1. Use the ion charges to determine the most likely product.
sodium’s ion charge of 1+ and chlorine’s ion charge of 1-
  1. use the cross-over method to determine the most binary ionic compound formed between the 2
the product after the cross-over is:                                NaCl
  1. balance the equation if necessary
Na(s) + Cl2(g)  NaCl

2Na(s) + Cl2(g) 2NaCl(s)




  • a decomposition reaction is a chemical reaction in which a compound breaks down (decomposes) into 2 or more simpler compounds or elements
  • decomposition reactions can be represented by a general chemical equation as: AB  A+B
  • the products may be any combination of elements and compounds, but the reactant will always be a compound
    • therefore, a decomposition reaction is the opposite of a synthesis reaction
  • an essential component in the development of hydrogen as a fuel source is the ability to produce sufficient amounts of hydrogen; one way hydrogen gas is produced is through the decomposition of water through an electric current (electrolysis)
    • 2H2O(l)  2H2(g) + O2(g)
  • the tremendous forces associated with the explosion are the result of a decomposition of trinitrotoluene (TNT); it’s a common explosive
    • 2C7H5N3O6(s)  3N2(g) + 5H2O(g) +7CO(g) + 7C(s)
      • the products contain gases; the rapid expansion of the gases push material away from the blast site, creating the explosion
  • during the decomposition of a binary ionic compound, electrons transfer back to the atoms of the metal, and each element becomes electrically neutral; the steps are:





AgCl(s)  ???

  1. because it’s a decomposition reaction, the reactant will decompose into the 2 elements that it is composed of
the products will be silver and chlorine
  1. write the chemical equation using the information given in the question, and the info found in step 1 (pay attention to diatomic molecules—they have a subscript of 2)
AgCl(s)  Ag(s) + Cl2(g)
  1. balance the equation if necessary
AgCl(s)  Ag(s) + Cl2(g)

2AgCl(s) 2Ag(s) + Cl2(g)




  • a component of concrete is the ionic compound calcium oxide (CaO), and it’s made by decomposing another ionic compound calcium carbonate (CaCO3)
    • decomposition of calcium carbonate to form calcium oxide: CaCO3(s)  CaO(s) + CO2(g)
    • when needed to make mortar or plaster, the calcium oxide is combined with water:                                                            CaO(s) + H2O(l)  Ca(OH)2(aq)


Chapter 5.2—Displacement Reactions


  • is a chemical reaction in which one element takes the place of another element in a compound
    • a reactive element (metal/non-metal) and a compound produce another element and another compound
  • general chemical equation: A + BC  B + AC (if A is metal)       OR      A + BC  C + BC (if A is non-metal)
  • the single displacement reaction when aluminum is placed into copper (II) chloride forms aluminum chloride and copper
    • 2Al(s) + 3CuCl2(aq) 2AlCl3(aq) + 3Cu(s)
  • displacing metals with metals: Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 3Cu(s)
    • when copper is put into aqueous silver nitrate, the solution turns blue because copper atoms from the wire turn into ions and dissolve into the solution
    • the copper displaces the silver from the compound
    • because a metal forms a positive ions, it must displace the positive ion from the compound so that a new ionic compound can form (therefore metal must only displace metal)



  • is a list of elements organized according to their chemical reactivity’ the most reactive element appears at the top, and the least reactive appears at the bottom
  • not all elements are equally reactive
  • activity series for metals (from most to least reactive):
    • lithium
    • potassium
    • calcium
    • sodium
    • magnesium
    • aluminum
    • zinc
    • iron
    • nickel
    • tin
    • hydrogen
    • copper
    • silver
    • platinum
    • gold
  • a reactive metal will displace a metal in a compound that is below it in the activity series
    • ex: since copper is more reactive than silver, copper will displace silver                                                                               Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 3Cu(s)
  • a metal will never displace another metal in a compound, if the element metal is less reactive than the compound metal
  • hydrogen is on the activity series even though it isn’t a metal; since hydrogen atoms can be positively charged, metals can take the place of hydrogen in compounds
    • ex: Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)



  • the ion of a non-metal is negatively charged, so it can only be replaced by another non-metal (negatively charged)
  • in the reaction between fluorine and sodium iodide, fluorine replaces the iodine to produce iodine and sodium fluoride
    • F2 + 2NaI(s) I2(s) + 2NaF(s)
  • there is a separate activity series for non-metals



  • is a chemical reaction in which the positive ions of two different compounds exchange places, resulting in the formation of 2 new compounds—one of which may be a precipitate
    • the positive ions of 2 compounds change places, to form 2 new compounds
  • the reactants of a double displacement reaction are often aqueous solutions of ionic compounds
  • in this chapter, only the double displacement reactions of aqueous solutions that result in a precipitate as one of the products will be studied
  • double displacement reactions general equation: AB + CD  CB + AD
    • “A” and “C” are cations while “B” and “D” are anions
  • when silver nitrate is added to potassium chromate, silver chromate is formed
    • 2AgNO3(aq) + K2CrO4(aq) Ag2CrO4(s) + 2KNO3(aq)
      • a precipitate was formed



  • sulphur dioxide is used to preserve the colour of dried fruits
    • sulphur dioxide gas is produced from a double displacement, followed by a decomposition reaction
      • Na2SO3(aq) + 2HCl(aq) 2NaCl(aq) + H2SO3(aq)
      • H2SO3 H2O(l) + SO



  • summary


Reaction General Equation Characteristics
synthesis A + B  AB 2 reactants join to form a single compound
decomposition AB  A + B single compound breaks apart into 2 or more products
single displacement A + BC  B + AC (if A is metal)

A + BC  C + BA (if A is non-metal)

reactive element takes the p-lace of a less reactive element in a compound
double displacement (with precipitate) AB + CD  AD + CB 2 ionic compounds in a solution switch ions and form 2 new compounds (where 1 compound is a precipitate



Chapter 5.3—Reactions and Environmental Issues


  • if oil is spilt onto water, it spreads over the surface in a very thin layer
    • a quick response is needed to minimise environmental harm, as oil is a mixture of many chemicals, and when time passes, some of the chemicals will evaporate, and the remaining chemicals become a thick, sticky mess
  • cleaning up oil spills involves several processes—they include initial containment, which can involve using special materials that absorb the oil; use biological agents that hasten oil degradation (chemical reactions that occur in the cells help to break down molecules into simpler, and less toxic molecules)



  • our society has become heavily reliant on burning of fossil fuels for heating, energy production, and transportation
  • in a car’s engine, gasoline burns and reacts with oxygen
    • 2C8H18(l) + 25O2 16CO2(g) + 18H2O(g)
  • however, conditions in an engine don’t always provide the right conditions for complete combustion—incomplete combustion occurs when there’s not enough oxygen present, and can produce poisonous carbon monoxide gas and carbon (soot); it can also cause unburned gasoline vapours to exit exhaust
    • the exhaust also includes a variety of nitrogen oxides (NO and NO2)
      • although N2 fairly non-reactive, the high temperatures in an engine provide enough energy to break the bonds between nitrogen atoms, allowing nitrogen to undergo a synthesis reaction with oxygen gas in the air, to produce nitrogen oxides which contribute to greenhouse gases
  • a catalyst is a substance that increases the rate of a reaction and is regenerated at the end of the reaction
    • it makes a reaction happen faster, and isn’t used up in the reaction
  • catalytic converters are an important device for helping to reduce the harmful emissions in a car’s exhaust
    • all exhaust pass through the converter before exiting vehicle
    • the catalytic converter contains precious metals (platinum, palladium, rhodium) which decompose the NO and NO2 gas back into the elements N2 and O2
  • catalytic converters also help change the unburned gasoline into carbon monoxide and water




  • the mining and recovery of gold relies on several reactions to separate small particles of gold from the rock/ore
  • one method used to retrieve gold involves reactions with cyanide ions (CN) and with zinc metal
    • cyanide compounds are highly toxic, and even at low levels can be lethal; the use of cyanide ions being released are at risk
  • in gold mining, the rock that makes up the gold ore is crushed into a fine power to expose as much gold as possible; the powder is placed in piles and sprayed with sodium cyanide causing the gold to form a soluble compound
    • 4Au(s) + 8NaCN(aq) + O2(g) + 2H2O(l) 4Na[Au(CN)2](aq) 4NaOH(aq)
      • soluble compound 4Na[Au(CN)2](aq) is drained away from the ore; this process is leaching (a technique used to extract a metal by dissolving the metal in an aqueous solution)
      • this reaction doesn’t fit neatly into any of the 4 reactions
  • one way to recover the gold is to react the 4Na[Au(CN)2](aq) with zinc
    • 4Na[Au(CN)2](aq) + Zn(s) 2NaCN(aq) + Zn(CN)2(aq) + 2Au(s)
  • sometimes, other chemicals (bromine and chlorine) can be used to leach gold’ micro-organisms have also been used to remove gold biologically; but cyanide ion leaching is the most cost-effective method



  • to keep pools safe, chemicals must be used to prevent the growth of bacteria and other organisms that could cause illness
    • the most common chemicals used for this are chlorinating agents (chemicals that release chlorine when dissolved in water)
      • levels of these chemical must be closely monitored, as just because a chemical is commonly used, doesn’t mean that you can be careless with it; these chemicals have the potential to cause harm
  • different types of chlorinating agents aren’t compatible with each other—in some cases, adding 2 different chlorinating chemicals can cause an explosive mixture
    • some of these chemicals are also corrosive, so contact with skin can cause chemical burns
  • before working around these chemicals, you should examine the Material Safety Data Sheet (MSDS) for each chemical



  • there are chemicals in common household products, and these chemicals can put you at risk if you’re not aware of the product warning labels and don’t take necessary precautions
  • there is a system of warning labels for consumer products, called the Hazardous Household Product Symbols(HPPS)
    • WHKMIS symbols are safety precautions for the lab only


  • there are dangers associated with mixing certain products—for example, bleach and ammonia can result in the production of toxic substances (main active ingredient in household bleach is sodium hypochlorite; when added to ammonia, reaction produces chlorine gas)
    • 2NH3(aq) + 2NaClO(aq) 2NaONH3(aq) + Cl2(g)
  • chlorine gas is very dangerous—was used in WWI and WWII as a chemical weapon; when inhaled, chlorine is highly reactive with molecules in a person’s respiratory system, which can cause considerable damage and even death
  • other reactions that can occur between ammonia and bleach which produce chloramines (toxic compounds that contain nitrogen and chlorine)
    • NH3(aq) + 3NaClO(aq) 3NaOH(aq) + NCl3(g)
    • NH3(aq) + NaClO(aq) NaOH(aq) + NH2Cl(g)


Handout—Catalytic Converter

  • is located under the hood (past the engine)
  • it reduces the toxic gasses that are released from the exhaust by converting it into non-toxic gasses
  • converter needs to be at 1300 degrees F to work (needs approximately 6 miles before it begins to work)
  • honey comb shaped, as it has many micro ducts (allows for more surface area)