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Lesson 6 of 8Chemical Reactions
Unit 2 · Lesson 6 of 8

Single Displacement, Double Displacement and Combustion

Predicting single displacement reactions with the activity series, predicting double displacement reactions that form a precipitate, and writing balanced equations for the complete and incomplete combustion of hydrocarbons.

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Key things to remember

  1. 1Single displacement: A + BC → AC + B; it happens only if A is above the element it replaces in the activity series.
  2. 2Metals above hydrogen release from acids; the halogen order is .
  3. 3Double displacement: AB + CD → AD + CB; it happens when a precipitate, a gas or water forms.
  4. 4Complete combustion of a hydrocarbon: .
  5. 5Incomplete combustion (limited oxygen) also produces carbon monoxide and soot.

Drop an iron nail into blue copper(II) sulfate solution and within minutes the nail grows a coat of pink-brown copper, and over time the blue colour fades. Mix two clear, colourless solutions of lead(II) nitrate and potassium iodide and a bright yellow solid appears instantly. Light a gas barbecue and propane burns with a blue flame. These are the other three reaction patterns: single displacement, double displacement and combustion. Each has a telltale pattern you can learn to spot, and each lets you predict products before you ever mix anything.

Single displacement: A + BC → AC + B

In a single displacement reaction, an element reacts with a compound and takes the place of one of the elements in it. A metal replaces another metal (or the hydrogen in an acid or in water), and a non-metal, usually a halogen, replaces another halogen. Whether it happens depends on which element is more reactive. Chemists ranked metals by testing them against each other's compounds, producing the activity series: a metal higher in the list can displace any metal below it from a compound in solution, but not the reverse.

Activity Seriesmost reactiveLiKBaCaNaMgAlZnFeNiSnPb(H₂)CuAgAuleast reactiveRule for single displacement:A free metal will displace any metallisted below it from a compound in solution.e.g. Zn + CuSO₄ → ZnSO₄ + CuZn is above Cu → reaction proceedsCu + ZnSO₄ → no reaction (Cu is below Zn)Halogen Reactivity SeriesF₂Cl₂Br₂I₂most reactiveleast reactivea free halogen displaces any halide listed below it, e.g. Cl₂ + 2NaBr → 2NaCl + Br₂
Fig 1The metal activity series from most reactive (Li, K, Ba, Ca, Na) to least reactive (Cu, Ag, Au), with hydrogen placed between lead and copper, and the rule that a free metal displaces any metal below it: Zn + CuSO₄ → ZnSO₄ + Cu happens, but Cu + ZnSO₄ does not. A separate halogen series runs F₂, Cl₂, Br₂, I₂ from most to least reactive.
  • A metal displaces any metal below it in the activity series from a compound in solution. If it is below, write "no reaction". (The most reactive metals, such as sodium and potassium, react with the water in a solution instead, so they are not used this way.)
  • Metals above hydrogen react with acids such as HCl(aq) to release hydrogen gas; metals below hydrogen (copper, silver, gold) do not.
  • The most reactive metals (lithium, potassium, barium, calcium, sodium) react with cold water to form a metal hydroxide and hydrogen gas.
  • Halogens: a halogen displaces any halogen below it in the order .
ReactantsPredictionBalanced equation
zinc + copper(II) sulfateZn is above Cu: reactsZn + CuSO₄ → ZnSO₄ + Cu
copper + zinc sulfateCu is below Znno reaction
magnesium + hydrochloric acidMg is above H: reactsMg + 2HCl → MgCl₂ + H₂
copper + hydrochloric acidCu is below Hno reaction
sodium + watervery reactive metal2Na + 2H₂O → 2NaOH + H₂
chlorine + sodium bromideCl₂ is above Br₂: reactsCl₂ + 2NaBr → 2NaCl + Br₂
iodine + sodium chlorideI₂ is below Cl₂no reaction
Using the activity series to predict single displacement reactions.

Double displacement: AB + CD → AD + CB

In a double displacement reaction, two ionic compounds in solution swap partners: the cation of each compound pairs up with the anion of the other. To predict the products, keep each positive ion and switch the negative ions, then write each new formula from the ion charges (do not just copy the old subscripts). A double displacement reaction actually happens when one of the products is a precipitate (an insoluble solid), a gas, or water. If both possible products stay dissolved, the ions just mix and there is no reaction.

  • Compounds of group 1 ions (Li⁺, Na⁺, K⁺) and ammonium (NH₄⁺) are soluble.
  • All nitrates (NO₃⁻) and acetates are soluble.
  • Most chlorides are soluble; silver chloride (AgCl) is the common insoluble one, and lead(II) chloride is only slightly soluble in cold water (more dissolves in hot water).
  • Most carbonates, phosphates and hydroxides are insoluble, except those of group 1 and ammonium.
  • If a question tells you which product is insoluble, give that product the state symbol (s) and the others (aq).
Lead(II) iodide is insoluble, so it forms the yellow precipitate; potassium nitrate stays dissolved.

Combustion: fuel + O₂ → CO₂ + H₂O

Combustion is the rapid reaction of a fuel with oxygen, releasing energy as heat and light. Many of the fuels we burn are hydrocarbons, compounds of only carbon and hydrogen, such as methane (, natural gas), propane () and octane (, in gasoline). With plenty of oxygen, complete combustion turns all the carbon into carbon dioxide and all the hydrogen into water, with a clean blue flame. With too little oxygen, incomplete combustion also makes carbon monoxide (CO) and soot (C), with a yellow, smoky flame and less energy released.

Complete combustion of methane (left) and one form of incomplete combustion (right).
Carbon monoxide is a silent danger
Carbon monoxide has no colour, odour or taste. It binds to the hemoglobin in red blood cells more than 200 times as strongly as oxygen does, so it stops blood from carrying oxygen. A furnace, water heater or fireplace that is not burning properly can produce it, which is why Ontario's Fire Code requires carbon monoxide alarms in homes with a fuel-burning appliance, a fireplace or an attached garage.
Identify the reaction typeAsk the questions in order; the first yes gives the type.O2 reacts with a fuel(a hydrocarbon, CxHy)?yesCombustionCxHy + O2 → CO2 + H2OC3H8 + 5O2 → 3CO2 + 4H2OnoOnly one reactant?(one compound breaks apart)yesDecompositionAB → A + B2H2O → 2H2 + O2noTwo or more reactantsbut only one product?yesSynthesisA + B → AB2Na + Cl2 → 2NaClnoAn element + a compound?(the element swaps places)yesSingle displacementA + BC → AC + BZn + CuSO4 → ZnSO4 + CunoTwo compounds: an acidand a base (a hydroxide)?yesNeutralizationacid + base → salt + waterHCl + NaOH → NaCl + H2OnoDouble displacementAB + CD → AD + CBAgNO3 + NaCl → AgCl + NaNO3Neutralization is a specialdouble displacement that makes water.
Fig 2A flowchart for classifying reactions: oxygen with a hydrocarbon fuel is combustion; one reactant is decomposition; one product is synthesis; an element plus a compound is single displacement; an acid plus a base is neutralization; otherwise two compounds swapping partners is double displacement. Each branch shows an example equation.

Worked examples

Reveal one step at a time and try to beat it to the next line.

Example 1
A copper wire is placed in silver nitrate solution. Predict whether a reaction occurs and, if so, write the balanced equation. (Copper forms Cu²⁺ in this reaction.)
Example 2
Predict the result of adding (a) zinc and (b) silver to hydrochloric acid.
Example 3
Barium chloride solution is mixed with sodium sulfate solution. Barium sulfate is insoluble. Write the balanced equation.
Example 4
Write the balanced equation for the complete combustion of propane, .

Where marks get lost

  • Predicting a single displacement without checking the activity series. If the free metal is below the metal in the compound, the answer is "no reaction".
  • Copying old subscripts into double displacement products, such as writing instead of NaCl. Rebuild each new formula from the ion charges.
  • Swapping the wrong parts in a double displacement, like pairing two cations. A cation always pairs with an anion.
  • Forgetting that oxygen is in combustion, or forgetting water as a product when the fuel contains hydrogen.
  • Balancing oxygen before carbon and hydrogen in a combustion equation; doing oxygen last saves a lot of rework.

Flashcards 8

Single displacement
Double displacement
Activity series rule
Halogen activity order
+ 4 more

Practice

1
Which metal will displace copper from copper(II) sulfate solution?
2
Chlorine gas is bubbled through sodium iodide solution. What are the products?
3
Silver nitrate solution is mixed with sodium chloride solution, and a white precipitate forms. Write the formula of the precipitate (type it on one line, e.g. type CaCO3 for calcium carbonate).
4
Name the two products of the complete combustion of a hydrocarbon.
5
Balance the complete combustion of butane: , using the lowest whole numbers. What coefficient goes in front of ?
6
Classify the reaction .
7
A furnace is burning natural gas with too little air. Which products are most likely besides water?
8
What type of reaction is ?
9
Match each general pattern to its reaction type.
A + B → AB
AB → A + B
A + BC → AC + B
AB + CD → AD + CB
CₓHᵧ + O₂ → CO₂ + H₂O

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