C. Chemical Reactions · Chemical ReactionsPredicting Products: Synthesis Reactions
C2 C3About 70 minutes
Learning goals- I can predict the product of a metal reacting with a non-metal, including metals with more than one ion charge.
- I can predict the products when metal oxides and non-metal oxides react with water.
- I can predict the salt formed when a metal oxide reacts with a non-metal oxide.
- I can write balanced equations, with states, for synthesis reactions and connect them to real situations like rust, acid rain and lime.
Words to know
synthesis reaction
Two or more reactants combine to form a single product.
binary compound
A compound made of exactly two elements, such as NaCl or Al₂O₃.
basic oxide
A metal oxide; it reacts with water to form a metal hydroxide (a base).
acidic oxide
A non-metal oxide; it reacts with water to form an acid.
oxyacid
An acid containing hydrogen, oxygen and another element, such as H₂SO₄ or H₂CO₃.
multivalent metal
A metal that can form more than one ion, such as iron (Fe²⁺ or Fe³⁺). The Roman numeral in the name gives the charge.
Building something new
A rusty shovel left out in a snowy Canadian winter, the brilliant flash of a magnesium sparkler, and the slaked lime used to treat acidic soil and water all start with a synthesis reaction: simple substances joining to make one new compound.
In this lesson you will learn four families of synthesis reactions. For each family there is a rule that lets you predict the product. Your job is to recognize which family a reaction belongs to and then apply its rule.
Family 1: metal + non-metal → binary ionic compound
A metal gives electrons to a non-metal. The ions that form pack into an ionic crystal. To predict the product:
- Write the ions each element forms.
- Combine them so the total charge is zero.
- Write the skeleton equation (remember diatomic non-metals).
- Balance.
Examples: 2Na(s) + Cl₂(g) → 2NaCl(s); 2Mg(s) + O₂(g) → 2MgO(s); 3Ca(s) + N₂(g) → Ca₃N₂(s).
Multivalent metals. Iron, copper, lead, tin and others can form more than one ion. The question must tell you which ion forms, usually through the name of the product, for example iron(III) oxide. If both form, they are different reactions: 2Fe(s) + O₂(g) → 2FeO(s) makes iron(II) oxide, while 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s) makes iron(III) oxide. Rust is mostly hydrated iron(III) oxide.
Family 2: non-metal + non-metal → molecular compound
Two non-metals share electrons to form a molecular compound: S₈(s) + 8O₂(g) → 8SO₂(g), H₂(g) + Cl₂(g) → 2HCl(g), N₂(g) + 3H₂(g) → 2NH₃(g).
These are harder to predict, because two non-metals can often form several compounds (carbon forms CO and CO₂; sulfur forms SO₂ and SO₃). You need to be told the product, or use a well-known one. With plenty of oxygen, carbon forms carbon dioxide.
Family 3: oxides + water
Oxides react with water in two opposite ways, and this pattern is one of the most useful in chemistry.
Metal oxide + water → metal hydroxide (a base). Metal oxides are called basic oxides.
- CaO(s) + H₂O(l) → Ca(OH)₂(s) (quicklime becomes slaked lime, releasing a lot of heat)
- Na₂O(s) + H₂O(l) → 2NaOH(aq)
To predict: the metal keeps its ion charge and pairs with hydroxide, OH⁻.
Non-metal oxide + water → acid. Non-metal oxides are called acidic oxides.
- CO₂(g) + H₂O(l) → H₂CO₃(aq) (carbonic acid, in sparkling water and rain)
- SO₂(g) + H₂O(l) → H₂SO₃(aq) (sulfurous acid)
- SO₃(g) + H₂O(l) → H₂SO₄(aq) (sulfuric acid)
To predict: add up all the atoms of the oxide and the water, and write them in acid order: H first, then the non-metal, then O. For example, SO₃ + H₂O has 2 H, 1 S and 4 O: H₂SO₄. Sometimes you need more water: N₂O₅(g) + H₂O(l) → 2HNO₃(aq).
These reactions are why sulfur dioxide and nitrogen oxides from smelters, power plants and vehicles cause acid rain. You will look at that problem in the lesson on industrial reactions.
Family 4: metal oxide + non-metal oxide → salt
A basic oxide and an acidic oxide can combine directly into an ionic compound with a polyatomic ion:
- CaO(s) + CO₂(g) → CaCO₃(s)
- CaO(s) + SO₂(g) → CaSO₃(s)
The non-metal oxide plus an oxide ion builds the polyatomic ion: CO₂ + O²⁻ gives carbonate, CO₃²⁻; SO₂ + O²⁻ gives sulfite, SO₃²⁻. The second reaction is used in some scrubbers that remove sulfur dioxide from smokestack gases.
Worked exampleMetal + non-metal with a multivalent metal
Copper metal is heated in chlorine gas to form copper(II) chloride. Write the balanced equation.
- Copper(II) means Cu²⁺. Chloride is Cl⁻. Two chloride ions balance one copper(II) ion: CuCl₂.
- Chlorine gas is diatomic: Cl₂.
- Skeleton: Cu(s) + Cl₂(g) → CuCl₂(s). Count: Cu 1 = 1, Cl 2 = 2. Already balanced.
Cu(s) + Cl₂(g) → CuCl₂(s)
Worked examplePredicting from the reactants only
Predict the product and write a balanced equation: aluminium + bromine.
- Aluminium is a metal and bromine is a non-metal: Family 1, a binary ionic compound.
- Ions: Al³⁺ and Br⁻. Neutral formula: AlBr₃ (aluminium bromide).
- Bromine is diatomic and a liquid at room temperature: Br₂(l).
- Skeleton: Al(s) + Br₂(l) → AlBr₃(s). Bromine is 2 vs 3, so use 3Br₂ and 2AlBr₃, then 2Al.
2Al(s) + 3Br₂(l) → 2AlBr₃(s)
Worked exampleOxide + water
Predict the products: (a) potassium oxide + water; (b) dinitrogen trioxide + water.
- (a) Potassium oxide, K₂O, is a metal oxide, so it forms a hydroxide. Potassium is K⁺, so KOH. Skeleton: K₂O + H₂O → KOH. Two K need 2KOH; then H 2 = 2 and O 2 = 2.
- (b) N₂O₃ is a non-metal oxide, so it forms an acid. Add the atoms: N₂O₃ + H₂O has 2 H, 2 N and 4 O. That splits evenly into two molecules of HNO₂ (nitrous acid).
- Check (b): left N 2, O 3 + 1 = 4, H 2. Right 2HNO₂: H 2, N 2, O 4. ✔
(a) K₂O(s) + H₂O(l) → 2KOH(aq) (b) N₂O₃(g) + H₂O(l) → 2HNO₂(aq)
Worked exampleTwo oxides together
Predict the product: magnesium oxide reacts with carbon dioxide.
- A metal oxide plus a non-metal oxide gives a salt with a polyatomic ion.
- CO₂ plus the oxide ion gives carbonate, CO₃²⁻. Magnesium is Mg²⁺, so the salt is MgCO₃, magnesium carbonate.
- Count: Mg 1, C 1, O 1 + 2 = 3 on the left; MgCO₃ has Mg 1, C 1, O 3. ✔
MgO(s) + CO₂(g) → MgCO₃(s)
Hands-on activityFast rusting: a synthesis you can feel
You need: a pad of plain steel wool (the kind with no soap in it, from a hardware store), white vinegar, a small jar, rubber gloves, a kitchen or outdoor thermometer, and a grown-up.
Safety: wear the gloves, because steel wool sheds tiny sharp fibres. Work on newspaper and wrap the used steel wool in paper before putting it in the garbage. Do not rub your eyes. Keep steel wool away from flames, sparks and batteries: fine steel wool is flammable.
- Put the thermometer in the empty jar for two minutes and record the temperature.
- Soak half a pad of steel wool in vinegar for one minute. (The vinegar removes the protective coating on the iron.) Squeeze out the extra vinegar.
- Wrap the steel wool around the bulb of the thermometer, put both in the jar and close the lid loosely.
- Record the temperature every minute for 5 minutes.
- Leave a small piece of the damp steel wool on a saucer for two days and look at it again.
What to expect: the temperature rises by a few degrees, because iron combining with oxygen is exothermic. After two days the steel wool shows orange-brown rust.
Questions: Which reactants combined? Write the simplified equation for iron(III) oxide forming. Why do road salt and wet snow make cars rust faster in Canadian winters? (Salt water conducts electricity and speeds up the reactions that corrode iron.)
Just for funRust is just iron's way of saying it has been in a long-term relationship with oxygen. Fun fact: a lot of the iron mined in the world comes from rocks containing iron oxides, which is basically ancient rust. Steelmakers spend huge amounts of energy separating iron and oxygen again, and given the chance, the iron slowly bonds right back.
Your turn
1Predict the product and balance: lithium + oxygen.
4Li(s) + O₂(g) → 2Li₂O(s). Li⁺ and O²⁻ give Li₂O; 2 O atoms need 2Li₂O, which need 4 Li.
2Predict the product and balance: calcium + chlorine.
Ca(s) + Cl₂(g) → CaCl₂(s)
3Write a balanced equation for iron reacting with oxygen to form iron(II) oxide.
2Fe(s) + O₂(g) → 2FeO(s). Iron(II) is Fe²⁺, so the formula is FeO.
4Predict the product: barium oxide + water.
BaO(s) + H₂O(l) → Ba(OH)₂(aq). A metal oxide gives a metal hydroxide; Ba²⁺ needs two OH⁻.
5Predict the product: sulfur trioxide + water.
SO₃(g) + H₂O(l) → H₂SO₄(aq), sulfuric acid. A non-metal oxide gives an acid.
6Predict the product: calcium oxide + sulfur trioxide.
CaO(s) + SO₃(g) → CaSO₄(s), calcium sulfate. SO₃ plus an oxide ion makes sulfate, SO₄²⁻.
More practice: warm-up
1What type of compound forms when a metal reacts with a non-metal?
2Is CO₂ an acidic oxide or a basic oxide?
Acidic oxide, because carbon is a non-metal. With water it forms carbonic acid.
3Is MgO an acidic oxide or a basic oxide?
Basic oxide, because magnesium is a metal. With water it forms magnesium hydroxide.
4Balance: K(s) + Br₂(l) → KBr(s)
5Name the product of 2Mg + O₂ → 2MgO.
More practice: on your own
1Write a balanced equation: aluminium + sulfur → aluminium sulfide. (Use S₈ for sulfur.)
16Al(s) + 3S₈(s) → 8Al₂S₃(s). Al³⁺ and S²⁻ give Al₂S₃. 3 S₈ = 24 S = 8 Al₂S₃, which need 16 Al. (If your teacher writes sulfur as S, the answer is 2Al + 3S → Al₂S₃.)
2Write a balanced equation for tin reacting with oxygen to form tin(IV) oxide.
Sn(s) + O₂(g) → SnO₂(s). Tin(IV) is Sn⁴⁺, which needs two O²⁻.
3Predict the acid formed when diphosphorus pentoxide (P₂O₅) reacts with water, and balance.
P₂O₅(s) + 3H₂O(l) → 2H₃PO₄(aq), phosphoric acid. Each H₃PO₄ has 1 P, so 2 P give 2 acid molecules, which need 6 H (3 water) and 8 O (5 + 3). ✔
4Predict the product and balance: sodium oxide + carbon dioxide.
Na₂O(s) + CO₂(g) → Na₂CO₃(s), sodium carbonate.
5Explain why the product of carbon + oxygen cannot be predicted with certainty, but the product of sodium + chlorine can.
Carbon and oxygen are both non-metals and can form more than one compound (CO or CO₂, depending on how much oxygen there is). Sodium only forms the Na⁺ ion and chlorine only forms Cl⁻ with metals, so the only product is NaCl.
Challenge
1Ammonia for fertilizer is made by a synthesis reaction. Write it, and explain why it matters for Canadian farms.
N₂(g) + 3H₂(g) → 2NH₃(g) (the Haber process). Plants need nitrogen compounds to grow, but cannot use N₂ from air directly. Ammonia and fertilizers made from it supply usable nitrogen for crops such as canola, wheat and corn.
2Quicklime (CaO) is spread on some fields and added to water treatment. Write the reaction of quicklime with water and explain why bags of quicklime warn users to keep it dry and away from skin and eyes.
CaO(s) + H₂O(l) → Ca(OH)₂(s). The reaction is strongly exothermic, so wet quicklime can get very hot, and the product, calcium hydroxide, is a base that irritates and can burn skin and eyes. Moisture on skin or in eyes is enough to start the reaction.
3Explain how the sequence 2SO₂ + O₂ → 2SO₃ followed by SO₃ + H₂O → H₂SO₄ turns smokestack gas into acid rain. Classify both reactions.
Sulfur dioxide released by burning or smelting is oxidized in the air to sulfur trioxide (a synthesis). Sulfur trioxide then dissolves in cloud droplets and combines with water to form sulfuric acid (another synthesis: a non-metal oxide + water gives an acid). The acid falls as rain or snow, lowering the pH of lakes and soils.
Quiz
1. What is the product when calcium oxide reacts with water?
2. Which product forms when sulfur dioxide reacts with water?
3. What is the balanced equation for sodium + oxygen forming sodium oxide?
4. Iron reacts with chlorine to form iron(III) chloride. What is the product's formula?
5. A metal oxide reacting with a non-metal oxide produces:
6. Which oxide forms an acid in water?
The key idea is that synthesis reactions come in families, each with a rule. Ask your teen to sort a list of reactions into the four families before predicting anything.
Common difficulties:
- Wrong ionic formulas, especially for multivalent metals (iron(II) vs iron(III)). Keep a table of ions handy.
- Forgetting that oxygen, chlorine, bromine and the others are diatomic.
- Mixing up which oxides make acids and which make bases. Memory hook: Metal oxides make More basic solutions; Non-metal oxides make acids (think Nitric, sulfuric).
Safety for the activity: steel wool sheds sharp fibres, so gloves are needed, and a grown-up should supervise. Vinegar is safe but should be kept away from eyes.