Chemistry 1.            


Test Review--Chemical Equations & Types of Reactions

Clich Here for Review Practice Test


On the test students should be able write balanced chemical equations, identify the type of reaction in a chemical change or chemical equation, complete a chemical equation by predicting the products of various types of reactions.


Writing Chemical equations:


Useful steps in completing a balanced chemical equation:


Step 1: Know the facts about the reactions including all reactants and products. This information may come from (a) scientific inquiry, ie. experiments and observations, (b) theoretical knowledge of types of reaction, periodic trends and element families and classes of compounds, or (c) information given in the question or problem.


Step 2: Write a word equation. Give the name of each reactant separated by “+” signs followed by a yields sign “———>” . Then write the name of each product separately.


ex.1: sodium + chlorine ———> sodium chloride

ex.2: calcium hydroxide———>calcium oxide and water vapor


Step 3: Use the rules for writing chemical formulas to write the correct formula for each reactant and product.


ex.1: Na + Cl2 ———> NaCl

ex.2: Ca(OH)2 ———> CaO + H2O(g)


Practice writing balanced equations:


§         ___  FeCl3 +   _____ H2S    ———>   ___  FeCl2   +  ____ S   +  ____ HCl


§         lithium hydroxide  ———>   lithium oxide  +  water


§         calcium hydroxide  +  acetic acid  ———>  water  +  calcium acetate


§         ferric nitrate  +  ammonia  + water  ———>  ferric hydroxide  +  ammonium nitrate


§         hydrogen  +  oxygen  ———>  water


Note 1: remember that the 7 diatomic elements: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine (HOFBrINCl). When writing the formulas for these elements only use the subscript 2. ex. H2  or Br2


Note 2: remember to apply the rules for ionic compounds, molecular compounds and acids when writing the formulas. Each formula must be correct regardless of how each is rearranged during the chemical change. Check each ion and use “cross-over” when required.


Note 3: additional symbols may be used to give extra information about the reaction.

These are some common examples:


§         (g) gas or vapor 

§         (l) liquid

§         (s) solid or precipitate

§         (aq) aqueous or dissolved in water


β     precipitate


α     gas bubbles



Electric Current or electrolysis






Heated to 650oC








Step 4: Use the law of conservation of mass to balance the equation by placing coefficients in front of each formula as required. Do not change the formulas. Do not change or add subscripts. Only use coefficients to balance at this step. If you have problems you might want to check to make sure you did step 3 correctly.


The number and type of each atom in the reactant must be the same as the atoms in the products.


ex.1 unbalanced equation  H2O  ———> H2 + O2

            balanced equation 2H2O ———> 2H2 + O2

            the correct coefficients are 2,2,1


ex.2 unbalanced equation Pb(NO3)2  + KI ———> KNO3 + PbI2 (s)  

            balanced equation   Pb(NO3)2 + 2KI ———> 2 KNO3 + PbI2 (s)

            the correct coefficients are 1,2,2,1


Predicting Reactions:

These reaction types are somewhat simplified. There are many variations and combinations, as well as other reactions not included here. For this test students will be required to know the basic types and are not responsible for variants or exceptions, etc.


Oxidation-Reduction Reactions (Redox reactions)

In these reactions the oxidation state of one atom in reduced and another is increased. Oxidation occurs when an atom achieves a higher oxidation state (more positive) by losing electrons. Reduction occurs when an atom decreases its oxidation state (becomes more negative) by gaining electrons.

§         Li ———> Li+ + 1e-  (oxidation)

§         O2 + 4e- ———> 2 O -2   (reduction)


There are various types of oxidation-reduction reactions including: synthesis, decomposition, single replacement, and combustion.


Synthesis Reactions (also called combination and composition): two or more reactants are put together to form one product.

§         element + element ———> compound

§         compound + compound ———> compound (often not redox)


general equation A + X ---> AX (A is oxidized, X is reduced)


Example 1

§         sodium  + chlorine ———> sodium chloride

§         2Na + Cl2 ———> 2 NaCl


§         calcium oxide  +  carbon dioxide ———>  calcium carbonate

§         CaO  + CO2 ———>CaCO3 (not redox)


Decomposition Reactions: one compound breaks apart into two or more products. Often this process requires heat, thermal decomposition, or an electric current, electrolysis.

§         compound ———> element + element

§         compound ———> compound + compound (often not redox)


general equation AX ———> A  + X (A is reduced, X is oxidized)


Example 1:

§         HgO ———> Hg + O2


Exampe 2:

§         calcium hydroxide is decomposed by heat to produce water and calcium oxide

§         calcium hydroxide -----> calcium oxide and water











(Not redox)



Single Replacement Reactions: an active element replaces a less active element in a compound, usually this element is an active metal but may also be a halogen.


replacement by metals       

§         active metal  + metallic compound ———> less active metal + metallic compound

general equation A + BX ———> B + AX           (A is oxidized, B is reduced)


§         iron + cupric chloride   ———>          copper  +  ferric chloride

§         2 Fe(s) + 3 CuCl2(aq) ———> 3 Cu(s) + 2 FeCl3(aq)


replacement by halogens

§         active halogen + metallic halide ———>less active halogen + metallic halide

general reaction:

§         Y + BX ----> X + BY (Y is reduced, X is oxidized)


§         chlorine gas + aqueous sodium iodide ———> solid iodine and aqueous sodium chloride

§         Cl2(g) + 2 NaI (aq)  ———>  I2 (s) +  2 NaCl (aq)


Combustion Reactions: commonly called burning and most often associated with burning hydrocarbons, although other fuels like carbohydrate and lipids are also examples. Hydrocarbon fuels contain only carbon and hydrogen atoms, like methane, propane, butane and octane.


§         Complete combustion

§         fuel + oxygen ———> carbon dioxide and water

§         (the carbon in the fuel is oxidized, oxygen is reduced)



§         methane  + oxygen  ———>  carbon dioxide and water

§         CH4   +   2 O2  ———>CO2  +   2 H2O


Incomplete combustion: when there is insufficient oxygen available all of the carbon is not oxidized and a mixture of products are possible including carbon, carbon monoxide and carbon dioxide. (incomplete combustion is not covered on test)


Reactions which are not redox.


Double Replacement reactions: two compound  react to produce two new compounds. These reactions are typically in aqueous solutions and are called ionic reactions. There are two general types precipitation (in which one of the products precipitates--forms an insoluble solid) and acid-base neutralization (in which an acid and base produce salt and water)


compound + compound ———> compound + compound

general reaction       

§         AX  +   BY  ———>   AY  +    BX




aqueous compound  + aqueous compound ———> solid compound  + aqueous compound



§         lead nitrate  +  potassium iodide  ———> lead iodide  +  potassium nitrate

§         Pb(NO3)2 (aq)  +  2 KI (aq)  ———>  PbI2 (s)   + 2 KNO3 (aq)



acid base neutralization

§         acid   +   base   ———>    water   +  salt


ex. 2

§         hydrochloric acid  + sodium hydroxide ———> sodium chloride  + water

§         HCl  + NaOH ———>NaCl  +  H2O


note : in acid - base reactions   H+   +  OH -  ———>  (HOH ) written as  H2O



§         zinc hydroxide   + nitric acid   ———>  zinc nitrate  +  water

§         Zn(OH)2 (aq)  +  2 HNO3 (aq)  ———>  Zn(NO3)2 (aq)  +  2 H2O (l)


Web Page by Ken Smith, Pattonville High School, 2004.
    E-mail @ Ken Smith