Chemical Reactions, Stoichiometry, and Chemical kinetics

1. TYPES OF CHEMICAL REACTION AND THEIR STOICHIOMETRY

There are thousands of different chemical reactions. It would be impossible to memorize them all. However, most chemical reactions can be categorised into five major groups. Understanding these categories of reactions can help you predict how reactants will react and what products will form.

Chemical reactions are processes in which substances (reactants) are transformed into new substances (products).

There are five major types of chemical reactions:

1. Combination (Synthesis) Reactions
2. Decomposition Reactions
3. Single Displacement (Replacement) Reactions
4. Double Displacement (Metathesis) Reactions
5. Combustion Reactions

 

Combination (Synthesis) Reactions

A synthesis reaction is a type of chemical reaction in which two or more different substances (elements or compounds) combine and form one compound. Synthesis means “putting together”. You can recognise a synthesis reaction because two or more reactants form only one product.

The general format of a synthesis reaction is shown below.

A + B → AB

Examples:

·         2H₂ ​+ O₂ ​→ 2H_2​O

·         CaO + CO₂ ​→ CaCO₃

·         2Na + Cl₂ → 2NaCl

 

Decomposition Reactions

In a decomposition reaction, one compound breaks down into two or more simpler substances. Notice that decomposition is the reverse of synthesis. You can recognise a decomposition reaction because one reactant forms two or more products. The general format of a decomposition reaction is shown below.

AB → A + B

Examples:

• Decomposition of water by electrolysis
  2H₂O → 2H₂ + O₂​
• Decomposition of calcium carbonate
  CaCO₃ → CaO + CO_2​
• Decomposition of hydrogen peroxide
  2H₂O₂ → 2H₂O + O₂​

 

Single Displacement (Replacement) Reactions

In a single replacement (also called single displacement), one element replaces another element in a compound. In this type of reaction, an element and a compound react to form a different element and a different compound. The general format of a decomposition reaction is shown below.

A + BC → AC + B
(Where A is a more reactive element that displaces B)

Examples:

·         Zinc replaces hydrogen from hydrochloric acid
Zn + 2HCl → ZnCl₂ + H₂​

·         Iron replaces copper from copper (II) sulfate
Fe + CuSO₄ → FeSO₄ ​+ Cu

Note: Reactivity is based on the activity series of metals.

 

Double Displacement (Metathesis) Reactions

In a double replacement (also called double displacement), the positive ions in two compounds switch places, forming two new compounds. In this type of reaction, two compounds react and form two new compounds. The general format of a double replacement reaction is shown below.

AB + CD → AD + CB

Examples:

• Reaction between sodium sulfate and barium chloride

Na₂SO₄ + BaCl₂ → BaSO₄(s) + 2NaCl

• Neutralisation of hydrochloric acid with sodium hydroxide

HCl + NaOH → NaCl + H₂O

 

Combustion Reactions

Combustion is a chemical reaction in which a substance reacts with oxygen and releases energy. This energy is usually released as thermal energy and light energy. For example, burning is a common combustion reaction. The general format of a combustion reaction of a hydrocarbon (a compound made of hydrogen and carbon) is shown below. The products of the combustion of a hydrocarbon are always CO₂ and H₂O.

Cx​Hy​ + O₂ ​→ CO₂​ + H_2​O

Examples:

• Combustion of methane
  CH₄ + 2O₂ → CO₂ + 2H₂O
• Combustion of propane
  C₃H₈ + 5O₂ → 3CO₂ + 4H₂O 

 

Special Cases and Additional Types of Reaction

• Redox Reactions: Involve transfer of electrons (oxidation-reduction).
• Precipitation Reactions: A solid forms from the mixing of two aqueous solutions.
• Acid-Base Reactions: A specific type of double displacement involving proton transfer.

    

                            REACTION STOICHIOMETRY

A balanced chemical equation provides a great deal of information in a very succinct format. Chemical formulas provide the identities of the reactants and products involved in the chemical change, allowing classification of the reaction. Coefficients provide the relative numbers of these chemical species, allowing a quantitative assessment of the relationships between the amounts of substances consumed and produced by the reaction. These quantitative relationships are known as the reaction’s stoichiometry; a term derived from the Greek words stoicheion (meaning “element”) and metron (meaning “measure”).

Stoichiometry: It is the study of the quantitative relationship conveyed by a chemical equation.

You must master the following chemistry concepts to solve stoichiometry problems:

1. Balancing a chemical equation

2. Converting between grams and moles

3. Calculating molar mass

4. Calculating mole ratio

 

Stoichiometric coefficient: Also known as the Stoichiometric number, is the number of molecules participating in the reaction. In simplicity, it is the number in front of atoms, molecules, and ions.

Note: Stoichiometric number can be a whole number or a fraction.

 

 

 

 

 

CHEMICAL ANALYSIS

Definition of terms used in volumetric analysis.

1. Mass concentration: The mass concentration of a solution is the amount of solute present in a given volume of the solution. It is expressed in g/dm³ or g/dm³.

Thus,

Mass concentration = molar concentration X molar mass

2. Molar concentration: The molar concentration (M) of a solution is the number of moles of solutes per dm³ of solution. It can also be defined as the concentration of a solution in moles per dm³.

The following equations are useful in calculations involving molar concentration.

·         Number of moles of a substance = Number of particles    mol^-1

         6.02 x 10²³

·         Number of moles of a substance = Mass of substance    mol^-1

        Molar mass

 

In volume per cm³ of solution, we have

·         Number of moles of solute = Volume    X molar concentration

          1000

3. Standard solution: A standard solution is a solution of known concentration.

4. Molar solution:  A molar solution of a compound contains one mole of the molar mass of the compound in one dm³ of the solution.

 

STOICHIOMETRIC PLAY QUESTIONS

Mass‐Mass relationship

1) What mass of barium chloride is required to react completely with 10.0 g of aluminium sulfate?

2) What mass of chlorine gas is required to react with 10.0 g of aluminium metal?

3) Reaction of hydrochloric acid with a sample of zinc hydroxide gave 0.555 g of zinc chloride. What was the mass of the zinc hydroxide sample?

4) When a barium chloride solution is mixed with a solution containing excess aluminium sulfate, 0.888 g of barium sulfate is obtained. What mass of barium chloride was contained in the solution?

5) 1.00 g of aluminium metal is treated with a solution containing 7.00 g of zinc chloride. What mass of metallic zinc will form?

 

Mass-mole\mole-mass relationship

6) Tin metal reacts with hydrogen fluoride to produce tin (II) fluoride and hydrogen gas, according to the following balanced equation.

Sn(s)+2HF(g)→SnF₂(s)+H₂(g)

How many moles of hydrogen fluoride are required to react completely with 75.0g of tin?

7) Hydrogen sulfide gas burns in oxygen to produce sulfur dioxide and water vapour: 2H₂S(g)+3O₂(g)→2SO₂(g)+2H₂O(g)

What mass of oxygen gas is consumed in a reaction that produces 4.60 mol SO₂?

8) 1.50 mol of KClO₃ decomposes according to this equation (2KClO₃→2KCl + 3O₂). How many grams of O₂ will be produced?

9) If 80.0 grams of O₂ were produced, how many moles of KClO₃ decomposed?

10) How many grams of H₂O are produced when 2.50 moles of oxygen are used?

2H₂ + O₂ → 2H₂O 

Mole-mole relationship

11) When 2.00 mol of N₂ reacts with sufficient H₂ (N₂ + 3H₂ → 2NH₃), how many moles of NH₃ will be produced?

12) Suppose 6.00 mol of H₂ reacted with sufficient nitrogen. How many moles of ammonia would be produced?

13) How many moles of water are produced when 3 moles of methane (CH₄) are combusted?

14) If 3 moles of aluminium react, how many moles of aluminium chloride (AlCl₃) are formed?

(2Al+3Cl₂​→2AlCl₃​)

 

15) How many moles of hydrogen gas will be produced from 10 moles of zinc?

(Zn+2HCl→ZnCl₂​+H₂​)

Mole-Volume relationship

16) What volume of oxygen gas is needed to completely combust 5 moles of methane according to this equation: CH₄​ + 2O₂ ​→ CO₂ ​+ 2H₂O

17) What volume of oxygen gas is produced when 4 moles of potassium chlorate decompose?

18) Na + H₂O → NaOH + H₂​ from the given equation, what volume of hydrogen gas is produced from 1.2 moles of sodium?

19) If 2.0 moles of sodium react with water, what volume of hydrogen gas is released at STP?

 

Volume-Mole relationship

20) You collected 22.4 L of oxygen gas from the decomposition of potassium chlorate. How many moles of O₂ did you collect?

21) You collected 67.2 L of CO₂ gas at STP during the reaction. How many moles of calcium carbonate (CaCO₃) decomposed? (CaCO₃ → CaO+CO₂)