Characteristics of Chemical Reactions

A chemical reaction is a process in which reactants are converted into products. Chemical equations are used to illustrate chemical reactions. In a chemical reaction, the conversion of reactants into products results in some observable features known as characteristics of chemical reactions. The important characteristics are the Evolution of a gas, formation of a precipitate, Change in colour, Change in temperature and Change in the state. Anyone of these general characteristics can tell us whether a reaction had taken place or not.

What is a Chemical Reaction?

When two or more molecules combine to produce a new product, it is called a chemical reaction (s). Reactants are chemicals that combine to generate new compounds, whereas products are newly produced compounds. 

Chemical reactions are important in a variety of businesses, cultures, and even our daily lives. They are always occurring in our environment, such as rusting of iron, pottery, and wine fermentation, to name a few. A chemical change must occur in a chemical reaction, which is commonly observed with physical changes such as precipitation, heat generation, colour change, and so on.

A reaction can occur between two atoms, ions, or molecules, in which they establish a new link without destroying or creating an atom, yet a new product is generated from reactants. The rate of reaction is influenced by variables such as pressure, temperature, and reactant concentration.

Characteristics of Chemical Reactions

In a chemical reaction, the transformation of reactants into products is frequently accompanied by specific characteristics that may be seen. Chemical reactions have a number of significant properties like:

Evolution of a Gas

Some chemical reactions are characterized by the evolution of gas.

• The chemical reaction between sodium sulfite and dilute hydrochloric acid is characterized by the evolution of sulfur dioxide gas.

Na₂SO₃ + 2HCl ⇢ 2NaCl + H₂O + SO₂

• The chemical reaction between zinc and sulfuric acid is characterized by the evolution of sulfur dioxide hydrogen gas.

Zn + H₂SO₄ ⇢ ZnSO₄ + H₂

Formation of a Precipitate

Some chemical reactions are characterized by the formation of a precipitate.

• The chemical reaction between copper sulfate and sodium hydroxide is characterized by the formation of a blue precipitate of copper hydroxide.

CuSO₄ + 2NaOH ⇢ Na₂SO₄ + Cu(OH)₂

• The chemical reaction between silver nitrate and sodium chloride is characterized by the formation of a white precipitate of silver chloride.

AgNO₃ + NaCl ⇢ NaNO₃ + AgCl

Change in Color

Some chemical reactions are characterized by changes in colour.

• The chemical reaction between citric acid and potassium permanganate solution is characterized by a change in colour from purple to colourless. Citric acid oxidizes potassium permanganate because it is a reducing agent.

3KMnO₄ + C₆H₈O₇ ⇢ 3HMnO₄ + C₆H₅K₃O₇

• The chemical reaction between sulfur dioxide and acidified potassium dichromate solution is characterized by a change in colour from orange to green.

SO₂ + K₂Cr₂O₇ + 3H₂SO₄ ⇢ K₂SO₄ +Cr₂(SO₄)₃ + 3H₂O

Change in Temperature

• The chemical reaction between quicklime and water to form slaked lime is characterized by a change in rising in temperature.

CaO + H₂O ⇢ Ca(OH)₂

• The chemical reaction between barium hydroxide and ammonium chloride to form barium chloride, ammonia and water are characterized by a change in fall in temperature.

BaCl₂ + NH₄OH ⇢ Ba(OH)₂ + NH₄Cl

Change in State

Some chemical reactions are characterized by changes in state.

• The combustion reaction of candle wax is characterized by a change in state from solid to liquid and gas.

C_XH_{Y +} O₂ ⇢ CO₂ + H₂O + Heat and Light

Sample Questions

Question 1: State the various characteristics of chemical reactions.

Answer:

The characteristics of a chemical reaction are: Evolution of gas, Formation of a precipitate, Change in colour, Change in temperature, Change in state etc.

Question 2: State two characteristics of the chemical reaction which takes place when diluting sulfuric acid is poured over zinc granules.

Answer:

When the dilute sulfuric acid is poured over zinc granules, hydrogen gas is evolved and the mixture becomes hot. Hence, “evolution of a gas” and “change in temperature” are the two characteristics of this chemical reaction.

Question 3: Give two characteristics of the chemical reaction which occurs on adding potassium iodide solution to lead nitrate solution. 

Answer:

On adding potassium iodide solution to lead nitrate solution, yellow precipitates of lead iodide are formed and the colour of the solution changes from colourless to yellow. Hence, “formation of a precipitate” and “change in colour” are the two characteristics of this chemical reaction.

Question 4: State one characteristic of chemical reaction which takes place when lemon juice is added gradually to potassium permanganate solution.

Answer:

When lemon juice is added gradually to potassium permanganate solution, there is a change in colour from purple to colourless.  Hence, this reaction is characterized by a “change in colour”.

Question 5: State one characteristic of chemical reaction which takes place when quick lime is treated with water.

Answer: 

When quicklime is treated with water, slaked lime is formed with the evolution of a large amount of heat. Hence, this reaction is characterized by a “change in temperature”.

Question 6: State one characteristic of chemical reaction which takes place when the wax is burned in the form of a candle.

Answer: 

When the wax is burnt in the form of a candle, it produces water and carbon dioxide.  Hence, this reaction is characterized by the “evolution of gas”.

Question 7: State one characteristic of chemical reaction which takes place when diluting hydrochloric acid is added to sodium carbonate.

Answer:

When sodium carbonate is added to dilute hydrochloric acid, carbon dioxide gas evolves. Hence, this reaction is characterized by the “evolution of gas”.