Types of Chemical Reactions Online Test 10th Science Lesson 10 Questions in English

A chemical reaction involves breaking of old chemical bonds and formation of new chemical bonds. This change may happen spontaneously or it may be facilitated by external forces or energy. A chemical reaction does not involve in formation of a substance which has a property other than that of reactants.

Human gets energy from the digestion of the food you eat. Plants grow by absorbing nutrients from the Earth and get their food by photosynthesis. The combustion of a fuel makes the car to move. Oxidation of iron causes rusting.

In chemical reaction process chemical changes i.e. the materials, which undergo changes are converted into some other new materials. For example, by burning petrol, the hydrocarbons present in it are converted into carbon dioxide and water.

In a chemical reaction following changes happen: In a chemical reaction, the atoms of the reacting molecules or elements are rearranged to form new molecules. Old chemical bonds between atoms are broken and new chemical bonds are formed. Bond breaking absorbs energy whereas bond formation releases energy

In the chemical equation, the chemicals of the reaction are represented by their chemical formulas. Compounds or elements, which undergo reactions (reactants) are shown to the left of an arrow and the compounds formed (products) are shown to the right of the arrow. The arrow indicates the direction of the reaction.

When methane reacts with oxygen, it forms carbon dioxide and water. How can you represent this reaction? It can be written as a word equation as shown below: Methane + Oxygen → Carbon dioxide + Water CH4 + O2 → CO2 + H2 O

CH4 + O2 → CO2 + H2 O This is also an incomplete chemical equation. Because, the law of conservation of matter states that matter cannot be created or destroyed. You cannot create new atoms by a chemical reaction. In contrast, they are rearranged in different ways by a chemical reaction to form a new compound. So, in a chemical equation, the number of atoms of the reactants and that of the products must be equal.

A balanced chemical equation is the simplified representation of a chemical reaction which describes the chemical composition, physical state of the reactants and the products, and the reaction conditions.

The phases or the physical state of the substances in a chemical reaction are denoted in short form within a bracket, as the subscript of the formula, of the respective substances.

When solid potassium reacts with liquid water, it produces hydrogen gas and potassium hydroxide solution. All the information of the reaction is given in the chemical equation as shown below: 2K(s) + 2H2 O(I) → 2KOH (aq) + H2(g)

A combination reaction is a reaction in which two or more reactants combine to form a compound. It is otherwise called 'synthesis reaction' or 'composition reaction'.

Hydrogen gas combines with chlorine gas to form hydrogen chloride gas. H2(g) + Cl2(g) → 2HCl(g)

Depending on the chemical nature of the reactants, there are three classes of combination reactions: Element + Element → Compound Compound + Element → Compound Compound + Compound → Compound

When solid sulphur reacts with oxygen, it produces sulphur dioxide. Here both the reactants are non-metals. S(s) + O2(g) → SO2(g)

Sodium, a silvery-white metal, combines with chlorine, a pale-yellow green gas, to form sodium chloride, an edible compound. Here one of the reactants is a metal (sodium) and the other (chlorine) is a nonmetal. 2Na(s) + Cl2(g) → 2NaCl(s)

A compound reacts with an element to form a new compound. For instance, phosphorous trichloride reacts with chlorine gas and forms phosphorous pentachloride. PCl3(I) + Cl2(g) → PCl5(s)

In the following reaction, silicon dioxide reacts with calcium oxide to form calcium silicate. SiO2(s) + CaO(s) → CaSiO3(s) Most of the combination reactions are exothermic in nature. Because, they involve the formation of new bonds, which releases a huge amount of energy in the form of heat.

In a decomposition reaction, a single compound splits into two or more simpler substances under suitable conditions. It is the opposite of the combination reaction

Breaking of bonds is the major phenomenon in a decomposition reaction and hence it requires energy to break the bonds, depending on the nature of the energy used in the decomposition reaction. There are three main classes of decomposition reactions. They are Thermal Decomposition Reactions Electrolytic Decomposition Reactions Photo Decomposition Reactions

A solution of slaked lime is used for white washing walls. Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls.

Calcium carbonate is formed aft er two to three days of white washing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO3.

In Thermal Decomposition Reactions, the reactant is decomposed by applying heat. For example, on heating mercury (II) oxide is decomposed into mercury metal and oxygen gas. As the molecule is dissociated by the absorption of heat, it is otherwise called ‘Thermolysis’. It is a class of compound to element/element decomposition. i.e. a compound (Hg O) is decomposed into two elements (Hg and Oxygen).

When calcium carbonate is heated, it breaks down in to calcium oxide and carbon dioxide. It is a type of compound to compound/compound decomposition.

In thermal decomposition reaction, heat is supplied to break the bonds. Such reactions, in which heat is absorbed, are called ‘Endothermic reactions’

In some of the decomposition reactions, electrical energy is used to bring about the reaction. For example, decomposition of sodium chloride occurs on passing electric current through its aqueous solution. Sodium chloride decomposes in to metallic sodium and chlorine gas. Th is process is termed as ‘Electrolysis’.

Light is another form of energy, which facilitates some of the decomposition reactions. As the decomposition is caused by the light, this kind of reaction is also called ‘Photolysis’.

Th e yellow coloured silver bromide turns into grey coloured silver metal when placed in sunlight. It is also a compound to element/element decomposition.

Single Displacement Reactions is a reaction between an element and a compound. When they react, one of the elements of the compound-reactant is replaced by the element-reactant to form a new compound and an element. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

2NaCl(aq) + F2(g) → 2NaF(aq) + Cl2(g) 2NaF(aq) + Cl2(g) → 2NaCl(aq) + F2(g) Th e first reaction involves the displacement of chlorine from NaCl, by fluorine. In the second reaction, chlorine displaces fluorine from NaF. Out of these two, the second reaction will not occur. Because, fluorine is more active than chlorine and occupies the upper position in the periodic table.

When two compounds react, if their ions are interchanged, then the reaction is called double displacement reaction. Th e ion of one compound is replaced by the ion of another compound. Ions of identical charges are only interchanged, i.e., a cation can be replaced by other cations. Th is reaction is also called ‘Metathesis Reaction’

For a double displacement reaction to take place, one of the products must be a precipitate or water. By this way, there are major classes of double displacement reactions. They are: Precipitation Reactions Neutralization Reactions

When the clear aqueous solutions of potassium iodide and lead (II) nitrate are mixed, a double displacement reaction takes place between them. Pb(NO3 )2(aq) + 2KI(aq) → PbI2(s)↓ + 2KNO3(aq) Potassium and lead displace or replace one other and form a yellow precipitate of lead (II) iodide

In your lower classes, you have learned the reaction between an acid and a base. It is another type of displacement reaction in which the acid reacts with the base to form a salt and water. It is called 'neutralization reaction' as both acid and base neutralize each other.

Reaction of sodium hydroxide with hydrochloric acid is a typical neutralization reaction. Here, sodium replaces hydrogen from hydrochloric acid forming sodium chloride, a neutral soluble salt. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

A combustion reaction is one in which the reactant rapidly combines with oxygen to form one or more oxides and energy (heat). So, in combustion reactions one of the reactants must be oxygen. Combustion reactions are majorly used as heat energy sources in many of our day to day activities. For instance, we use LPG gas for domestic cooking purposes.

LPG is a mixture of hydrocarbon gases like propane, butane, propylene, etc. All these hydrocarbons burn with oxygen to form carbon dioxide and water. C3 H8(g) + 5O2(g) → 3CO2(g) + 4H2 O(g) + Heat

Since heat is evolved, it is an exothermic reaction. As oxygen is added, it is also an oxidation. So, combustion may be called as an exothermic oxidation.

Digestion of Food Rusting of iron None of the reaction is combustion reaction, because digestion food is endothermic reaction and rusting of iron does not produce energy.

You know that innumerable changes occur every day around us. For example, liquid water freezes into ice, but then ice melts into liquid water. In other words, freezing is reversed. So, it is not a permanent change. Moreover, it is a physical change. Physical changes can be reversed easily.

Let us consider the burning of a wood. The carbon compounds present in the wood are burnt into carbon dioxide gas and water. Can we get back the wood immediately from carbon dioxide and water? We cannot. So, it is a permanent change. In most of the cases, we cannot. But, some chemical reactions can be reversed. Our mobile phone gets energy from its lithium-ion battery by chemical reactions. It is called discharging. On recharging the mobile, these chemical reactions are reversed. Thus, chemical reactions may be reversed under suitable conditions. Hence, they are grouped into two categories such as reversible and irreversible reactions.

A reversible reaction is a reaction that can be reversed, i.e., the products can be converted back to the reactants. A reversible reaction is represented by a double arrow with their heads in the direction opposite to each other.

The forward reaction is the decomposition of PCl5 and the backward reaction is the combination of PCl3 and Cl2. Initially, the forward reaction proceeds faster than the backward reaction. Aft er sometimes, the speed of both the reactions become equal. So, PCl5 cannot be completely converted into the products as the reaction is reversed. It is a reversible reaction. Th e actual measurements of the given reaction show that the reaction is at equilibrium, but the amount of PCl5 is more than that of PCl3 and Cl2.

If hydrogen peroxide is poured on a wound, it decomposes into water and oxygen. Th e gaseous oxygen bubbles away as it is formed and thus prevent the formation of H2 O2. Thus, more amount of product can be obtained in a reversible reaction by the periodical removal of one of the products or the periodical addition of the reactants.

The reaction that cannot be reversed is called irreversible reaction. Th e irreversible reactions are unidirectional, i.e., they take place only in the forward direction. Consider the combustion of coal into carbon dioxide and water. C(s) + O2(g) → CO2(g) + Heat In this reaction, solid coal burns with oxygen and gets converted into carbon dioxide gas and water. As the product is a gas, as soon as it is formed it escapes out of the reaction container. It is extremely hard to decompose a gas into a solid. Th us, the backward reaction is not possible in this case. So, it is an irreversible reaction.

Where, [A] – Concentration of A [B] – Concentration of B The negative sign indicates the decrease in the concentration of A with time. The positive sign indicates the increase in the concentration of B with time.

Faster the reaction, more will be the amount of the product in a specified time. So, the rate of a reaction is important for a chemist for designing a process to get a good yield of a product. Rate of reaction is also important for a food processor who hopes to slow down the reactions that cause food to spoil.

The rate of a reaction can be changed. For example, iron gets rusted faster in an acid than in water. Some factors influence the rate of a reaction.

Important factors that affect rate of a reaction are Nature of the reactants Concentration of the reactants Temperature Catalyst Pressure Surface area of the reactants

The reaction of sodium with hydrochloric acid is faster than that with acetic acid. Hydrochloric acid is a stronger acid than acetic acid and thus more reactive. So, the nature of the reactant influence the reaction rate. 2Na(s) + 2HCl(aq) → 2NaCl(aq)+ H2 (g) (fast) 2Na(s) + 2CH3 COOH (aq) → 2CH3 COONa (aq)+ H2(g)(slow)

Changing the amount of the reactants also increases the reaction rate. The amount of the substance present in a certain volume of the solution is called ‘concentration’. More the concentration, more particles per volume exist in it and hence faster the reaction. Granulated zinc reacts faster with 2M hydrochloric acid than 1M hydrochloric acid.

Most of the reactions go faster at higher temperature. Because adding heat to the reactants provides energy to break more bonds and thus speed up the reaction. Calcium carbonate reacts slowly with hydrochloric acid at room temperature. When the reaction mixture is heated the reaction-rate increases.

Food kept at room temperature spoils faster than that kept in the refrigerator. In the refrigerator, the temperature is lower than the room temperature and hence the reaction rate is less.

A catalyst is a substance which increases the reaction rate without being consumed in the reaction. In certain reactions, adding a substance as catalyst speeds up the reaction. For example, on heating potassium chlorate, it decomposes into potassium chloride and oxygen gas, but at a slower rate. If manganese dioxide is added, it increases the reaction rate.

When solid reactants are involved in a reaction, their powdered form reacts more readily. For example, powdered calcium carbonate reacts more readily with hydrochloric acid than marble chips. Because, powdering of the reactants increases the surface area and more energy is available on collision of the reactant particles. Thus, the reaction rate is increased.

Although pure water is often considered as a non-conductor of electricity, precise measurements show that it conducts electricity to a little extent. This conductivity of water has resulted from the self-ionisation of water. Self-ionisation or auto ionisation is a reaction in which two like molecules react to give ions. In the process of ionisation of water, a proton from one water molecule is transferred to another water molecule leaving behind an OH— ion. The proton gets dissolved in water forming the hydronium ion as shown in the following equation:

Initially the rate of the forward reaction is greater than the rate of the backward reaction. However, during the course of reaction, the concentration of the reactants decreases and the concentration of the products increases. Since the rate of a reaction is directly proportional to the concentration, the rate of the forward reaction decreases with time, whereas the rate of the backward reaction increases.

Not only chemical changes, physical changes also may attain equilibrium. When water kept in a closed vessel evaporates, it forms water vapour. No water vapour escapes out of the container as the process takes place in a closed vessel. So, it builds up the vapour pressure in the container. At one time, the water vapour condenses back into liquid water and when the rate of this condensation becomes equal to that of vapourisation, the process attains equilibrium. At this stage, the volume of the liquid and gaseous phases remain constant. Since it is a physical change, the equilibrium attained is called ‘Physical Equilibrium’. Physical equilibrium is a state of a physical change at which the volume of all the phases remain unchanged.

Characteristics of equilibrium: In a chemical equilibrium, the rates of the forward and backward reactions are equal. The observable properties such as pressure, concentration, colour, density, viscosity etc., of the system remain unchanged with time. The chemical equilibrium is a dynamic equilibrium, because both the forward and backward reactions continue to occur even though it appears static externally. In physical equilibrium, the volume of all the phases remain constant.

Aerated soft drinks contain dissolved carbon dioxide in a pop bottle (Soda). When the bottle is sealed, the dissolved carbon dioxide (in the form of carbonic acid) and gaseous CO2 are in equilibrium with each other. When you open the bottle, the gaseous CO2 can escape. So, the dissolved CO2 begins to un-dissolve back to the gas phase trying to replace the gas that was lost, when you opened the bottle. That’s why if you leave it open long enough, it will goes ‘flat’. All the CO2 will be gone, blown away in the air.

The hydronium ion formed is a strong acid and the hydroxyl ion is a strong base. So as fast as they are formed, they react again to produce water. Thus, it is a reversible reaction and attains equilibrium very quickly. So, the extent of ionisation is very little and the concentration of the ions produced is also very less.

The product of the concentration of the hydronium ion and the hydroxyl ion is called ‘ionic product of water’. It is denoted as ‘Kw’. It is mathematically expressed as follows: Kw = [H3 O+] [OH-] Its unit is mol^2 dm^-6. At 25° C, its value is 1.00 × 10^-14.

All the aqueous solutions may contain hydrogen and hydroxyl ions due to self-ionisation of water. In addition to this ionisation, substances dissolved in water also may produce hydrogen ions or hydroxyl ions. The concentration of these ions decides whether the solution is acidic or basic.

Explanation pH scale is a set of numbers from 0 to 14 which is used to indicate whether a solution is acidic, basic or neutral. Acids have pH less than 7 Bases have pH greater than 7 A neutral solution has pH equal to 7 The pH is the negative logarithm of the hydrogen ion concentration. i.e. pH = –log10[H+]

pH scale is a scale for measuring the hydrogen ion concentration in a solution. The 'p' in pH stands for ‘Potenz’ in German meaning 'power'. pH notation was devised by the Danish biochemist Sorensen in 1909.

The pH of a solution can be determined by using a universal indicator. It contains a mixture of dyes. It comes in the form of a solution or a pH paper. A more common method of measuring pH in a school laboratory is by using the pH paper. A pH paper contains a mixture of indicators. It shows a specific colour at a given pH. A colour guide is provided with the bottle of the indicator or the strips of paper impregnated with it, which are called pH paper strips. Th e test solution is tested with a drop of the universal indicator, or a drop of the test solution is put on the pH paper. The colour of the solution on the pH paper is compared with the colour chart and the pH value is read from it. The pH values thus obtained are only approximate values.

Our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change. Different body fluids have different pH values. For example, pH of blood is ranging from 7.35 to 7.45. Any increase or decrease in this value leads to diseases. The ideal pH for blood is 7.4.

It is very interesting to note that our stomach produces hydrochloric acid. It helps in the digestion of food without harming the stomach.

During indigestion the stomach produces too much acid and this causes pain and irritation. pH of the stomach fluid is approximately 2.0.

pH of the saliva normally ranges between 6.5 to 7.5. White enamel coating of our teeth is calcium phosphate, the hardest substance in our body. When the pH of the mouth saliva falls below 5.5, the enamel gets weathered. Toothpastes, which are generally basic are used for cleaning the teeth that can neutralise the excess acid and prevent tooth decay.

In agriculture, the pH of the soil is very important. Citrus fruits require slightly alkaline soil, while rice requires acidic soil and sugarcane requires neutral soil.

The pH of rain water is approximately 7, which means that it is neutral and also represents its high purity. If the atmospheric air is polluted with oxide gases of sulphur and nitrogen, they get dissolved in the rain water and make its pH less than 7. Th us, if the pH of rain water is less than 7, then it is called acid rain. When acid rain fl ows into the rivers it lowers the pH of the river water also.

The pH and pOH of a water solution at 25 degree C, are related by the following equation. pH + pOH = 14 If either the pH or the pOH of a solution is known, the other value can be calculated.