Signs of Chemical ReactionsThere are four main signs that indicate a chemical reaction has taken place: solid liquid Gas change in color change in state Evolution of gas change in temperature
CHEMICAL EQUATIONS when a magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide. This description of a chemical reaction in a sentence form is quite long. It can be written in a shorter form. The simplest way to do this is to write it in the form of a word-equation. The word-equation for the above burning of magnesium ribbon reaction would be –Magnesium + Oxygen → Magnesium oxide
CHEMICAL EQUATIONS Reactants – the substances that exist before a chemical change (or reaction) takes place. Products – the new substance(s) that are formed during the chemical changes. CHEMICAL EQUATION indicates the reactants and products of a reaction. REACTANTS PRODUCTS
A word-equation shows change ofreactants to products through anarrow placed between them. The reactants are written on the left-hand side (LHS) with a plus sign (+)between them. Similarly, products arewritten on the right-hand side (RHS) witha plus sign (+) between them. The arrowhead points towards theproducts, and shows the direction ofthe reaction.
Writing a Chemical Equation Mg + O2 → MgOCount and compare the number of atoms ofeach element on the LHS and RHS of thearrow. Is the number of atoms of eachelement the same on both the sides? If not,then the equation is unbalanced becausethe mass is not the same on both sides ofthe equation. Such a chemical equation is askeletal chemical equation for a reaction.Equation is a skeletal chemical equation forthe burning of magnesium in air.
Balanced Chemical EquationsIn other words, the number of atoms ofeach element remains thesame, before and after a chemical reaction.Hence, we need to balance askeletal chemical equation.
Unbalanced and Balanced Equations H Cl Cl H H Cl H Cl Cl Cl H ClHHH2 + Cl2 HCl (unbalanced) H2 + Cl2 2 HCl (balanced) reactants products reactants products H 2 1 H 2 2 Cl 2 1 Cl 2 2
Let us try to balance thefollowing chemicalequation –Step I: To balance achemical equation, first drawboxes around each formula.Do not change anything insidethe boxes while balancing theequation.
Step II: List the number of atomsof different elements present in theunbalanced equation
Step III: It is often convenient tostart balancing with the compoundthat contains the maximum number ofatoms. It may be a reactant or aproduct. In that compound, select theelement which has the maximumnumber of atoms. Using these criteria,we select Fe3O4 and the element oxygenin it. There are four oxygen atoms on theRHS and only one on the LHS.
Step IV: Fe and H atoms are still notbalanced. Pick any of these elementsto proceed further. Let us balancehydrogen atoms in the partly balancedequation.To equalise the number of Hatoms, make the number of molecules ofhydrogen as four on the RHS.
(partly balanced equationStep V: Examine the above equationand pick up the third element which is notbalanced. You find that only one elementis left to be balanced, that is, iron.To equalise Fe, we take three atoms of Feon the LHS.
Step VI: Finally, to check the correctness of the balanced equation, we count atoms of each element on both sides of the equation. (balanced equationThe numbers of atoms of elements on bothsides are equal. This equation is nowbalanced. This method of balancingchemical equations is called hit-and-trialmethod as we make trials to balancethe equation by using the smallest whole
q1. Why should a magnesium ribbonbe cleaned before burning in air?
q2. Write the balanced equation forthe following chemical reactions.(i) Hydrogen + Chlorine → Hydrogenchloride(ii) Barium chloride + Aluminiumsulphate → Barium sulphate +Aluminium chloride(iii) Sodium + Water → Sodium hydroxide+ Hydrogen
q3. Write a balanced chemical equation withstate symbols for the following reactions.(i) Solutions of barium chloride and sodiumsulphate in water react to give insolublebarium sulphate and the solution of sodiumchloride.(ii) Sodium hydroxide solution (in water)reacts with hydrochloric acid solution (inwater) to produce sodium chloride solutionand water.
During a chemical reaction atoms ofone element do not change into thoseof another element. Nor do atomsdisappear from the mixture or appearfrom elsewhere. Actually, chemicalreactions involve the breaking andmaking of bonds between atoms toproduce new substances.
Definition-A reaction in which a single product is formed from two or more reactants is known as a combination reaction. For ex-Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.In this reaction, calcium oxide and watercombine to form a single product, calciumhydroxide.
combination reaction (Synthesis) 2 Na + Cl2 2 NaCl Na Cl Cl NaGeneral form: A + B AB element or element or compound compound compound
Example of combination Reactions:-Photosynthesis 6 CO2 + 6 H2O C6H12O6 + 6 O2Formation of water 2 H2 + O2 2 H2OFormation of salt 2 Na + Cl2 2 NaClGeneral Form A + B C
Let us discuss some more examples ofcombination reactions.(i) Burning of coal(ii) Formation of water from H2(g) and O2(g)In simple language we can say that when two ormore substances (elements or compounds)combine to form a single product, the reactionsare called combination reactions.
Reactions in which heat isreleased along with the formation ofproducts are called exothermic chemicalreactions. Other examples of exothermicreactions are –(i) Burning of natural gas
(ii) Do you know that respiration is an exothermicprocess?We all know that we need energy to stay alive. We getthis energy from the food we eat. During digestion, foodis broken down into simpler substances. For example,rice, potatoes and bread contain carbohydrates. Thesecarbohydrates are broken down to form glucose. Thisglucose combines with oxygen in the cells of our bodyand provides energy. The special name of this reactionis respiration.iii) The decomposition of vegetable matter into compost is alsoan example of an exothermic reaction.
Definition-A reaction in which a single reactant is break down to give simpler product. For example-Ferrous sulphate crystals (FeSO4, 7H2O) lose water when heated and the colour of the crystals changes. It then decomposes to ferric oxide (Fe2O3),sulphur dioxide (SO2) and sulphur trioxide (SO3). Ferric oxide is a solid, while SO2 and SO3 are gases.
Example of Decomposition Reactions:-Hydrogen Peroxide 2 H2O2 2 H2O + O2Electrolysis of water 2 H2O electricity H2 + 2 O2Nitrogen triiodide 2 NI3 N2 + 3 I2General Form AB A + B
Decomposition of calcium carbonate to calciumoxide and carbon dioxide on heating is animportant decomposition reaction used invarious industries. Calcium oxide is called lime orquick lime. It has many uses – one is in themanufacture of cement. When a decompositionreaction is carried out by heating, it is called
Other example of thermal decomposition reactionare:-You will observe the emission of brown fumes.These fumes are of nitrogen dioxide (NO2). Thereaction that takes place is –
Let us perform some more decompositionreactions;
You will see that white silver chloride turns grey insunlight. This is due to the decomposition of silverchloride into silver and chlorine by light.Silver bromide also behaves in the same way.The above reactions are used in black and whitephotography.
We have seen that the decompositionreactions require energy either in theform of heat, light or electricity forbreaking down the reactants.Reactionsin which energy is absorbed are knownas
q1. A solution of A substAnce ‘X’ is usedfor white washing.(i) nAme the substAnce ‘X’ And write itsformula.(ii) write the reAction of the substAnce ‘X’named in (i) above with water.
q2. Why is the amount of gas collected inone of the test tubes in Activity1.7 double of the amount collected in theother? Name this gas.
Definition-A reaction in which a one element displaced or replaced or removed another element, is known as a displacement reaction. For example-The reaction between iron & copper sulphate.In this reaction, iron has displaced or removedanother element,copper, from copper sulphate
Other examples ofdisplacement reactions areZinc and lead are more reactiveelements than copper. Theydisplace copper from itscompounds.
Definition-A reactions in which there is an exchange of ions between the reactants are called double displacement reactions. For example-Formation of barium sulphate and sodium chloride from sodium sulphate and barium chloride.
Any reaction that produces aprecipitate can be called aprecipitation reaction.
Single and Double displacementReactions Single-replacement reaction Mg + CuSO4 MgSO4 + Cu General form: A + BC AC + B Double-replacement reaction CaCO3 + 2 HCl CaCl2 + H2CO3 General form: AB + CD AD + CB
The surface of copper powder becomescoated with black copper(II) oxide. Why hasthis black substance formed? This isbecause oxygen is added to copper andcopper oxide is formed.If hydrogen gas is passed over thisheated material (CuO), the black coating onthe surface turns brown as the reversereaction takes placeand copper is obtained.
If a substance gains oxygen during a reaction, itis said to be oxidised. If a substance loses oxygenduring a reaction, it is said to be reduced. Duringthis reaction,the copper(II) oxide is losing oxygenand is being reduced. The hydrogen is gainingoxygen and is being oxidised.In other words, onereactant gets oxidised while the other getsreduced during a reaction. Such reactions arecalled oxidation-reduction reactionsor redox reactions.
Some other examples ofredox reactions are:In reaction (1) carbon is oxidised to COand ZnO is reduced to Zn.In reaction (2) HCl is oxidised to Cl2whereas MnO2 is reduced to MnCl2.From the above examples we can say thatif a substance gains oxygenor loseshydrogen during a reaction, it is oxidised. Ifa substance loses oxygen or gainshydrogen during a reaction, it is reduced.
Definition-When a metal is attacked bysubstances around it such as moisture, acids, etc.,it is said to corrode and this process is calledcorrosion. The black coating on silver and thegreen coating on copper are other examples ofcorrosion.•AFFECTS-Corrosion causes damage to carbodies, bridges, iron railings, ships and to allobjects made of metals, specially those of iron.Corrosion of iron is a serious problem. Every yearan enormous amount of money is spent toreplace damaged iron.
Definition-When fats and oils are oxidised, theybecome rancid and their smell and taste change.Usually substances which prevent oxidation(antioxidants) are added to foods containing fatsand oil. Keeping food in air tight containers helpsto slow down oxidation. Do you know that chipsmanufacturers usually flush bags of chips with gassuch as nitrogen to prevent the chips from gettingoxidised ?
q1. Why does the colour of coppersulphate solution change when an ironnail is dipped in it?
q2. Give an example of a doubledisplacement reaction other than theone given in Activity 1.10.
q3. Identify the substances that areoxidised and the substances that arereduced in the following reactions.(i) 4Na(s) + O2(g) → 2Na2O(s)(ii) CuO(s) + H2(g) → Cu(s) + H2O(l) THE*******END