Chemical Reactions

What are Chemical Reactions?

nature and identity of initial substance changes so, whenever a change occurs, a chemical reaction occurs When a chemical reaction takes place, the following happens change in state of the initial object (solid, liquid, gas) change in color evolution of a gas ( as a result of the reaction) change in temperature

What are Chemical Equations?

a shorter form of describing a chemical reaction the simplest way is a word-equation        Magnesium + Oxygen --> Magnesium Oxide 2 parts of the equation Left side -reactants Right side - product        

Using Chemical Formula

a shorter way of representing the equation use the chemical symbols of the reactants and products          Mg + O2 --> MgO

Skeletal Chemical Equations

number of atoms in LHS is not equal to no of atoms in RHS the equation is not balanced 

Balanced Chemical Equations

Mass can neither be created nor destroyed the total mass of reactants = total mass of products no. of atoms remain the same - before and after the chemical reaction

Steps to balance equation

Fe + H2O --> Fe3O4 + H2 Step 1: Identify each formula in the equation             Fe             H2O             Fe3O4             H2 

Step 2: Identify the elements in the equation List the number of atoms in LHS and RHS Fe --> LHS = 1 atom ; RHS =1 atom H --> LHS = 2 atoms ; RHS =2 atom O --> LHS =1 atom ; RHS = 4 atoms no. of atoms differ because of the chemical formula 3Fe needs to combine with 4O to form a stable ferrous oxide molecule

Step 3: Identify the compound with maximum no. of atoms (can be on any side of the equation) Fe3O4 

Step 4: Choose an element in the compound O

Step 5: no. of atoms in LHS of O --> 1 no. of atoms in RHS of O --> 4 balance in LHS (as it is less) to balance 1x4 H2O ( as O is present in that compound) Equation now becomes  Fe + 4H2O --> Fe3O4 + 4H2

Step 6:  Repeat steps 4 and 5 for all elements in the equation For H no. of atoms in LHS --> 8 (4x2) no. of atoms in RHS --> 2 balance RHS ( as it is less) to balance  1x4 H2 (4x2=8)         Equation becomes: Fe +4H2O --> Fe3O4 + 4H2 For Fe no. of atoms in LHS --> 1 no. of atoms in RHS --> 3 balance LHS ( as it is less) to balance 1x3 Fe Equation becomes : 3Fe + 4H2O --> Fe3O4 +4H2  

Completing the equation

Follow these final steps: Count number of atoms on both sides- they should be equal Write the physical states of the compound -s,l,g Add temp, pressure, catalyst if required

Types of Chemical Equation

There are the following types: Combination Reaction: single product formed with two or more reactions Decomposition Reaction: single, complex reactant gives simpler products Displacement Reaction:  the reactant replaces a compound in another reactant Exothermic chemical Reaction: reaction produces a lot of heat Thermal decomposition: decomposition reaction in the presence of heat  Endothermic reactions: reaction in which energy (heat, light, electricity) is absorbed Double displacement reaction: displacement of reactant and exchange of ions as well Precipitation reaction: any reaction that forms a precipitate (insoluble substance) Oxidation Reaction: if a reactant gains oxygen during a reaction Reduction Reaction: if a reactant loses oxygen during a reaction Redox Reaction: if both oxidation and reduction occur

Examples

Combination Reaction CaO(s) + H2O(l) → Ca(OH)2(aq) C(s) + O2(g) --> CO2(g) 2H2(g) + O2 --> 2H2O 2Mg + O2 --> 2MgO

Exothermic Reaction CaO(s) + H2O(l) → Ca(OH)2(aq) CH4(g) + 2O2(g) --> CO2(g) + 2H2O(g) C6H12O6 (aq)  +6O2 (aq) --> 6CO2(aq) + 6H2O (l) +energy (respiration) decomposition of vegetable into compost

Decomposition Reaction 2FeSO4(s) (Heat)→  Fe2O3(s) + SO2(g) + SO3(g) (on heating) CaCO3(s) (Heat)→CaO(s) + CO2(g) (on heating) 2Pb(NO3)2(s) (Heat)→ 2PbO(s) + 4NO2(g) + O2(g) (on heating) 2AgCl(s) (Sunlight) → Ag(s) + Cl2(g) 2AgBr(s) (Sunlight) → 2Ag(s) + Br2(g)

Endothermic Reactions 2FeSO4(s) (Heat)→  Fe2O3(s) + SO2(g) + SO3(g) (on heating) CaCO3(s) (Heat)→CaO(s) + CO2(g) (on heating) 2Pb(NO3)2(s) (Heat)→ 2PbO(s) + 4NO2(g) + O2(g) (on heating) 2AgCl(s) (Sunlight) → Ag(s) + Cl2(g) 2AgBr(s) (Sunlight) → 2Ag(s) + Br2(g) breaking of bonds in decomposition requires energy, hence they are always endothermic

Displacement Reaction Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) Zn(s) + CuSO4(aq)→ ZnSO4(aq) + Cu(s) Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)

Double Displacement Reaction Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)

Oxidation Reaction 2Cu + O2→ (Heat) 2CuO in real life - corrosion and rancidity (spoiling of food) Reduction Reaction Cuo + H2 --> (Heat) Cu +H2O  

Redox Reaction Cuo + H2 --> (Heat) Cu +H2O ZnO + C → + Zn CO  MnO2 + 4HCl --> MnCl2 +  2H2O +Cl2 

Effects of oxidation

corrosion rancidity

What happens when....?

Burn magnesium ribbon in air Observation: magnesium ribbon burns with a dazzling white flame and changes into a white powder.  Conclusion: MgO is formed due to the reaction between magnesium and oxygen present in the  air.

Add potassium iodide solution to lead nitrate Observation: lead nitrate reacts with potassium iodide to give lead iodide and potassium nitrate. The color of the products is different from that of the reactants. Conclusion: Pb(NO3)2 + 2KI → PbI2 + 2KNO3. The change in colour is because lead iodide and potassium nitrate have been formed after the reaction. PbI2 is a yellow ppt. KNO3 is a colourless solution

Add hydrochloric acid or sulphuric acid to zinc granules A gas is evolved and the conical flask is warm  Conclusion: From this, we conclude that when a chemical reaction takes place there is a change in the state, colour, and temperature. Zn + 2HCl → ZnCl2 + H2↑ + heat    Zn + H2SO4 → ZnSO4 + H2↑ + heat

Water is added to calcium oxide Observation: The beaker becomes hot (exothermic reaction). It reacts vigorously with water.  Slaked lime is formed by the reaction of the combination of calcium oxide with water CaO(s)              +            H2O(l)              →               Ca(OH)2(aq) + heat (quicklime)                                                                 (Slaked lime)

Ferrous sulphate crystals are heated The green colour of the ferrous sulphate crystals has changed there is also the characteristic odour of burning sulphur. 2FeSO4(s) + Heat      → Fe2O3 + SO2(g) + SO3(g)

Lead nitrate is heated We observe the emission of brown fumes.  These fumes are of nitrogen dioxide (NO2). The reaction that takes place is 2Pb(NO3)2(s)        ---Heat→   2PbO(s)            +       4NO2(g)              +       O2(g) (Lead nitrate)                           (Lead oxide)           (Nitrogen dioxide)        (Oxygen)

Electrolysis of water ( separating hydrogen and oxygen) The test tube containing hydrogen gas has double the volume of the test tube containing oxygen gas. Hydrogen gas burns with a light blue flame with a pop sound.  its a decomposition reaction 2 H2O(l) → 2 H2(g) + O2(g)

Silver chloride is kept in sunlight Observation: White silver chloride turns grey in sunlight. Conclusion: This is due to the decomposition of silver chloride into silver and chlorine by light 2AgCl(s)   ----Sunlight→   2Ag(s) + Cl2(g)

Iron nails are immersed in a copper sulphate solution.  the iron nail dipped in the copper sulphate solution in test tube A becomes brownish in colour and the blue colour of copper sulphate solution fades in test tube A. Wh

Sodium Sulphate and Barium Chloride are mixed together: A white substance, which is insoluble in water, is formed. This is a double displacement reaction shown by the equation  Na2SO4(aq)          +     BaCl2(aq)                →          BaSO4(s)         +       2NaCl(aq) (sodium sulphate)        (Barium chloride)                (Barium sulphate)  (Sodium chloride)