This page will help us revise the first chapter of science – Chemical Reactions and Equations.
This chapter is very important, as the basics of all your future chapters are covered in this chapter.
Let us now look at the weightage of this chapter. On analysing previous years’ papers, we can say that there is a higher probability of a 1 or 2 mark question being asked from this chapter. The questions that have appeared in previous years from this chapter are:
- Balance the following chemical equation [CBSE, 2008]
- Why is respiration considered an exothermic process? [CBSE, 2008]
- What changes in the colour of iron nails and copper sulphate solution do you observe after keeping the iron nails dipped in copper sulphate solution for about 30 minutes? [CBSE, 2010]
- Give an example of a decomposition reaction. Describe an activity to illustrate such a reaction by heating. [CBSE, 2008]
- I. What is observed when a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube?
II.What type of reaction of this?
III.Write a balanced chemical equation to represent the above reaction. [CBSE, 2008]
- What happens when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride? State the physical conditions of reactants in which the reaction between them will not take place. Write the balanced chemical equation for the reaction and name the type of reaction. [CBSE, 2010]
Now if you notice, you will see that from this chapter, some specific types of questions are asked each time. For answering all the questions correctly, you need to know the following 4 things:
- Balancing a chemical equation: Whenever you write any chemical equation, make sure you balance it. If you want to revise balancing equations, you can watch Avanti video ‘How to Balance a Chemical Equation’. <add hyperlink>
- Colour of precipitate and gas: If any precipitate is being formed in the reaction or a gas is being evolved, make sure you mention it and remember their colour.
- Reaction type – for all the reactions, remember whether they are combination/ decomposition/ displacement/ double-displacement or oxidation-reduction reactions. Also remember if they are endothermic or exothermic.
- Formulae and state of all elements and compounds – remember to write down the correct formulae of all elements and compounds and mention their states (solid/liquid/gas/aqueous) correctly.
By following these 4 points, you can answer any given question to you correctly. Let us now see how we solve a question from this chapter.
Q. What happens when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride? Write the balanced chemical equation for the reaction. State the physical conditions of reactants in which the reaction between them will not take place. [CBSE, 2010]
In this question, we are given the reactants. Let us first write down their formulae and states correctly.
- Sodium sulphate: Na2SO4 (aq)
- Barium chloride: BaCl2(aq)
We write down (aq) as we are given that both the compounds are in solution form. So the reaction will start like: Na2SO4(aq) + BaCl2(aq)
Now if you remember, these substances react together to give a double displacement reaction: Na2SO4 (aq) + BaCl2 (aq) BaSO4(s)+ 2NaCl (aq)
And during the reaction, a white precipitate of BaSO4 is formed.
Now, the physical conditions in which no reaction will take place, will be solid – i.e., if both our reactants are in solid form instead of aq, no reaction will be observed.
Q. What changes in the colour of iron nails and copper sulphate solution do you observe after keeping the iron nails dipped in copper sulphate solution for about 30 minutes?
In this question, the reaction involved is:
In this reaction, iron has displaced or removed another element, copper, from copper sulphate solution. This reaction is known as displacement reaction.
Why some metals displace some metals will be covered in chapter 3 – metals and non-metals of chemistry in more detail.
Thus, changes that take place are: iron nail become brownish in colour and the blue colour of copper sulphate solution fades.
For your reference, we have made a list of all balanced reactions of this chapter and mentioned their reaction type. Make sure you definitely study these, as there is a high probability that some or the other reaction from this list will be asked, as per previous papers of CBSE.
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