Chemistry Chapter 6 Class 11 Notes

[Evelina Browder]

Scienceis a subject that explains how the world around us is made and the chemical reactions that make things happen around us. From rust to decomposition, chemical reactions provide a more in-depth insight into how molecular interaction and changes occur. Chapter 1 of CBSE class 10 Science explains how a substance changes form.

Learn more about chemical reactions and equations by exploring CBSE Notes for Class 10 Science Chapter 1. These CBSE notes are comprehensive and detailed yet concise enough to glance through for exam preparations.

A chemical reaction occurs when one or more reactants are changed into one or more products. The constituent atoms of the reactants are rearranged in a chemical reaction, resulting in the formation of various substances as products.

A chemical reaction can be determined with the help of any of the following observations.

a) Evolution of a gas

b) Change in temperature

c) Formation of a precipitate

d) Change in colour

e) Change of state

A chemical reaction is a process that causes one set of chemical components to change into another. Chemical reactions are defined as changes in the locations of electrons in the formation and breaking of chemical bonds between atoms, with no change in the nuclei, and are described using a chemical equation. At a given temperature and chemical concentration, chemical reactions occur at a predictable rate. Reaction speeds often increase as the temperature rises because more thermal energy is available to attain the activation energy required to break bonds between atoms.

A symbol is a chemical code for an element. Each element has a one or two-letter atomic symbol, which is, in most cases, the abbreviated form of its name.

Valency is the combining capacity of an element. It can be considered as the number of electrons lost, gained or shared by an atom when it combines with another atom to form a molecule.

According to the Law of Conservation of Mass, no atoms can be created or destroyed in a chemical reaction, so the number of atoms for each element on the reactants side has to balance the number of atoms that are present on the products side.

In other words, the total mass of the products formed in a chemical reaction is equal to the total mass of the reactants participating in a chemical reaction.

Chemical equations are balanced using coefficients. A coefficient is a numerical value that is added to the front of a chemical symbol or formula. It indicates the number of atoms or molecules of the material involved in the process.

Hit and trial method: While balancing the equation, change the coefficients (the numbers in front of the compound or molecule) so that the number of atoms of each element is the same on each side of the chemical equation.

A single reactant decomposes on the application of heat or light, or electricity to give two or more products.

Types of decomposition reactions:

a. Decomposition reactions which require heat-thermolytic decomposition or thermolysis.

One of the best examples of precipitation reactions is the chemical reaction between potassium chloride and silver nitrate, in which solid silver chloride is precipitated out. This is the insoluble salt formed as a product of the precipitation reaction. The chemical equation for this precipitation reaction is provided below.

A redox reaction occurs when the oxidation states of the substrate change. The loss of electrons or an increase in the oxidation state of a chemical or its atoms is referred to as oxidation. The gain of electrons or a decrease in the oxidation state of a chemical or its atoms is referred to as reduction.

It refers to the oxidation of fats and oils in food that is kept for a long time. It gives foul smell and bad taste to food. Rancid food causes stomach infections during consumption.

Prevention:

(i) Use of air-tight containers

(ii) Packaging with nitrogen

(iii) Refrigeration

(iv) Addition of antioxidants or preservatives

In the electrolysis of water (acidified), the gases that are evolved at the anode and cathode, respectively, are oxygen and hydrogen. Hydrogen ions gain electrons from the cathode and form hydrogen gas, and oxygen ions give electrons to the anode and form oxygen gas.

Note: This video is designed to help the teacher better understand the lesson and is NOT intended to be shown to students. It includes observations and conclusions that students are meant to make on their own.

Students will observe a copper and an aluminum cube of the same volume placed on a balance. They will see that the copper has a greater mass. Students will try to develop an explanation, on the molecular level, for how this can be. Students are then given cubes of different materials that all have the same volume. Students determine the density of each cube and identify the substance the cube is made from.

Students will be able to calculate the density of different cubes and use these values to identify the substance each cube is made of. Students will be able to explain that the size, mass, and arrangement of the atoms or molecules of a substance determines its density.

For this lesson, you will need a set of cubes of different materials that are all the same volume. These sets of cubes are available from a variety of suppliers. You will want a set of cubes that contains Copper, Brass, Steel, Aluminum, PVC, Nylon, Oak, and Pine or Poplar. In the activity, each group will need to measure the mass of each of the eight cubes. Groups will need to measure and record their data for a cube and pass it along to another group until each group has used each of the cubes.

This is the first lesson in which students see models of molecules that are more complex than a water molecule. Some of these molecules may look a little intimidating. Let students know that they do not need to memorize or draw these molecules. For the purpose of this chapter, students only need to think about the size and mass of the atoms that make up the molecule and how they are arranged in the substance.

Tell students that both cubes are exactly the same size, and both are solid with no hollow spots. Explain that the aluminum cube is made of only aluminum atoms and the copper cube is made of only copper atoms.

Students will record their observations and answer questions about the activity on the activity sheet. The Explain It with Atoms & Molecules and Take It Further sections of the activity sheet will either be completed as a class, in groups, or individually, depending on your instructions. Look at the teacher version of the activity sheet to find the questions and answers.

Explain to students that the copper and aluminum atoms are arranged in the same way in their cubes. Copper atoms are a little larger than aluminum atoms. This means there are fewer copper atoms in the copper cube than aluminum atoms in the aluminum cube. But copper atoms have much more mass than aluminum atoms. So even though there might not be as many copper atoms, their extra mass makes up for it and makes the copper cube heavier than the aluminum cube of the same size and shape (volume).

Note: There are different ways of measuring the size of atoms, and in close cases the results are not always in agreement. This is true with copper and aluminum. Some sources report copper as larger by some measures and some report aluminum as larger. For the purposes of this lesson, we will treat copper as the larger atom.

Explain to students that this idea of how heavy something is compared to the amount of space it takes up is called density. The density of an object is the mass of the object compared to its volume. The equation for density is: Density = mass/volume or D = m/v. Each substance has its own characteristic density because of the size, mass, and arrangement of its atoms or molecules.

Explain to students that volume is a measure of the amount of space an object takes up. It is always in three dimensions. To find the volume of an object like a cube or a box, you measure the length, width, and height and then multiply them (V = l w h). If measured in centimeters, the answer will be in cubic centimeters (cm3).

Note: Students often confuse volume and area. Check their understanding to make sure they know the difference. Make sure they understand that area is measured in two dimensions (length width) with an answer in cm2. Area is a measure of the amount of surface. But volume is measured in three dimensions (length width height) with an answer in cm3. Volume is a measure of the entire object, including the surface and all the space the object takes up.

Student groups will not need to measure the volume of the cubes. The volume of each cube is the same, 15.6 cm3 , and is given in their chart on the activity sheet. They will need to measure the mass of each of the eight different cubes and calculate their densities. Students will use their values for density to identify each cube.

Note: The densities students calculate may not be exactly the same as the given densities in this chart. However, their calculations will be close enough that they should be able to identify most of the cubes.

Expected results: Student values for density for each cube will not be exact, but will be close enough that they should be able to identify each of the cubes. You may notice that the approximate densities given for each cube in this lesson are slightly different than those given in the cube set. Most of this difference is probably due to the value for the volume of each cube. Since it is likely that these are 1-inch cubes, each side should be 2.54 cm. We rounded to 2.5 cm because students can make this measurement more easily.

Explain to students that each substance has its own density because of the atoms and molecules it is made from. The metal, plastic, and wood cubes that students measured each have their own unique density. In general, the density of metal, plastic, and wood can be explained by looking at the size and mass of the atoms and how they are arranged.

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