Chemistry_Grade_10-12 (1).pdf

This type of reaction is known as an endothermic

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from the surroundings. This type of reaction is known as an endothermic reaction. In the earlier part of this chapter, most decomposition reactions were endothermic, and heating was needed for the reaction to occur. Most of the synthesis reactions were exothermic, meaning that energy was given off in the form of heat or light. More simply, we can describe the energy changes that take place during a chemical reaction as: Total energy absorbed to break bonds - Total energy released when new bonds form So, for example, in the reaction... 2 Mg + O 2 2 MgO Energy is needed to break the O-O bonds in the oxygen molecule so that new Mg-O bonds can be formed, and energy is released when the product (MgO) forms. Despite all the energy changes that seem to take place during reactions, it is important to remember that energy cannot be created or destroyed. Energy that enters a system will have come from the surrounding environment, and energy that leaves a system will again become part of that environment. This principle is known as the principle of conservation of energy. Definition: Conservation of energy principle Energy cannot be created or destroyed. It can only be changed from one form to another. Chemical reactions may produce some very visible, and often violent, changes. An explosion, for example, is a sudden increase in volume and release of energy when high temperatures are generated and gases are released. For example, NH 4 NO 3 can be heated to generate nitrous oxide. Under these conditions, it is highly sensitive and can detonate easily in an explosive exothermic reaction. 11.4 Conservation of atoms and mass in reactions The total mass of all the substances taking part in a chemical reaction is conserved during a chemical reaction. This is known as the law of conservation of mass . The total number of atoms of each element also remains the same during a reaction, although these may be arranged differently in the products. We will use two of our earlier examples of chemical reactions to demonstrate this: 217
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11.4 CHAPTER 11. PHYSICAL AND CHEMICAL CHANGE - GRADE 10 The decomposition of hydrogen peroxide into water and oxygen 2 H 2 O 2 2 H 2 O + O 2 O O H H O O H H O H H O H H + O O Left hand side of the equation Total atomic mass = (4 × 1) + (4 × 16) = 68 u Number of atoms of each element = (4 × H) + (4 × O) Right hand side of the equation Total atomic mass = (4 × 1) + (2 × 16) + (2 × 16) = 68 u Number of atoms of each element = (4 × H) + (4 × O) Both the atomic mass and the number of atoms of each element are conserved in the reaction. The synthesis of magnesium and oxygen to form magnesium oxide 2 Mg + O 2 2 MgO Mg Mg + O O Mg O Mg O Left hand side of the equation Total atomic mass = (2 × 24.3) + (2 × 16) = 80.6 u Number of atoms of each element = (2 × Mg) + (2 × O) Right hand side of the equation Total atomic mass = (2 × 24.3) + (2 × 16) = 80.6 u Number of atoms of each element = (2 × Mg) + (2 × O) Both the atomic mass and the number of atoms of each element are conserved in the reaction.
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