5.5 Rearranging atoms

• Crucible: A crucible is a heat-resistant container typically made of ceramic or metal, used for heating substances to high temperatures in a laboratory setting. Crucibles are commonly used for processes such as melting, calcination, and chemical reactions where high temperatures are required.
• Endothermic Reactions: Endothermic reactions are chemical reactions that absorb heat from their surroundings, causing a decrease in temperature. In these reactions, energy is taken in from the environment to break chemical bonds in the reactants and form new bonds in the products. Examples of endothermic reactions include the dissolution of ammonium nitrate in water and the decomposition of baking soda (sodium bicarbonate) into sodium carbonate, water, and carbon dioxide gas.
• Exothermic Reaction: In contrast to endothermic reactions, exothermic reactions release heat into their surroundings, causing an increase in temperature. Energy is released as chemical bonds in the reactants are broken and new bonds are formed in the products. Common examples of exothermic reactions include combustion reactions, such as the burning of wood or the reaction between hydrogen gas and oxygen gas to form water.
• The Law of Conservation of Energy: The law of conservation of energy states that energy cannot be created or destroyed in an isolated system. Instead, energy can only change forms or be transferred from one object to another. In the context of chemical reactions, this law means that the total amount of energy in the universe remains constant before and after a reaction, although it may change form between potential energy, kinetic energy, heat, or other forms.
• The Law of Conservation of Mass: The law of conservation of mass states that the mass of the reactants in a chemical reaction is equal to the mass of the products. In other words, matter cannot be created or destroyed
• When calcium carbonate is added to hydrochloric acid in an open flask, the mass of the flask and its contents decreases over time
• According to the law of conservation of mass, the total mass of the reactants should be equal to the total mass of the products in a chemical reaction
• The observed decrease in mass during the reaction is attributed to the escape of carbon dioxide gas produced during the reaction
• Carbon dioxide gas escapes into the air through the open flask, and its mass is not accounted for in the measurements
• Despite the apparent decrease in mass, the total mass of the system (including reactants, products, and any gases released) remains constant, as required by the law of conservation of mass
• When magnesium is heated in a crucible, the resulting magnesium oxide weighs more than the original mass of the magnesium
• Despite the appearance of the ashes being smaller than the original magnesium, the mass of the ashes is greater due to the combination of magnesium with oxygen from the air during the reaction
• The word equation for the reaction is: magnesium + oxygen → magnesium oxide
• The increase in mass was first observed by French scientist Antoine Lavoisier in 1772 during his experiments on combustion
• Lavoisier proposed that substances combine with a gas from the air during burning, which he later identified as oxygen
• This discovery laid the foundation for understanding the role of oxygen in combustion and respiration