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CHM 131 Lecture Notes - Lecture 4: Mass Number, Ionic Bonding, Chemical Formula

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peachwoodchuck721

9 Aug 2018

School

University of Rochester

Department

Course

Professor

Benjamin Hafensteiner

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Document Summary

The law of conservation of mass : mass is neither created nor destroyed. The mass before a chemical reaction always equals the mass after a chemical reaction. The law of definite proportion: a given compound always contains exactly the same proportion of elements by mass. Water is always comprised of hydrogen and oxygen in a. The composition of a substance is contingent on the number of atoms of each constituent element comprising it (which is given by the empirical or molecular formula of the compound). It is not, however, contingent on the composition of the mixture from which it was created. A molecule is a group of atoms held together in a specific arrangement by chemical bonds. An ion is an atom or a group of atoms which has a net positive or negative charge. In covalent bonding, electrons are shared between two elements.

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Chemical Reactions and Equations Learning objectives To observe physical and chemical properties associated with chemical changes To be able to give evidence for the occurrence of a chemical reaction To write a balanced chemical equation To identify areaction as a combination, decomposition, precipitation, single or double replacement, or combustion reaction ntroduction When a substance undergoes a physical change, it changes its appearance but not its composition. For example, when ice melts to form water, it undergoes a physical change from solid to liquid. In a chemical change, the composition of a substance is changed to give a new substance with different physical and chemical properties. For example, when silver tarnishes, the shiny silver (Ag) changes to a dull gray silver sulfide (AgaS). Not only has the chemical formula changed from Ag to Agus, but also the physical properties (shininess) of the substance have changed. Some examples of physical and chemical changes are listed in Table 1 below. Table 1. Comparison of Physical and Chemical Changes Examples of Physical changes Examples and Evidence of Chemical Changes Change in state (melting, freezing, evaporating Formation of a gas (bubbles) subliming) Formation of a solid (precipitate) Change in size Tearing Change in Color Heat is given off or absorbed A solid dissolving in a solvent In a chemical reaction, atoms in the reactants are rearranged to produce new combinations of atoms in the products. However, the total number of atoms of each element in the reactants is equal to the total number of atoms in the products. This is true according to the Law of Conservation of Mass, which states that the total mass remains constant during a chemical reaction. In other words, matter cannot be created or destroyed in a chemical reaction. In a chemical equation, the reactants are shown on the left and the products are on the right. An arrow in between indicates that a chemical reaction occurs. Finally, the physical state of each compound is designated in parentheses after the chemical formula. Solid, liquid, gas, and aqueous (dissolved in water) are shown as(s,(n,(g), and (aq), respectively.