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CHE 131 Chapter 2: Chapter 2.2

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1 Sep 2016

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CHE 131 Full Course Notes

CHE 131 Full Course Notes

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2. 2- the observations that led to an atomic view of matter. 3 mass laws- the law of mass conservation, the law of definite (constant) composition and the law of multiple proportions: mass conservation. Definition- the total mass of substances does not change during a chemical reaction. The number of substances and their properties can change, but the total amount of matter cannot be changed. In other words, mass cannot be created or destroyed. Einstein showed that after a chemical reaction, some of the mass is converted into energy (or the other way around: definite composition. Definition- no matter what it"s source, a particular compound is made of the same elements in the same parts by mass. Fraction by mass- the fraction of the mass an element contributes to a compound. Found by dividing the mass of the element by the total mass of the compound. Percent by mass- fraction by mass as percent.

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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.