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CHM1022: Summary - Chemical and structural formulae, stoichiometry

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Monash University

THE LANAGUAGE OF CHEMISTRY Significant Figures o The last significant figure of a measurement may not always be accurate. o The number of significant figures is equal to the number known for certain, plus one that is estimated. o The number of significant figures is the same regardless the units. o How accurate a measure is refers to how close the value is to the correct value. o How precise a measure is when a calculation is made by giving the same answer at different times. Chemical Formulae o A Chemical formula describes the combination of any substance. It shows the relative number of each type of atom in a substance. It contains elemental symbols to represent atoms and subscripted numbers to show the number of atoms of each type. o Molecular formula describes the types and numbers of atoms present in the molecule. o Binary compounds are the compounds that only contain two elements. o Subscripted numbers refers ONLY to the atom before it. o The difference between covalent and ionic compounds is the type of bonding. o Ionic compounds are made up of only ions which are held together by attractive forces between ions of opposite charges. o Covalent compounds are bonding that involves the sharing of electrons between neighbouring atoms. o Anhydrous is when the compounds contains no water molecules. Structural formulae o Structural formulas tell us structural information as it shows how atoms in a molecule are bonded together. o Structural formulas do not show accurate geometry of a compound because it is hard to accurately show the 3D molecules in 2D. o Catenation is the linking of identical atoms to form massive molecules. This is due to the covalent inorganic compounds which are small molecules that exist as rings and chains. o Isomers are molecules that have the same chemical formula but different chemical structures. Ball-and-Stick models o The balls represent atoms. o The sticks represent the bonds. Space-filling models o The distorted sphere represents the volume occupied by its electrons. o The spheres joins together to build a molecule. Nomenclature o The system for naming compounds. Functional Groups o A group of one or more atoms within a molecule that is bonded together in a particular way. Aldehyde and Ketones o Carbonyl Group (C=O) o Aldehydes always have the carbon atom of the carbonyl group bonded to at least one H atom, hence it must always be at the end of a carbon chain. o Ketones is always bonded to two other C atoms, thus it must never be at the end of a carbon chain. Carboxylic Acids o Carboxyl acid (-COOH) o Always found at the end of a carbon chain. The nomenclature of Alkanes o Alkanes are molecules that contain carbon and hydrogen where carbon are joined by single bonds. Constitutional isomerism in Alkanes o Compounds with the same chemical formula but a different order of attachment of the constitute atoms. Chemical Reactions and Stoichiometry Chemical equations o A chemical reaction is when new substances are form when two or more chemical species are mixed. o Reactants are on the left of a chemical equation. o Products are on the right of a chemical equation. o Reversible reaction is when the reactions of both reverse and forward occur. o Qualitative analysis is when we find out what substances are present in a sample without measuring their amounts. o Quantitative analysis is where we measure the amounts of various substances in a sample. o Stoichiometry is the relative amounts of products and reactants in a chemical reaction. o Stoichiometric coefficients are the numbers infront of the formulae which shows how many molecules of each kind among reactants and products. They can also refer to atoms and ions. o Stoichiometric coefficients are used to ensure the equation acts according to the law of conservation mass. Atoms can’t be destroyed or created in a chemical reaction, thus we must keep the equal before and after a reaction. Mole o One mole is the amount of substance contains the same number as atoms in 12g of Carbon. Empirical Formulae o Empirical formulae is the simplest whole number ratio of atoms within a compound. o M = m/n The determination of chemical formulae o Describing the relative masses of the elements in a compound is a list of percentages by mass called the compound’s percentage composition. o % by mass of element = (mass of element present in a sample/mass of whole sample) x 100 Determination of empirical formulae o Molecular formula is when a formula gives the composition of one molecule. o It is rare to get the mass of every element in a compound by the use of just one weighed sample so two or more analyses on different samples are needed. o Three steps to determine empirical formulae: 1. 100g of compound individual mass % become actual masses of the elements in 100g of compound. 2. Convert the ratio of elements by mass to a ratio by amount – divide the mass of each element by molar mass. 3. Divide the resulting numbers by the smallest, which gives the smallest whole-number ratios of each
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