Review for Chemical Changes PDF

Review for Chemical Changes Physical Changes Chemical Changes No new substances are formed. One or more new substances are formed. Eg. change of state, dissolving The changes can usually be reversed. The changes are usually not e...

Review for Chemical Changes Physical Changes Chemical Changes No new substances are formed. One or more new substances are formed. Eg. change of state, dissolving The changes can usually be reversed. The changes are usually not easily reversed. Constituent substances can be obtained The reactants cannot be obtained easily. using separation techniques. What happens in a chemical reaction (a) “Law of conservation of mass” which states that in a chemical change, matter is neither created nor destroyed. (b) “Law of conservation of energy” which states that in a chemical change, energy is neither created nor destroyed, they are either transformed or transferred from one form to another. Any chemical change involves only a rearrangement of the atoms. Eg. The reaction between hydrogen and oxygen is also known as combustion After the reaction, hydrogen and oxygen atoms do not disappear, they are rearranged and can be found in the new compound, water. Some common types of chemical reaction (need to know how to identify) Type of General representation of reaction Definition of reaction reaction and examples Thermal decomposition involves AB → A + B Thermal the breakdown of a compound into heating sugar to form carbon and water decomposition simpler compounds/elements heating calcium carbonate to form upon heating. calcium oxide and carbon dioxide Combustion involves the burning 2 A + O2 → 2 AO Combustion of a substance in the presence of lighting a match oxygen to form new compound(s). burning petrol in a car engine AB(aq) + CD(aq) → CB(aq) + AD(s) Precipitation involves the reaction of sodium carbonate solution Precipitation formation of a solid when two with calcium chloride solution to form solutions are mixed. calcium carbonate precipitate and sodium chloride solution  HA(aq) + MOH(aq) → MA(aq) + H2O(l) Neutralisation The reaction of an acid and alkali eating antacids to treat gastic pain brushing teeth with toothpaste Oxidation rusting (iron + oxygen + water → reactions Oxidation is the addition of oxygen hydrated iron(III) oxide) (other than to a substance. respiration (glucose + oxygen → carbon combustion) dioxide + water) Reaction in which carbon dioxide and water is converted to glucose carbon dioxide + water → glucose + Photosynthesis and oxygen in presence of light oxygen and chlorophyll Additional note: Need to know how chemical reactions can benefit our lives (e.g., cooking, respiration) and cause harm to our health and environment (e.g., rusting, decay, burning) Stability of noble gases, and why other elements react Noble gases are generally inert as they have completely filled valence electron shell (stable duplet or octet electronic configurations), so they do not need to gain, lose or share valence electrons. Other elements generally tend to gain, lose or share valence electrons when they undergo chemical reactions, until they obtain completely filled valence electron shell (stable duplet or octet electronic configurations). Dot–and–cross diagrams of ionic compounds Generally metal and non–metal form ionic compounds. Examples Sodium chloride, NaCl Magnesium oxide, MgO Sodium oxide, Na2O Lithium fluoride, LiF Magnesium chloride, MgCl2 Potassium oxide, K2O Steps to draw dot–and–cross diagrams using sodium oxide, Na2O as example 1. Identify the Group that element is in, which will indicate the number of valence electrons the element has. Eg Na is in Group 1 so it has 1 valence electron, O is in Group 16 so it has 6 valence electrons 2. Identify the number of electrons that the element need to lose to achieve duplet or octet configuration, and determine the charge of the ion formed. Eg Na needs to lose 1 valence electron, so it will form Na+ O needs to gain 2 electrons so it will form O2– 3. Put a bracket around element that forms cation, and indicate the charge of the cation. Draw the valence electrons around the element that forms anion(ensure different symbol for electrons present initially and the electrons gained) and indicate the charge of the anion Dot–and–cross diagrams of covalent substances Generally non–metal and non–metal form covalent substances. Examples Methane, CH4 Chlorine, Cl2 Water, H2O Ammonia, NH3 Silicon tetrafluoride, SiF4 Steps to draw dot–and–cross diagrams using water, H2O as example 1. Identify the Group that element is in, which will indicate the number of valence electrons the element has. Eg O is in Group 16 so it has 6 valence electron, H has 1 valence electron 2. Identify the number of electrons that the element need to share to achieve duplet or octet configuration. Eg O needs to share 2 valence electrons, so O form 1 covalent bond each with two H atoms H needs to share 1 valence electron, so each H will form 1 covalent bond with O 3. Draw dot–and–cross diagram to show sharing of electron between H and O Checking of answers for dot–and–cross diagrams (for both ionic and covalent substances) 1. Check that the correct number of valence electrons is drawn for each atom/ion 2. Check the all the atoms/ions have duplet or octet configuration after transfer/sharing of electrons Formula and nomenclature of ionic compounds Nomenclature for ionic compounds 1. Name the cation first followed by the anion. For simple cations, the name of the cation is the same as the element. For simple anions, change the ending of the non-metal to “ide”. E.g. NaCl: sodium chloride. 2. For metals that can form ions with more than one possible charge, roman numerals are included in the name to indicate the ionic charge of the metal. E.g. FeO: iron(II) oxide. Chemical formulae for ionic compounds 1. The formula of the cation is written first followed by that of the anion. 2. The total positive and negative charges must be balanced. Reminder: The charge of a simple ion is based on the number of electrons that the atom of the element will gain or lose to achieve the electronic configuration of a noble gas. 3. If a polyatomic ion occurs more than once in the formula, it must be enclosed in brackets. Eg calcium nitrate, Ca(NO3)2 List of polyatomic ions Name Formula Name Formula ammonium ion NH4+ sulfate ion SO42– hydroxide ion OH– carbonate ion CO32– nitrate ion NO3– Formula and nomenclature of covalent substances Nomenclature and formula for covalent substances The first element is listed using the element’s name. The second element usually ends with ‘-ide’. Prefixes are used to indicate the number of atoms of each element present in the molecule. See examples given in the table below. "Mono" is not used to name the first element. Eg. name of NO2 is nitrogen dioxide. Prefixes used in naming covalent compounds Prefix Number of atoms Example Name mono– 1 NO Nitrogen monoxide di– 2 CO2 Carbon dioxide tri– 3 SO3 Sulfur trioxide tetra– 4 SiCl4 Silicon tetrachloride Note: The above rules does not apply for certain covalent substances like water (H2O), ammonia (NH3) and methane (CH4) List of elements that exist as diatomic molecules Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine Writing chemical equations Steps to follow Example 1 Write a word equation. magnesium + oxygen → magnesium oxide 2 Write the chemical equation using Mg + O2 → MgO the correct formulae for all the reactants & products. 3 Balance the equation. For O atom: (NOTE: Formulae of reactants and 2 atoms on the left and one atom on the right, products are not to be altered.) We need 2 MgO on the right to make the number of O atoms equal. i.e. Mg + O2 → 2 MgO The numbers “2” in front of Mg & For Mg atom: MgO are used to balance the equation and is called coefficients. Now 1 atom on the left and two atoms on the right. We need 2 Mg on the left to balance the number of magnesium atoms on the right. i.e. 2 units of Mg react with 1 unit of i.e. 2 Mg + O2 → 2 MgO oxygen produce 2 units of MgO. The equation is now balanced 4 Add the state symbols. 2 Mg (s) + O2 (g) → 2 MgO (s) State symbols Acids and Alkalis Examples of common acids used in the laboratory name chemical formula hydrochloric acid HCl sulfuric acid H2SO4 nitric acid HNO3 Examples of common alkalis used in the laboratory name chemical formula sodium hydroxide NaOH potassium hydroxide KOH calcium hydroxide Ca(OH)2 Salts A salt consists of a positively charged ion (cation) [except hydrogen ion] and a negatively charged ion (anion) [except oxide and hydroxide]. Examples of salts: sodium chloride, ammonium sulfate, potassium nitrate pH scale Indicators (Litmus paper) Moist blue litmus paper will turn red if the substance is acidic, ie having a pH < 7. Moist red litmus paper will turn blue if the substance in alkaline, ie having a pH > 7. Indicators (Universal Indicator) Chemical Reactions of acids 1) Acids + reactive metals → salt + hydrogen Eg. Magnesium ribbon reacts with dilute hydrochloric acid to produce magnesium chloride solution and hydrogen gas. Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g) Note: copper, silver, gold are examples of unreactive metals. Observations for H2 gas: Effervescence of colourless odourless gas. Test for H2 gas: Gas produced extinguish a burning splint with a ‘pop’ sound. 2) Acids + carbonates → salt + carbon dioxide + water Eg. Limestone (calcium carbonate) reacts with dilute hydrochloric acid to produce calcium chloride solution, carbon dioxide gas and water. CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2(g) + H2O(l) Observations for CO2 gas: Effervescence of colourless odourless gas. Test for CO2 gas: When gas is bubbled through limewater, a white precipitate is formed. 3) Acids + alkalis → salt + water. Eg. Dilute hydrochloric acid reacts with aqueous sodium hydroxide solution to produce sodium chloride solution and water. NaOH (aq) + HCl (aq) → NaCl (aq) + H2O(l) Collection of gases Collection Method Solubility of Density of Gas Examples Gas in Water Water Displacement insoluble to density does not hydrogen, slightly soluble affect gas oxygen, carbon collection dioxide Downward Delivery can be insoluble denser than air chlorine, or soluble hydrogen chloride, sulfur dioxide Upward Delivery can be insoluble less dense than ammonia or soluble air Note: You do not have to memorise the solubility and densities of gases. Drying of gases Drying Agent Gases that can be dried concentrated sulfuric acid Neutral and acidic gases, like hydrogen or hydrogen chloride can be dried. Alkaline gases like ammonia cannot be dried using this method as they will react with sulfuric acid. quicklime (calcium oxide) Neutral and alkaline gases, like oxygen or ammonia can be dried. Acidic gases like chlorine, hydrogen chloride, nitrogen dioxide, sulfur dioxide, carbon dioxide cannot be dried using this method as they will react with calcium oxide. fused calcium chloride Most gases except ammonia. Ammonia will react with calcium chloride. Note: You do not have to memorise the acidity and alkalinity of gases.