OCR Chemistry A-Level - Modules 1-4 Questions and Answers

OCR Chemistry A-Level - Modules 1-4 Questions and Answers what is water of crystallisation? - Correct answer water molecules that make up part of the crystal structure of a solid" "what are there exceptions to the octet rule? - Correct answer - NO (odd number of electrons) - BF3 (boron has less than 8 outershell electrons) - SF6 (sulfur has more than 8 outershell electrons)" "why can sulfur have more than 8 electrons in its outershell? - Correct answer it has an empty 3d subshell" "define a dative covalent bond - Correct answer a shared pair of electrons which has been provided by only one of the bonding atoms" "give some common examples of ions with a dative bond - Correct answer NH4+ H3O+" "what is a base? - Correct answer proton (H+) acceptors" "what is the shape of an atom determined by? - Correct answer - the number of electron pairs - electron pairs repel to become as far apart as possible - lone pairs of electrons repel more strongly than bonding pairs" "define electronegativity - Correct answer the ability of an atom to attract the bonding electrons in a covalent bond" "what creates a polar bond? - Correct answer - lone pairs offset symmetry by 102.5 degrees each - asymmetrical molecule - dipoles do not cancel out - bonded atoms with different electronegativities" "what are London forces? - Correct answer - uneven distribution of electrons causes a temporary dipole on one molecule - this induces an opposite dipole on the next molecule" "define relative isotopic mass - Correct answer the ass of an atom of an isotope of an element relative to one twelfth the mass of an atom of C-12" "why is relative atomic mass written as a decimal? - Correct answer it is a weighted average mass" "define molecular formula - Correct answer the actual number of atoms of each element in a compound" "define what an orbital is - Correct answer a region within an atom that can hold up to two electrons of opposite spins" "what shape are S orbitals? - Correct answer spherical" "what shape are orbitals? - Correct answer figure of 8" "how can you make out the electron structure of an element using the periodic table? - Correct answer period = energy level columns = electrons group = subshell" "show the oxidation of sodium and the reduction of chlorine - Correct answer Na - Na+ + e- Cl + e- - Cl- ions always go on the RHS whether oxidation or reduction" "give the properties of giant ionic compounds - Correct answer - high melting and boiling points as the lattice of strong ionic bonds requires a lot of energy to overcome - soluble in polar solvents as the ions interact with the dipole on water molecules - do not conduct when solid as the ions are fixed in place in the lattice - conduct when molten/aqueous solution as the ions are mobile" "why does boiling point increase down group 17? - Correct answer - the number of electrons in the molecules increase - larger dipoles are induced which means stronger London forces - more energy is required to overcome the intermolecular forces" "give the properties of simple covalent molecules - Correct answer - low melting and boiling points as intermolecular forces are weak - do not conduct as they have no mobile ions or delocalised electrons - nonpolar molecules will dissolve in nonpolar solvents" "why is ice less dense than water? - Correct answer - when water freezes a network of hydrogen bonds form which holds the H2O molecules apart in open lattice structure - this spacious lattice gives ice a lower density than water" "why does water have a RELATIVELY high boiling point? - Correct answer - between water molecules there are London forces as well as hydrogen bonds - a lot of energy is required to overcome these bonds" "give the general oxidation states - Correct answer group 1 element: +1 group 2 element: +2 group 3 element: +3 fluorine: ALWAYS -1 hydrogen: +1 (except with metals when it's -1) oxygen: -2 (except with F when it's +2) chlorine: -1 (except with O or F when it's positive) roman numerals: used to show the positive oxidation state of an element when it can have more than one oxidation state" "define ionic bond - Correct answer the electrostatic force of attraction between positive and negative ions" "ionic bonding occurs between metals and non-metals but which ion is the exception? - Correct answer NH4+" "define covalent bond - Correct answer the strong electrostatic force of attraction between the shared pair of electrons and the nuclei of the bonded atoms" "when non-metals covalently bond, what is their aim? - Correct answer the octet rule: - to reach a noble gas configuration (8 electrons in the outer shell)" "which type of molecules are London forces common to? - Correct answer - all molecules - however London forces are most obvious in nonpolar molecules" "what are permanent dipole-dipole forces? - Correct answer - the permanent dipole on one molecule will attract the opposite permanent dipole in a neighbouring molecule - this includes polar molecules" "what is hydrogen bonding? - Correct answer the attraction between the positive dipole on an H atom and the lone pair of electrons on an O, F or N on a different molecule" "between which type of molecules do intermolecular forces occur? - Correct answer simple covalent molecules" "what is a concentrated solution? - Correct answer a solution which contains a high number of moles of solute dissolved in a given volume of solution" "give the ideal gas equation and its units - Correct answer PV=nRT P = pressure in Pa V = volume in m3 n = moles R = 8.314 T = temperature in K" "what is the relationship between atom economy and sustainability? - Correct answer low atom economy gives poor sustainability" "how is sustainability of a reaction increased? - Correct answer - find a use for waste products - develop an alternative process with a high atom economy - sell waste products" "what is an acid? - Correct answer a proton donor" "what is the formula for ethanoic acid? - Correct answer CH3COOH" "what are alkalis? - Correct answer bases that are soluble in water" "what is the ionic equation for neutralisation reactions? - Correct answer H+ + OH- = H2O" "how are salts formed? - Correct answer by neutralising acids with metal hydroxides, metal carbonates, metal oxides" "what type of reaction is a metal plus acid? - Correct answer redox" "what makes a solution acidic? - Correct answer - high concentration of H+ ions - low pH - turns blue litmus paper pink" "how is the reaction between ammonia and an acid different? - Correct answer - no water produced, only an ammonium salt - ammonium ion is formed" "what is a strong acid? - Correct answer one that will fully dissociate, all of it reacts" "define periodicity - Correct answer the trend in properties that is repeated across a period" "why do elements in a group all react in a similar way - Correct answer they have the same number of outershell electrons" "an element in period 3 has the following successive ionisation energies, identify the element - Correct answer the largest increase in ionisation energy is between fourth and fifth which indicates that there are four outershell electrons and the fifth is being removed from a shell closer to the nucleus." "what are the factor affecting ionisation energy in terms of attractive force? - Correct answer - atomic radius - nuclear charge - shielding" "which element has the largest first ionisation energy? - Correct answer helium - no shielding - electrons are very close to the nucleus - more protons than hydrogen" "why is the second ionisation energy of rubidium larger than that of strontium? - Correct answer the second electron in rubidium is removed from a new shell closer to the nucleus" "state the factors affecting metallic bond strength - Correct answer - higher charge on metal cation, stronger attraction - as size of metal decreases, charge density increases - more delocalised electrons means greater attraction" "state the trend down group 2 - Correct answer - atomic radius increases - more shielding - nuclear attraction to outermost electrons decreases" "state the trends down group 2 - Correct answer - solubility increases - pH of solution increases (more OH- ions present) - alkalinity increases - reactivity increases" "give a use of calcium hydroxide - Correct answer to neutralise acidic soils to maximise crop yields" "give a use of magnesium hydroxide - Correct answer to neutralise excess acids in the stomach" "give the colours of the halogens - Correct answer bromine: orange in water/orange in cyclohexane iodine: brown in water/purple in cyclohexane chlorine: pale green in water/pale green in cyclohexane" "give two examples of disproportionation reactions - Correct answer Cl2 + H2O - HCl + HClO Cl2 + NaOH - NaCl + NaClO + H2O" "describe the qualitative tests in order - Correct answer 1. the carbonate test - add dilute HNO3 - if solution bubbles then there are CO32- ions present 2. the sulfate test - add excess Ba(NO3)2 - if white precipitate forms then SO42- ions are present 3. the halide test - add AgNO3 - if precipitate (white, cream, yellow) forms then halide ions are present" "describe the cation test - Correct answer NH4+ + OH- = NH3 + H2O - add aqueous sodium hydroxide to ammonium ions" "define structural isomerism - Correct answer compounds with the same molecular formula but different structural formula" "what are standard conditions? - Correct answer 298 K 100 Kpa" "what happens in an exothermic reaction? - Correct answer energy is transferred from the system to the surroundings" "define activation energy - Correct answer the minimum energy required to start a reaction by the breaking of bonds" "what happens in an endothermic reaction? - Correct answer energy is transferred from the surroundings into the system" "suggest three reasons why experimental values are often lower than accepted values - Correct answer - incomplete combustion - heat lost to surroundings - not done under standard conditions" "what is a sigma bond? - Correct answer a single covalent bond" "state the trend between number of branches in a molecule and boiling point - Correct answer the more branched the molecule, the lower the boiling point as there are less surface points of contacts so the London forces are weaker and less energy is required" "what is breaking a covalent bond called? - Correct answer bond fission (heterolytic and homolytic bond fission)" "what is a radical? - Correct answer a species with an unpaired electron" "state the trends across a period - Correct answer - number of protons increases so the nuclear charge increases - electrons are added to the same shell so there is no difference in shielding - the attraction between the nucleus and the outershell electron increases - ionisation energy increases - atomic radius decreases" "state the trends down a group - Correct answer - number of shells increases - shielding increases - the two outweigh the increase in nuclear charge - nuclear attraction to outershell electron decreases - ionisation energy decreases - atomic radius increases" "define first ionisation energy - Correct answer the energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions" "why do successive ionisation energies increase? - Correct answer - once the first electron has been removed, the next electrons have to be removed from positive ions and therefore the nuclear attraction to the outershell electron increases"