AQA A-level Chemistry Paper 1 Questions and Answers

AQA A-level Chemistry Paper 1 Questions and Answers Explain what is meant by the term co-ordinate bond a covalent bond in which both electrons are donated by one atom Define the term Ligand Atom, ion or molecule which can donate a pair of electrons to a metal ion What can lattice enthalpy defined as? (2 things) Lattice enthalpy of formation and lattice enthalpy of dissociation Why is the experimental value for lattice dissociation of some compounds greater than the theoretical value? the bonding is not 100% ionic/ there is some covalent character/ there is additional covalent bonding what does the perfect ionic model assume? The bonding in the compound is not 100% ionic. In the model the ions are perfect spheres/point charges are not distorted What 2 things might cause a different lattice enthalpy? Enthalpy of bond dissociation (eg halogens) Enthalpy of formation why is the lattice enthalpy greater for ions with a high charge? There is a stronger attraction Why is the lattice enthalpy greater for smaller ions? smaller ions are more closely packed in the lattice there is a stronger attraction what is the relationship between enthalpy of lattice dissociation and enthalpy of lattice formation? same numerical value but different signs State Hess' law Enthalpy change for any chemical reaction is independent of the route taken, and depends only on the initial and final states. Why is the second electron affinity endothermic? Repulsion of the negative ion with the negative electron being added Define enthalpy of hydration The enthalpy change when 1 mole of gaseous ions is converted to one mole of aqueous ions What does enthalpy of solution equal? enthalpy of solution = lattice dissociation + enthalpy of hydration (^H(solution)=^H(lattice dissociation)+^H(hydration)) what happens when and ionic compound dissolves in water? the ionic lattice breaks up (enthalpy of lattice dissociation ) the polar water molecules form bonds with the ions (enthalpy of hydration) define enthalpy of solution the enthalpy change when one mole of ionic compounds dissolves in water to form a solution of infinite dilution why do some compounds have covalent character? one of the ions is small and has a large charge this distorts the other ion the ions are not spherical define first electron affinity the enthalpy change when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of 1- ions. What is the relationship between enthalpy of atomisation and bond dissociation enthalpy Enthalpy of atomisation produces 1 mole of gaseous atoms Bond dissociation enthalpy produces 2 moles of gaseous atoms. (BDE=EAx2) In diatomic molecules, why can't the bond enthalpy be averaged across different compounds? The bond only occurs in the diatomic molecule. Define mean bond enthalpy the energy required to break one mole of covalent bond averaged over compounds containing that bond. (kJmol-1) Define Bond dissociation enthalpy the enthalpy change when one mole of covalent bond is broken in the gaseous state Which enthalpy changes are exothermic? 2nd electron affinity Enthalpy of formation (mostly) Which Enthalpy Changes are Endothermic? enthalpy of lattice dissociation enthalpy of atomisation 1st/2nd ionisation enthalpy 2nd electron affinity bond dissociation enthalpy Why is the phrase 'infinitely separated' used in enthalpy of lattice dissociation all bonding in the ions must be broken Define enthalpy of lattice dissociation the enthalpy change when 1 mole of ionic compound is (infinitely) separated to form its constituent gaseous ions Define enthalpy of lattice formation The enthalpy change when 1 mole of an ionic compound is formed from its component gaseous ions Define first ionisation enthalpy The enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous atoms to form 1 mole of gaseous +1 ions Define enthalpy of atomisation the enthalpy change when one mole of gaseous atoms are formed from the element in its standard states Define standard enthalpy of combustion the enthalpy change when one mole of a substance is burned completely in excess oxygen with all products and reactants in their standard states under standard conditions Define enthalpy of formation Enthalpy change when 1 mole of a substance is formed from its constituent elements under standard conditions (all reactants and products in standard states). How is ionisation achieved in a mass spectrometer? High voltage supply Which removes electrons from a substance This allows ions to be accelerated by an electric field Allows them to be detected by a detector Why is the enthalpy of hydration more exothermic for magnesium ions than calcium ions? Mg ions are smaller They have a greater charge density The positive ion attracts the delta negative O more strongly Why is the hydration more exothermic for fluoride ions than chloride ions? Fluoride ions are smaller They have a greater charge density The negative ion attracts the delta +veH ion more strongly Why is the enthalpy of hydration exothermic? Energy is released when ions are attracted to the polar water molecule and because bonds are made between the ions and the water molecules