OCR A level Chemistry Module 3 Questions and Answers

OCR A level Chemistry Module 3 Questions and Answers Periods Horizontal rows on the periodic table Ionisation The removal of one or more electrons from an atom. First ionisation energy The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions Equation for IE1 of Li Li(g) -- Li+(g) + e- Factors affecting ionisation energy -atomic radius -nuclear charge -electron shielding How does atomic radius affect ionisation energy? The larger the atomic radius, the smaller nuclear attraction, as they are further away from the nucleus. How does nuclear charge affect ionisation energy? The higher the nuclear charge, the larger the attractive force is on the outer electrons. How does electron shielding affect ionisation energy? Inner shells of electrons repel the outer shell electrons because they are negative, the more shells there are the smaller is the nuclear attraction experienced by the outer electrons Successive ionisation energies The measure of the amount of energy required to remove each electron in turn, each successive energy is higher than the one before. As each electron is removed there is less repulsion between remaining electrons and drawn closer to the nucleus. Trends across a period Across each period the number of protons increase, electrons are added to the same shell so the shell is drawn inwards slightly, electron shielding remains the same. First ionisation energy increases across a period. Trends down a group Moving down a group first ionisation energy decreases because the number of shells increase so the nuclear attraction is weaker and there is more shielding. Metallic bonding a bond formed by the attraction between positively charged metal ions and the electrons around them Giant metallic lattice structure A regular structure consisting of closely packed positive metal ions in a sea of delocalised electrons. Properties of giant metallic lattices -high melting and boiling points -malleable and ductile as there are no bonds -good thermal and electrical conductors -insoluble Exothermic reaction A reaction that releases energy in the form of heat bonds are broken to start the reaction then bonds are made which releases heat Enthalpy The energy associated with the making and breaking of bonds. Endothermic reaction A reaction in which energy is absorbed, more energy is needed to break bonds than is released when new bonds are formed Endothermic enthalpy profile Exothermic enthalpy profile activation energy the minimum amount of energy required to start a chemical reaction Standard conditions 100kPa, 298K, 1 mol dm-3 Standard Enthalpy change of reaction The enthalpy change when, the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions Standard Enthalpy change of formation the enthalpy change when, 1 mole of a compound is formed, from its elements in their standard states, under standard conditions Standard enthalpy change of combustion the energy change when one mole of the substance in its standard state is burned in excess oxygen, under standard conditions Standard enthalpy change of neutralisation The enthalpy change when 1 mole of water forms from a neutralisation reaction. q= mc∆T units of q J, Joules units of T K, Kelvin Average bond enthalpy The mean average energy required to break 1 mole of a given type of bond, in gaseous molecules Effect of concentration on rate of reaction Increasing the concentration of a reactant will increase the rate of reaction as there are more particles per unit volume, therefore more frequent collisions occur How to calculation rate with concentration change in concentration/time units of rate, from concentration v time data mol dm-3 s-1 Effect of temperature on rate of reaction ~ Molecules have more kinetic energy so move faster, more frequent collisions occur ~ More molecules have EEa of reaction, so more collisions are successful Effect of surface area on rate of reaction Increasing SA of a reactant will increase the rate of reaction as there are more particles in a unit volume, therefore more frequent collisions occur Effect of pressure on rate of reaction Increasing the pressure of a gas will increase the rate of reaction as there are more particles per unit volume, therefore more frequent collisions occur Effect of catalyst on rate of reaction A catalyst increases rate by providing an alternative reaction pathway, with a lower Ea, so more collisions are successful Collision theory For a reaction to occur, the particles must collide, with the appropriate orientation, and with sufficient energy - EEa. Homogeneous catalyst a catalyst that is in the same phase as all the reactants and products in a reaction system Heterogeneous catalyst A catalyst that is in a different phase from that of the reactant substances. Economic importance of catalysts -they lower the energy demands, by doing so they reduce costs -less carbon dioxide emissions -improves yields Boltzmann distribution The distribution of energies of molecules at a particular temperature, usually shown as a graph. (a skewed normal distribution, with no limit to energy of particles) Boltzmann distribution- temperature increase - kinetic energy of all molecules increases - maximum lowers and shifts to the right - faster particles, more frequent collisions - greater proportion of particles have EEa, more successful collisions Boltzmann distribution-catalysts -Ecat Ea -more particles have EEcat -more successful collisions Dynamic equilibrium Occurs when a reversible reaction in a closed system, has equal rates of forward and reverse reactions, and concentrations of species remains constant