AQA A-Level Chemistry Exam Questions and Answers

Explain why CF4 has a bond angle of 109.5°. -there are 4 bonding pairs and no lone pairs -repel equally and therefore spread apart as far as possible Topic 4 Energetics State and explain change in the volume of water in calorimetry that would cause a reduction in heat loss. -higher volume -smaller increase in temperature above room temperature / increased contact with calorimetry / smaller heat loss by evaporation Define the term standard enthalpy of combustion. The enthalpy change when one mole of a substance is burnt completely in oxygen under standard conditions with all reactants and products in their standard states. State why experimental values for enthalpy change is different to mean bond enthalpy. -heat loss / not all heat transferred to apparatus -reactants and products may not be in their standard states -MBE values are taken from a range of compounds -incomplete combustion -water may end up in a gaseous state -MBE refers to gaseous species whereas enthalpy change of combustion is liquid species Give the meaning of the term mean bond enthalpy. Enthalpy for dissociating covalent bonds, averaged for that bond type for a range of values. Describe the steps you would take to determine an accurate minimum temperature that is not influenced from heat from the surroundings. -start clock when compound is added -record every minute for 5 minutes -plot temperature vs time graph -extrapolate time back to the time of mixing to determine the temperature