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Thermodynamics
AQA A Level Chemistry
| Question | Answer |
|---|---|
| What does a negative free energy change tell us? | The process is spontaneous (feasible). |
| Define enthalpy change | The heat change at constant pressure |
| Define enthalpy of atomisation of an element. | The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state |
| Define enthalpy of hydration. | The standard enthalpy change when water molecules surround one mole of gaseous ions. |
| Define enthalpy of solution. | The standard enthalpy change when one mole of an ionic solid dissolves completely in sufficient water to form a solution in which the ions are far enough apart not to interact with each other |
| Define first electron affinity. | The standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions each with a single negative charge. |
| Define first ionisation energy. | The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge. |
| Define lattice dissociation enthalpy. | The standard enthalpy change when one mole of solid ionic compound is broken up into its free gaseous ions. |
| Define lattice formation enthalpy. | The standard enthalpy change when one mole of solid ionic compound is formed from its free gaseous ions. |
| Define mean bond enthalpy. | The enthalpy change when one mole of covalent bonds is broken, with all species in the gaseous state, average over a range of different compounds. |
| Define second electron affinity. | The standard enthalpy change when a mole of electrons is added to a mole of gaseous ions each with a single negative charge to form ions each with a double negative charge. |
| Define second ionisation energy. | The standard enthalpy change when one mole of gaseous unipositive ions is converted into a mole of gaseous ions each with a double positive charge. |
| Define standard enthalpy of atomisation of a compound. | The enthalpy change when one mole of a compound in its standard state is converted into its free gaseous atoms. |
| Define standard enthalpy of combustion. | The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all reactants and products being in their standard states. |
| Define standard enthalpy of formation. | The enthalpy change when one mole of a substance is produced from its constituent elements under standard conditions, all reactants and products being in their standard states. |
| Define the Perfect Ionic Model. | A mathematical calculation of the lattice formation enthalpy of a compound, which assumes that the positive and negative ions are perfectly spherical and that there is no covalent character in the compound. |
| State Hess's Law | The enthalpy change of a reaction depends only on the initial and final states of the reaction and is independent of the route by which the reaction occurs. |
| What are standard conditions? | A standard pressure of 100 kPa and 298K |
| What is entropy? | A measure of disorder |
| What is the entropy change? | The change in disorder of a system. An increase in disorder produces a positive entropy change. |
| Write an expression for ΔHf for sodium chloride in terms of the enthalpy changes in a Born Haber cycle (use symbols not words) | ΔHf= ΔHat Na + ΔHat Cl + ΔH 1stIE Na + ΔH 1stEA Cl + ΔHLE NaCl |
| Write an expression for ΔHf for Calcium Bromide in terms of the enthalpy changes in a Born Haber cycle (use symbols not words) | ΔHf= ΔHat Ca + 2ΔHatBr + ΔH 1stIE Ca + ΔH 2ndIE Ca + 2ΔH 1stEABr + ΔH LE CaBr2 |
| Write an expression for the lattice enthalpy of formation for Potassium Iodide in terms of the enthalpy changes in a Born Haber cycle (use symbols not words) | ΔHLE Kl= ΔHf - ΔHat K - ΔHat I - ΔH 1stIE K - ΔH 1stEA |
| Why is the lattice enthalpy of magnesium oxide so much greater than that of sodium chloride | "because in magnesium oxide, 2+ ions are attracting 2- ions; in sodium chloride, the attraction is only between 1+ and 1- ions. |
| " Why does the lattice enthalpy of the group 1 metal chlorides decrease down the group | "Attractions are governed by the distances between the centres of the oppositely charged ions, and that distance is greater as the positive ion gets bigger. |
| " Explain the relationship between charge density of the ions in an ionic lattice and the degree of covalent character of the bonding. | The greater the charge density of the ions the greater the covalent charater in the lattice because the greater the degree of polarisation of the anion by the cation. |
| How can you tell from looking at actual and theoretical lattice enthalpy data that a particular ionic compund has a significant degree of covalent character | The actual lattice enthalpy will be greater than the theoretical lattice enthalpy. The greater the difference the greater the degree of covalent character. |
| State the formula to calculate the enthalpy of solution of sodium iodide in terms of the energy changes in a Born Haber Cycle | ΔHsol= ΔHLE(dissociation) NaI + ΔHhydNa + ΔHhyd I- |
| State the formula to calculate the enthalpy of solution of calcium chloride in terms of the energy changes in a Born Haber Cycle | ΔHsol= ΔHLE(dissociation) CaCl2 + ΔHhydCa2+ + 2ΔHhydCl- |
| Fill in the missing word in this statement of the second law of thermodynamics Entropy tends to a --------------- | Maximum |
| Fill in the missing word in this description of what Entropy is the degree of -------- | Entropy is the degree of disorder. |
| Fill in the missing words entropy increases when solids ------, liquids …… , and ionic compounds ……. In water | boil, melt dissolve |
| What word goes in both these gaps. Entropy increases when the number of gas molecules ----- and the temperature ----- | increases |
| What word goes in this gap. Entropy increases when atoms and molecules are ………….. | larger |
| State the equation that links Free energy, enthalpy and entropy | "ΔG = ΔH - TΔS |
| "What is the value of ΔG for a system at equilibirum | 0 |
| What is the most likely reason that a reaction with a negative value of ΔG does not occur. | The activation energy of the reaction is too high. |
| "Calculate the entropy change when water turns to steam at 100°C (373K). Include the correct units for for entropy. The enthalpy of vaporisation of water is +44 kJ mol-1 | " " + 118 J K -1 mol -1 |
| "For this reactions state and explain the sign of ΔH and ΔS and hence ΔG C6H12(l) + 9O2(g) —> 6CO2(g) + 6H2O(g) | " " ΔS is positive due to increase in number of molecules ΔH is negative as combustion is exothermic Taking a +ive value away from a –ive value must give a –ive value ΔG must be NEGATIVE so the reaction is feasible |
| "This reaction is exothermic. Explain why it will only be feasible at low temperature. NH3(g) + HCl(g) —> NH4Cl(s) | " ΔS is negative due to the reduction in gas molecules. So ΔG will only be negative at low temperature because only at low temperature will -TΔS be less positive than ΔH is negative |
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davenantchemistry
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