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17 Cards in this Set
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Enthalpy profile diagram for and ENDOthermic reaction |
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Enthalpy profile diagram for an EXOthermic reaction |
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Exothermic reactions |
System RELEASES heat energy to surroundings Enthalpy of system DECREASES Temperature of surroundings INCREASES ∆H = negative |
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Endothermic reaction |
System ABSORBS heat energy from surroundings Enthalpy of system INCREASES Temperature of surroundings DECREASES ∆H = positive |
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Explain in terms of bond breaking and forming, why a particular reaction is EXOthermic |
Breaking bonds absorbs energy Forming bonds releases energy In an EXOthermic reaction more energy is released than required More energy is released by forming bonds than energy required breaking bonds. |
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Explain in terms of bond breaking and forming, why a particular reaction is ENDOthermic |
Breaking bonds absorbs energy Forming bonds releases energy In an ENDOthermic reaction more energy is required than released More energy is required for breaking bonds than released by forming bonds |
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Why is the value of an enthalpy change, calculated using mean bond enthalpies, different to its data book value? |
Mean bond enthalpy ar average values therefore they do not measure the exact value of the bond Bonds have different strengths in different environments |
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Why is the value of an enthalpy change of combustion calculated in an experiment different to its data book value? |
Incomplete combustion Heat loss Non standard conditions |
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Enthalpy change definition |
An enthalpy change is the amount of heat released (or absorbed) by a chemical reaction carried out at constant pressure. Units are KJmol-1 |
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Average bond enthalpy definition |
Average bond enthalpy is the average enthalpy change for breaking one mole of bonds in gaseous molecules by homolytic fission. |
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Exothermic reaction |
Where heat energy is released to the surroundings. ∆H is negative. |
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Endothermic reaction definition |
Is a reaction where heat energy is absorbed from the surroundings. ∆H is positive. |
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Standard conditions |
Temperature = 25°c / 298KPressure = 1 atm / 100kPaConcentration of solutions : 1.00moldm-3 |
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Standard enthalpy on neutralisation |
Is the enthalpy change the accompanies the formation of one mole of water from neutralisation, under standard conditions. |
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The standard enthalpy change of formation |
Is the enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions. |
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The standard enthalpy change of combustion |
Is the enthalpy change for complete combustion of one mole of a substance under standard conditions, all reactants and products being in their standard states. |
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Why is it difficult to determine the enthalpy change of formation directly from a hydrocarbon such as hexane? |
Many isomers of the hydrocarbon would form. |
