OCR A level Chemistry A Chains Energy and Resources, Energetics
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| What is a substance’s enthalpy content? | It’s energy or heat content.
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| How is change in enthalpy written? | ∆H
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| What is an exothermic reaction? | A reaction where heat is given out to its surroundings.
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| What is the ∆H for an exothermic reaction? | Negative.
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| What is an endothermic reaction? | A reaction where heat is taken in from the surroundings.
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| What is the ∆H for an endothermic reaction? | Positive.
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| How is ∆H calculated? | H(products) – H(reactants).
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| What are standard conditions? | 298K, 1atm, 1M.
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| What is standard enthalpy change of formation? | The enthalpy change when one mole of a substance is formed from its elements under standard conditions.
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| What is standard enthalpy change of combustion? | The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions.
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| What is Hess’ Law? | If a chemical reaction can take place by more than one route, the overall change in enthalpy for the chemical reaction is the same regardless of which route is used, provided the initial and final conditions are the same.
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| What is standard enthalpy change of atomisation? | The enthalpy change when one mole of gaseous atoms is formed from the element under standard conditions.
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| What is standard enthalpy change of neutralisation? | The enthalpy change when one mole of hydrogen atoms from an acid is neutralised by one mole of hydroxide ions from an alkali, to form one mole of water, under standard conditions.
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| What is calorimetry? | An experiment to measure enthalpy change.
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| What is needed for calorimetry? | A calorimeter and a thermometer.
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| What is the equation to find results of a calorimetry experiment? | E
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| What is the standard enthalpy change of reaction? | The enthalpy change when molar quantities of reactants as stated in the equation react together under standard conditions.
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| What is a bond enthalpy E? | The energy required to break one mole of a bond in a gaseous molecule.
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| Breaking bonds? | Requires energy – endothermic.
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| Making bonds? | Gives out energy – exothermic.
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| Are bond enthalpies positive or negative? | Positive.
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| Are bond enthalpies exact? | No, they are an average. Actual values depend on which molecule the bond is in, as they are affected by other bonds and atoms in the molecule.
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| Can bond enthalpies be compared for all molecules? | No, they can be compared for gaseous molecules only.
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| Are bond enthalpies easy to measure? | No, they are very difficult to measure directly.
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| What are bond enthalpies used for? | To estimate the enthalpy change for a reaction, to compare bond strengths, to understand structure and bonding, and to understand reaction mechanisms.
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