AS edexcel chemistry - energetics
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| What is enthalpy? | The total energy content of the reacting materials, symbol is H, units are kJ/mol
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| What is enthalpy change? | The amount of heat energy absorbed or released in a reaction, delta H, endothermic reaction has a positive value, exothermic has a negative value
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| What are standard conditions? | 298 K, 1 atmosphere pressure, 1 mol/dm3
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| What is Hess's law? | That the enthalpy change for a reaction is independent of the route taken to achieve the overall change
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| What is the standard enthalpy of reaction? | The enthalpy change for the reaction as written in the equation under standard conditions
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| What is the 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
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| What is the standard enthalpy change of formation for elements? | 0 - it is for the formation of a compound
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| Write the enthalpy of formation of pentanol | 6C (s) + 6H2 (g) + 1/2 O2 (g) -> C6H11OH
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| What is the standard enthalpy change of combustion? | The enthalpy change when 1 mol of a substance is burned completely in excess oxygen under standard conditions, all reactants and products are in standard states, always exothermic
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| What is the standard enthalpy of neutralisation? | The enthalpy change when 1 mol of water is formed by the neutralisation of an acid by excess base under standard conditions, always exothermic, concentrations must be in 1mol/dm3
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| What is the standard enthalpy of neutralisation similar for strong acids + strong alkalis? | Because the solutions are fully ionised so the ionic equation is H+ (aq) + OH- (aq) -> H20 (l)
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| Why is the standard enthalpy of neutralisation different for weak acids + weak alkalis? | They aren't fully ionised in solution so other enthalpy changes take place
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| What is standard bond enthalpy? | The enthalpy change when 1 mol of a particular chemical bond is broken in the gaseous state to give gaseous atoms, endothermic
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| What is mean bond enthalpy? | An average value for the standard bond enthalpy of a particular chemical bond across many compounds, used in Hess's law calculations, equals enthalpy change of bonds broken - enthalpy change of bonds made
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| What are limitations of mean bond enthalpy? | Inaccurate as it is only an average, only valid for gaseous reactants and products
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| How does bond enthalpy affect rate of reaction? | If all bond enthalpies are low, the reaction will proceed quickly as not much heat energy is needed to break bond
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| What is the standard enthalpy change of atomisation? | The enthalpy change when 1 mol of gaseous atoms is formed from an element in its standard state, endothermic
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| What is energy transferred equal to? | m x c x change in temp. m - mass of water being heated or cooled, excludes solute mass, 1 cm3 = 1g. c is specific heat capacity of water, 4.2 kJ/K/kg
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| What is delta H equal to in calculations by experiment? | energy transferred/ amount in mol of reactant not in excess
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| What are the assumptions made when calculating enthalpy change in an experiment? | No heat is lost to the surroundings. The specific heat capacity of the solution is approximately the same as water. The density of the solution is 1g/cm3
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| What are the possible sources of error when calculating enthalpy change in an experiment? | Heat energy may have been lost to surroundings in a slow reaction / absorbed by metal in system or thermometre
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| How do you conduct an experiment to find out the temperature change for the reaction? | You carry out a closed system reaction (no heat exchange) in a polystyrene cup with a lid as a calorimeter or with a spirit burner heating a known mass of water. Plot temperatures and extrapolate to find temperature change.
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| What is a calorimeter? | an apparatus for measuring the amount of heat involved in a chemical reaction
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| What is specific heat capacity? | The amount of energy required to raise the temperature of 1kg of water by one degree Celsius
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