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Chemistry 1.4

AS edexcel chemistry - energetics

QuestionAnswer
What is enthalpy? The total energy content of the reacting materials, symbol is H, units are kJ/mol
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
What are standard conditions? 298 K, 1 atmosphere pressure, 1 mol/dm3
What is Hess's law? That the enthalpy change for a reaction is independent of the route taken to achieve the overall change
What is the standard enthalpy of reaction? The enthalpy change for the reaction as written in the equation under standard conditions
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
What is the standard enthalpy change of formation for elements? 0 - it is for the formation of a compound
Write the enthalpy of formation of pentanol 6C (s) + 6H2 (g) + 1/2 O2 (g) -> C6H11OH
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
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
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)
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
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
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
What are limitations of mean bond enthalpy? Inaccurate as it is only an average, only valid for gaseous reactants and products
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
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
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
What is delta H equal to in calculations by experiment? energy transferred/ amount in mol of reactant not in excess
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
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
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.
What is a calorimeter? an apparatus for measuring the amount of heat involved in a chemical reaction
What is specific heat capacity? The amount of energy required to raise the temperature of 1kg of water by one degree Celsius
Created by: 11043