Energetics 1
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define Enthalpy change
The amount of heat energy taken in or given out during any change in a system provided the pressure is constant
What are standard conditions?
298K 100kPa
Define Standard enthalpy of formation
The enthalpy change when 1 mole of the compound is formed from its elements under standard conditions, all reactants being in their standard states.
Define standard enthalpy of combustion
the enthalpy change when 1 mol of substance is completely combusted in O2 under standard conditions, all reactants in their standard states.
Define the standard enthalpy of neutralisation
• the enthalpy change when an acid and an alkali react in their standard states and under standard conditions to form 1 mole of water.
define the standard enthalpy of atomisation
when 1 mole of gaseous atoms of a substance are formed from the substance in its standard state in standard conditions.
why might a data booklet value for Enthalpy of combustion be more exothermic than the value calculated using mean bond enthalpies?
incomplete combustion
Define Enthalpy change of reaction
the enthalpy change when the number of moles of reactants as specified in the balanced equation react together.
What does Hess’s law state? What is the equation?
• the total enthalpy change is independent of the route by which the chemical change takes place.
What are some reasons for an enthalpy change value being different from the data book value in the case of a neutralisation reaction?
• Heat loss to the surroundings • SHC of the apparatus not taken into account • the experiment may be carried out in non-standard conditions.
What are some reasons for an enthalpy change value being different from the data book value in the case of a combustion reaction?
• alcohol/water evaporated from apparatus • incomplete combustion
Why might the value for standard enthalpy change be more exothermic than the actual value?
• bond enthalpy might vary with environment
Why can’t the enthalpy change be determined directly in a reaction?
• not possible to react exact moles of the substances together.
Why might a decomposition reaction have a diff enthalpy change value?
• Heat loss to surroundings • Measured in non-standard conditions
Describe briefly an experimental method to determine the enthalpy change of reaction, given a known mass of CuSO4.5H20. Mention any apparatus and measurements you would make.
• Using a measuring cylinder, measure out a certain volume of water. • Pour this into a polystyrene cup. • Measure the initial temp of the water using thermometer. • Pour the known mass of CuSO4.5H20 into the water and stir. • Measure final temp of solution using a thermometer.