topic 8 - heat energy and enthalpy Flashcards
what 2 components make up chemical energy
explain what each mean
-kinetic energy, a measure of the motion of particles
-potential energy, a measure of how strongly particles interact with each other i.e. how strongly they repel or attract
what is the definition of enthalpy change
the heat energy that is transferred between the system and the surroundings at constant pressure
what is the definition of exothermic
heat energy is transferred from the system to the surroundings
so the temperature of the surroundings increases
there is a negative enthalpy change
what is the definition of endothermic
heat energy is transferred from the surroundings to the system
temperature of the surroundings decreases
there is a positive enthalpy change
a reaction has to be continuously heated so it can take place
is it endothermic or exothermic
endothermic
state the standard conditions of a reaction
100KPa at a stated temperature, usually 298K
these are the conditions needed to calculate the standard enthalpy change of a reaction
what is the definition of standard enthalpy change of combustion
the enthalpy change measured at 100KPa and a stated temperature, usually 298K
when 1 mole of a substance is completely burned in oxygen
what are the steps to writing out the equation to represent the standard enthalpy change of combustion
-make sure there is only 1 mole of the substance being combusted
-on the left hand side there should be the substance being combusted + oxygen
-balance the equation (but don’t add any molar ratios to the substance being combusted
-add state symbols
describe the experiment to work out the standard enthalpy of combustion of a liquid
-add the sample liquid to a spirit burner and weigh it
-add a known volume of water to a copper can
-measure the temperature of the water
-light the spirit burner and continuously stir the water
-once the temperature of the water has increased 20 degrees above the starting temperature extinguish the flame and immediately reweigh the spirit burner
-record the final temperature of the water
-work out the mass of liquid combusted and the temperature change of the water
4 possible sources of error when carrying out an experiement to measure standard enthalpy change of combustion
-some of the heat produced while burning the sample is transferred to the air not to the water
-incomplete combustion causes soot to form at the bottom of the copper can
-some of the heat produced is transferred to the copper can not the water
-the conditions were not standard
what are the steps to calculate standard enthalpy change of combustion
-work out the energy transferred using the equation Q=mcΔT
m is the mass of substance that changes temperature
the value of Q you get is in J so you need to convert it into KJ
-use the equation ΔH=Q/moles of substance combusted
-write the enthalpy change - ΔH=+/- number KJ mol-1
what are the steps to find the enthalpy change of combustion of a liquid
-weigh the initial mass of the spirit burner containing the sample liquid
-add a known volume of water to a copper can
-measure the initial temperature of the water with a thermometer
-light the spirit burner
-constantly stir the water with the thermometer
-when the temperature of the water has increased by 20 degrees above the original number, extinguish the flame and reweigh the spirit burner
-measure the final temperature of the water
in the experiment used to measure the enthalpy of combustion of a liquid, what 2 pieces of equipment are used to prevent heat dissipation
copper can and draught shield
in the experiment used to measure the enthalpy of combustion of a liquid, what are some sources of error
-some heat energy produced by burning the fuel is transferred to the air and copper can rather than the water, and some heat is transferred from the water to the surroundings
-incomplete combustion of the fuel, soot may form on the bottom of the copper can
-conditions may not be standard, e.g. water vapour not liquid water is produced
what are the steps to find the enthalpy change of neutralisation of an acid and alkali
-place a polystyrene cup into a glass beaker
-use a pipette to measure a set volume of acid and place into the polystyrene cup
-measure the initial temperature of the acid
-use a pipette to measure a set volume of the alkali and add to the cup
-constantly stir the solution with the thermometer
-at every 15 second interval record the temperature of the solution
-use the maximum temperature to work out the temperature increase
in the experiment used to measure the enthalpy of combustion of a liquid, what are some sources of error
-error interval of pipette and thermometer
-heat energy is transferred to the air, thermometer, polystyrene cup and glass beaker instead of the solution
what is the definition of standard enthalpy change of formation
the enthalpy change measured at 100kPa at a specified temperature, when 1 mole of a substance is formed from its elements in their standard states
what is the definition of standard enthalpy change of neutralisation
the enthalpy change measured at 100kPa and a stated temperature when 1 mole of water is produced by the neutralisation of an acid with an alkali
what is the definition of Hess’s Law
the enthalpy change of a reaction is independent of the route taken to convert reactants into products, provided the initial and final conditions are the same in each case
what is the definition of bond enthalpy
the enthalpy change when 1 mole of a bond in gaseous state is broken
