Measuring Enthalpy Changes Flashcards
What is standard molar enthalpy of formation
The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions with both reactants and products in standard states(state you expect to find it in standard conditions)
What is standard molar enthalpy of combustion
The enthalpy change when one mole of an element or compound reacts in excess oxygen under standard conditions with both reactants and products in standard states
What is the standard enthalpy change of reaction
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation under standard conditions in their standard states
What is temperature and how is it measured
The average KE of the particles in a system(as particles move faster their average KE increases and the temperature goes up), measured using a thermometer
Is temperature dependent or independent to the number of particles
Independent
What is heat
A measure of the total energy of all the particles present in a given amount of substances
Is heat dependent or independent pf the number of particles
Dependent on the number of particles present
Equation for energy transferred and units
Energy transfer = mass(g) pf solution or water x SHC of water (4.18J/g degrees Celsius) x change in temperature (degrees Celsius)
What is specific heat capacity
The amount of energy needed to raise the temperature of 1g of a substance by 1 degrees celcius
Specific heat capacity of water and solutions
4.18J/g degrees celcius
What is simple calorimetry
Used to compare the amount of energy produced by different fuels and foods by measuring the heat energy given out or taken in to heat up or cool down a known mass of water
Three measures you need to calculate enthalpy change using simple calorimetry
- Mass of substance being heated up which is often water
- Temperature change
- Specific heat capacity of substance
Why are the values found in simple calorimetry experiments usually not accurate
Usually a lot lower than the actual values as not all energy is transferred into the water as some is lost into the atmosphere
What is the bomb calorimeter and why is it more accurate than simple
Heat energy is trapped inside a sealed and insulated container to reduce heat loss to the surroundings and water is stirred continuously to make sure the heat energy is distributed evenly throughout
Do you use the total mass/volume of the mixture when calculating enthalpy change
Yes use the total volume or mass of the mixture
Units for enthalpy change
kJ/mol so divide energy change by moles of limiting reactant
What is enthalpy change of neutralisation
The enthalpy change when solutions of acid and alkali react together under standard conditions to produce one mole of water
How many g of water in 1cm3
1g=1cm3 of water as it’s density is 1g/cm3
How to calculate enthalpy change
kJ of energy change/moles of limiting reactant
Unit for energy change in E=mcdeltaH
J, divide by 1000 to get kJ
Do you need to convert temperature from degrees Celsius to kelvin in E=mcdeltaH
No as it is energy change
Which physical property should be kept constant when measuring enthalpy change
Pressure
Why is enthalpy change of neutralisation same for all strong acids and bases
As the number of moles of H+ ions and OH- ions are the same
Why are enthalpy changes not always the same for the same compound
As bond energies differ in different environments
Why are data book values different to experimental values
Data book uses bond energies calculated with all reactants and products in gaseous state while enthalpy change uses them in standard states
