4.3 Measuring enthalpy changes Flashcards
What is the standard molar enthalpy of formation
. Enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions, all the reactants and products in their standard states.
What is the standard molar enthalpy of combustion
. Enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions, all reactants and products in standard states
What is temperature
It is related to the average kinetic energy of the particles in a system.
As particles move faster, their average kinetic energy increases and the temp increases
Temperature isn’t affected by the number of particles present
Measure with thermometer
What is heat
Measure of the total energy of all the particles present in a given amount of a substance.
It does depend on the number of particles / how much substance there is
The energy of each particle is included, so eg a bath of lukewarm water has much more heat than a red hot nail because there are more particles in it.
Heat always flows from high to low temperature, so would still flow from the nail into the bathwater if the nail was placed in despite water having more heat than the nail
What is the enthalpy change of a reaction
It is the heat given out or taken in as the reaction proceeds
How do you measure enthalpy change of a reaction
There is no instrument that measures heat, so to measure enthalpy change you have to arrange for the heat to be transferred into a particular mass of a substance eg water.
You need to know 3 things:
. Specific heat capacity of substance
. Mass of substance that is being heated up or cooled down
. Temperature change
Then put into equation
Enthalpy change= Mass X SHC X temp change
What is specific heat capacity
. The amount of heat needed to raise the temperature of 1g of a substance by 1K
. Its units are joules per gram per kelvin
J/g-1/k-1
Eg shc of water is 4.18 meaning it takes 4.18 joules of energy to raise 1 gram of water by 1 kelvin
What does the simple calorimeter do
. It measures the approximate enthalpy change when fuel burns (combustion)
. You burn the fuel to heat a known mass of water, and then measure the temperature rise of the water
Assume that all the heat from the fuel goes into the water
so put a spirit burner flame over ethanol, and above it have a beaker of 200g water with a thermometer in it to measure temp change
What is the flame calorimeter, what special features does it have
It is an improved version of the simple calorimeter used to measure enthalpy change of combustion
It has these features to reduce heat loss:
. Spiral chimney made of copper coming out of the flame beaker
. The flame is enclosed so no wind can impact it
. The fuel burns in pure oxygen so complete combustion can occur
The spirit burner and ethanol is kept inside an expanded polystyrene beaker which has oxygen going in.
The beaker is in water to reduce heat transfer.
There is a chimney made of copper to filter pump out the CO2 and water
How do you measure enthalpy changes of reactions in solution
. It is relatively easy to measure heat changes for reactions occurring in solution
. The heat is generated in the solutions themselves and only has to be kept in a calorimeter
Expanded polystyrene beakers are usually used as insulators as they reduce heat loss through their sides, and have a low specific heat capacity so absorb very little heat.
What type of reaction are neutralisation reactions in solution
Exothermic so give out heat when an acid and alkali react to make a salt and water.
How do you use moles to calculate enthalpy change
After doing the equation to find energy change, use the quantities of moles given by the balanced equation
How would you get the mass of liquid in the equation finding energy change
. Find the total volume of the liquid in solution by adding together the volumes of the two reactants.
Eg if you get 100cm3 in total, it will have a mass of 100g because density of water/ dilute solutions is approx
1gcm-3
What is a displacement reaction
A metal that is more reactive than another will displace the less reactive one from a compound
If the compound dissolves in water, this reaction can be investigated using the polystyrene beaker, same as before
Eg zinc displaces copper from copper sulphate making zinc sulphate and copper.
This is an exothermic reaction
How do you measure enthalpy change in a displacement reaction
. If a solid is added to a solution
eg if zinc is added to copper sulphate:
First find the energy change of the copper sulphate by doing mass times by shc times by temp change.
Then , find moles of copper sulphate i solution by doing concentration times volume (make sure you’re in dm3).
Once you have the moles of copper sulphate, find moles of zinc added by doing ratio
Then finally divide the energy change by moles and remember to convert the answer to KJmol-1
