Unit 1: Section 4: Energetics Flashcards
What is enthalpy change
The heat energy transferred in a reaction at constant pressure
What is the notation for enthalpy change
ΔH
What are the units of enthalpy change
kj mol^-1
What are the standard conditions
100kPa or 1 atm of pressure
At what temperature are enthalpy changes measured at
298 K (25 degrees celcius)
What type of value will the enthalpy change be if the reaction is exothermic
Negative because the energy is given out to the surroudings
What type of value will the enthalpy change be if the reaction is endothermic
Positive because it’s taking in energy from the surroundings
What is bond enthalpy
The energy needed to break a bond
Is breaking bonds endothermic or exothermic
Need energy to break bonds so bond breaking is endothermic
What is the enthalpy change
The overall effect of these two changes
What is mean bond ethalpies
The average energy needed to break a cetain type of bond over a range of compounds
How do you calculate the enthalpy change
Enthalpy change = Totale energy absorbed - Total energy released
How do you find the total energy released by the reaction
Find the total energy needed to form all the new bonds
What are the three different types of entahlpy change
Standard enthalpy change of formation
Standard enthalpy change of combustion
Standard entahlpy change of reaction
Explain standard enthalpy change of formation
The enthalpy change when 1 mole of a compound is formed from its elements in their standard sates under standard conditions
Explain standard enthalpy change of combustion
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in the ir standard states
Explain standard entahlpy change of reaction
The enthalpy change when a reation occurs in molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states
What are the three things you need to know when measuring the enthalpy change in a lab
Number of moles of stuff that is reacting
Change in temperature
How much stuff you’re heating
What are experiments that measure heat given out by reaction called
Calorimetry experiments
Describe the apparatus needed in when measuring enthalpy changes in a lab
Polystyrene beaker
Solution inside beaker
Thermometer inside solution
Explain how you measure enthalpy change for reactions that happen in solution
Use the thermometer to measure the temperature at regular intervals
Use a polystryene beaker to insulate the reaction
Describe the apparatus needed to measure the enthalpy change of combustion
Stirrer
Thermometer
Water
Combustion chamber
Fuel (reactan)
Air tube
Calorimeter
Explain how you measure the enthalpy change of combustion
As the fuel burns it heats the water and you can measure the temperature change and find out how much heat the water has absorbed if you know the mass of the water and the specific capacity of the water
Explain why the method for working out the enthalpy change of combustion may give inaccurate results
Some heat may be lost to surroundings and there might be some incomplete combustion
Why is finding the starting value of the water and subtracting it from the final value not a good method of finding out the temperature change when measuring enthalpy change of combustion
Because some heat is lost fromp the calorimeter to the surroundings
How do calculate the temperature change when measuring enthalpy change of combustion
Record temperature at regular intervals (begin a couple of minutes before you start the reaction)
Plot a graph (temp against time)
Plot two lines of best fit one for the results before the reaction and one for after
The distance between the two lines at the time the reaction started is the accurate temperature change for the reaction
How do calculate the standard enthalpy change of combustion
Calculate the amount of heat lost
Calculate the number of moles of ffuel that caused thsi enthalpy change (number of moles = mass/Mr)
Use the equation - heat change/number of moles of fuel burned - to calculate
What is standard enthalpies of combustion measured in
kj mol^-1
What is the enthalpy of neutralisation
The energy change when 1 mole of water is formed by the reation of an acid and an alkali
How do you find the enthalpy of neutralisation
Add a known volume of acid to an insulated conatainer
Then add known volume of alkali and record the temperature at regular intervals
Stir solution to make sure it’s evenly heated
Work out the heat energy given out by reaction using the heatc change/number of moles equation
What is a dissolution reaction
When the reactants split up into it’s ions and makes it an aqueos solution
How do you measure the enthalpy change of a dissolution reaction
Add known mass of a solid to a known mass of water in a polystyrene cup and stir
Record the temperature regularly
How do you find out the molar enthalpy chanage of a dissolution reaction
Find the enthalpy change per mole of solute dissolved
Divide the heat energy change for the reactoin by the number of moles of solid that you started off with
What is Hess’s Law
The total enthalpy change for a reaction is independent of the route taken
What is the purpose of Hess’s Law
If you know the enthalpy change for one route of a reaction you can use that to find out the enthalpy change of the other route of the same reaction
What are entahlpy changes of formation
The heat change of going from and element to a compound
What does increasing the concentration of molecules in a reaction do to the Maxell-Boltzmann distribution curve
It increases the peak of the curve
