TOPIC 8 Chemical Energetics Flashcards
What is the first law of thermodynamics?
During a chemical, energy cannot be created or destroyed. However, one form of energy can be converted into another
What are the 2 components that make up chemical energy? What are they?
Kinetic energy - measure of the motion of particles in a substance
Potential energy - measure of how strongly these particles interact
What is heat energy?
The portion of potential energy and kinetic energy of a substance that is responsible for the temperature of the substance
It is directly proportional to its absolute temperature
What is enthalpy?
A measure of the total energy of a system
What is enthalpy change?
The heat energy that is transferred between the system and the surroundings at constant pressure
What are the standard conditions that all enthalpy changes should be quoted at?
Pressure = 100kPa Temperature = usually 298K
What is the enthalpy change measured at standard conditions called?
Standard enthalpy change
In standard enthalpy change of reaction, what does ‘per mole’ refer to?
One mole of equation and not one mole of any reactant or product
Draw the enthalpy level diagram for an exothermic and endothermic reaction.
Exothermic - final enthalpy is less than initial enthalpy
Endothermic - final enthalpy is more than initial enthalpy
What are some (4) points to remember when constructing enthalpy level diagrams?
Only need to label the vertical axis
The formulae of both the reactants and products should be given
The value for 🔼H should be given, including the correct sign
It is not essential to show the activation energy
Draw an enthalpy level diagram for the following reaction:
C(s) + O2(g) —> CO2(g)
🔼H = -394kJ mol-1
Reactants higher than products
Draw an enthalpy level diagram for the following reaction:
C(s) + CO2(g) —> 2CO(g)
🔼H = +172kJ mol-1
Reactants less than products
Draw an enthalpy level diagram for the following reaction:
CH4(g) + 2O2(g) —> CO2(g) + 2H2O(l)
🔼H = -890kJ mol-1
Reactants higher than products
What is the experiment method for finding the enthalpy change of combustion?
(1) a spirit burner containing the liquid under test is weighed
(2) a known volume of water is added to a copper can
(3) the temperature of the water is measured
(4) the burner is lit
(5) the mixture is constantly stirred
(6) when the temperature of the water is approximately 20 C above initial temperature , the flame is extinguished and the burner immediately reweighed
(7) the final temperature is measured
What are the 3 steps for calculating enthalpy change of combustion and neutralisation and the formulas that are part of these steps?
Step 1: calculate the heat energy , Q , transferred to the water in kJ
Q = mc🔼T
Step 2: calculate the amount , n , of liquid burned
n = m/M
Step 3: calculate enthalpy change of combustion
🔼H = -Q/n
What are the typical sources of error when calculating enthalpy change of combustion? (5)
Some of the heat energy produced is transferred to the air and not the water
Some of the liquid is not burned completely
Some of the heat energy produced is transferred to the copper can and not the water
The conditions are not standard (water vapour not liquid water is produced)
Not all of the heat energy transferred from the water to the surrounding is compensated for
What is the experiment method for finding the enthalpy change of neutralisation?
(1) using a pipette fitted with a safety filter , place 25.0 cm3 of 1.00 mol dm-3 acid into an expanded polystyrene cup
(2) measure the temperature of the acid
(3) using a pipette fitted with s safety filter , place 25.0 cm3 of the alkali into a beaker
(4) measure the temperature of the alkali
(5) add the alkali to the acid, stir with the thermometer and measure the maximum temperature reached
What is the typical bond enthalpy change for a diatomic molecule, XY?
XY (g) —> X (g) + Y (g)
How would you calculate the bond enthalpy of diatomic molecules?
Consider each bond separately
What is the process for calculating the enthalpy change of reaction using mean bond enthalpies?
Step 1: calculate the sum of the mean bond enthalpies of the bonds broken
Step 2: calculate the sum of the mean bond enthalpies of the bonds made
Step 3: calculate the enthalpy change by subtracting the sum of the bonds made from the sum of the bonds broken
