Topic 8 Flashcards
What is standard conditions?
Standard conditions are 100kPa (pressure) and 298K (kelvin for temperature).
What is the difference between heat and chemical energy?
Heat Energy
- Portion of potential energy and the kinetic energy of a substance that is responsible for the temperature of the substance
- The heat energy of the substance is proportional to its absolute temperature (Measured in Kelvin)
Chemical Energy
- Made up of two components
- Kinetic energy - Motion of the particles in a substance
- Potential energy - How strongly these particles interact with each other (attract + repel)
What is enthalpy change?
Enthalpy change is the heat energy being transferred between the system and the surroundings measured at a constant pressure
What is the difference between exothermic and endothermic reactions?
- Exothermic - heat energy SYSTEM → SURROUNDINGS
- Endothermic - heat energy SURROUNDINGS → SYSTEM
What do enthalpy level diagrams show?
The change in energy in exothermic and endothermic reactions
What is the diagram with exothermic and endothermic reactions?
Exothermic = High Initial → Low Final
Endothermic = Low Initial → High Final
- Horizontal axis doesn’t need labelling
- Enthalpy Change needs to be given in numbers
- +ΔH = endothermic
- -ΔH = Exothermic
What is the the enthalpy change of reaction?
The enthalpy change when the reaction occurs in the same molar quantities as the chemical equation, under standard conditions
What is the enthalpy change of formation
When 1 mole of a compound is formed from elements in their natural states under standard conditions
What is the enthalpy change of combustion
When 1 mole of substance is completely burned in oxygen under standard conditions
What is the enthalpy change of neutralisation
When an Acid and Alkali react together, under Standard conditions to form 1 mol of water
How do you find the enthalpy change of combustion of a liquid?
- Weight liquid in spirit burner
- Add known volume of water to copper can
- Measure initial temp of water
- Burner is lit whilst stirring mixture with thermometer
- Once water reaches 20 degrees flame extinguished
- Burner Reweighed
- Final temp of water measured
How do you calculate the enthalpy change?
What sources of error or assumptions were made?
Sources of Error
- Heat energy transferred to air instead of water (heat loss)
- Incomplete combustion of ethanol can occur meaning not all ethanol is turned into carbon dioxide and water
- Some heat energy is transferred to copper can rather than going to eating the water (heat loss)
- Inaccurate reading of thermometer
- Heat capacity isn’t actually 4.15
How do you find enthalpy change of neutralisation reaction?
Enthalpy Change of Neutralisation Experiment
- Pipette 25.0$cm^3$ of $1.00 mol dm^{-3}$ of acid into expanded polystyrene cup
- Measure initial temp of acid
- Pipette 25.0$cm^3$ of $1.00 mol dm^{-3}$ of into expanded polystyrene cup
- Measure temperature of alkali
- Add alkali to the acid and stir
- Measure the maximum temperature reached (this is your final)
How do you calculate the enthalpy change?
What are the rules for Cycle construction?
Reaction Type = Arrow Direction + Bottom Line
- Combustion = Arrow DOWN + $H_2O + CO_2$
- Formation = Arrow UP + Natural State
What are the steps for construction? (Combustion)
Steps
- Rewrite equation (With state symbols) + Write bottom line as $H_2O + CO_2$
- Calculate arrow values from data given (MUILPYING by BALANCE NUMBER)
- Construct calculation by going down and then up on all arrows
-
TO CHECK use the same calculation + reversed enthalpy value and it should = 0
This proves Hess’ law
What are the steps for construction? (Formation)
Steps
- Rewrite equation (With state symbols) + Write bottom line as $H_2O + CO_2$
- Calculate arrow values from data given (MUILPYING by BALANCE NUMBER)
- Construct calculation by going down and then up on all arrows
-
TO CHECK use the same calculation + reversed enthalpy value and it should = 0
This proves Hess’ law
What is the formula for calculating bond enthalpy with Bond enthalpy?
How do you work out enthalpy change?
How do you work out mean bond enthalpies?
