Energetics Flashcards
What is the definition of enthalpy change?
It’s the heat energy transferred in a reaction at constant pressure
True or False: The enthalpy change for a reaction is the same regardless of the pathway taken.
True
What is the standard enthalpy change of formation?
The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, where all reactants and products are found in their standard states.
What does Hess’s Law state?
Hess’s Law states that the total enthalpy change for a reaction is the same, no matter what route is taken, provided the initial and final conditions are the same.
What is the formula for calculating enthalpy change?
ΔH = H(products) - H(reactants)
Which of the following is an endothermic process? A) Combustion of methane B) Dissolving ammonium nitrate in water C) Respiration D) Neutralization reaction
B) Dissolving ammonium nitrate in water
True or False: In an exothermic reaction, heat is absorbed by the system.
False
What is the unit of enthalpy?
Kilojoules per mole (kJ/mol)
What type of reaction has a negative enthalpy change?
Exothermic reaction
Fill in the blank: The heat absorbed or released during a chemical reaction is known as ____.
enthalpy change
What is the standard enthalpy change of combustion?
The standard enthalpy change of combustion is the enthalpy change when one mole of a compound is burned completely in oxygen under standard conditions, where all reactants and products are in their standard states.
Which law is used to calculate the enthalpy change of a reaction based on known enthalpy changes of other reactions?
Hess’s Law
What is the sign of ΔH for an exothermic reaction?
Negative
What is a calorimeter used for?
A calorimeter is used to measure the heat changes during a chemical reaction.
True or False: In an exothermic reaction, the products have higher energy than the reactants.
False
What is the enthalpy change when water freezes?
It is an exothermic process with a negative enthalpy change.
Identify the type of reaction: A + B → C + heat.
Exothermic reaction
What is meant by the term ‘heat of reaction’?
The heat of reaction is the change in enthalpy that occurs during a chemical reaction.
Fill in the blank: The process of breaking bonds requires ____ energy.
input
What does a positive ΔH indicate?
That the reaction is endothermic.
Which of the following is an example of an exothermic reaction? A) Photosynthesis B) Respiration C) Electrolysis D) Melting ice
B) Respiration
What is the relationship between bond energy and enthalpy change?
The enthalpy change of a reaction can be calculated using the bond energies of the reactants and products.
Fill in the blank: In an endothermic reaction, the energy of the products is ____ than that of the reactants.
higher
What is the purpose of using standard conditions (298 K and 1 atm) in thermochemistry?
To provide a consistent basis for comparing enthalpy changes.
What is ‘The enthalpy change required to break one mole of a specific covalent bond in a gaseous molecule, under standard conditions.’ the definition of ?
Bond dissociation energy
True or False: The heat released in an exothermic reaction is equal to the heat absorbed in the reverse endothermic reaction.
True
What is the significance of a negative ΔH in a chemical reaction?
It indicates that the reaction releases energy to the surroundings.
What does the term ‘enthalpy of solution’ refer to?
The enthalpy change when one mole of a solute dissolves in a solvent.
Fill in the blank: The process of dissolving table salt in water is generally _____.
endothermic
Explain why experimental value differs from data book value?
mean bond enthalpies are average values taken from a range of compounds, not specific to the exact molecules in the reaction. Also, bond enthalpy calculations assume all species are in the gaseous state, which may not reflect the actual conditions.
What are the standard conditions used?
Pressure = 100 kilopascals
Temperature= 298K
Formula to calculate :
A) standard enthalpy of formation
B) standard enthalpy of combustion
A) ΔH = ΣΔHf(products) – ΣΔHf(reactants)
B) ΔH = ΣΔHc(reactants) – ΣΔHc(products)
Required Practical 2 – Measuring Enthalpy Changes
Method Summary:
• Use a polystyrene cup placed in a beaker for insulation.
• Measure a known volume (e.g. 50.0 cm³) of acid using a pipette or burette and transfer it into the cup.
• Measure the initial temperature of the acid using a thermometer and continue doing this and stirring with glass rod every minute until 4th minute.
• Measure a known volume of alkali and at the fourth minute , add the alkali to the acid and stir continuously. Do not measure temperature at fourth minute.
• At the fifth minute , record the temperature and then stir and do so for every minute until 15 minutes has been reached or you receive at least three concordance temperatures ( ie the temperature is no longer increasing).this will be the maximum temperature reached.
- plot a graph of temperature ( y axis) and time ( x axis) and draw two lines of best fit ( one from 1 to 3 minutes , other from 5 to 15 minutes or last data point)
- extrapolate both lines to the fourth minute . Then calculate temperature change ie temp at 4 - temp at 1.
• Calculate energy change:
q = mcΔT, where:
• m = total mass of solution (assume 1.00 g/cm³)
• c = specific heat capacity (4.18 J g⁻¹ K⁻¹)
• ΔT = temperature change
• Calculate moles of limiting reagent.
• Find enthalpy change per mole:
ΔH = –q / n
Common Sources of Error:
Heat loss to surroundings (not all heat goes into the solution)
• Incomplete mixing, causing inconsistent temperature
• Evaporation of liquids, especially in combustion experiments
• Assuming specific heat capacity and density of solution is that of water
• Reaction may be incomplete or take time, allowing more heat loss
Improvements:
Use a lid to reduce heat loss
• Use insulated equipment like a polystyrene cup
• Record temperature every 30 seconds and extrapolate back to find true maximum
• Use a flame calorimeter with a metal calorimeter for combustion experiments
• Use a digital thermometer or data logger for more precise readings
Why is the enthalpy change calculated lower than the data book value for this experiment ?
Model answer:
• Heat loss to surroundings
• Incomplete mixing
• Assumption that no heat is absorbed by the container
• Specific heat capacity assumed to be that of water
