Energetics

Question 1

What is the definition of enthalpy change?

Answer 1

It’s the heat energy transferred in a reaction at constant pressure

Question 2

True or False: The enthalpy change for a reaction is the same regardless of the pathway taken.

Answer 2

True

Question 3

What is the standard enthalpy change of formation?

Answer 3

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.

Question 4

What does Hess’s Law state?

Answer 4

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.

Question 5

What is the formula for calculating enthalpy change?

Answer 5

ΔH = H(products) - H(reactants)

Question 6

Which of the following is an endothermic process? A) Combustion of methane B) Dissolving ammonium nitrate in water C) Respiration D) Neutralization reaction

Answer 6

B) Dissolving ammonium nitrate in water

Question 7

True or False: In an exothermic reaction, heat is absorbed by the system.

Answer 7

False

Question 8

What is the unit of enthalpy?

Answer 8

Kilojoules per mole (kJ/mol)

Question 9

What type of reaction has a negative enthalpy change?

Answer 9

Exothermic reaction

Question 10

Fill in the blank: The heat absorbed or released during a chemical reaction is known as ____.

Answer 10

enthalpy change

Question 11

What is the standard enthalpy change of combustion?

Answer 11

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.

Question 12

Which law is used to calculate the enthalpy change of a reaction based on known enthalpy changes of other reactions?

Answer 12

Hess’s Law

Question 13

What is the sign of ΔH for an exothermic reaction?

Answer 13

Negative

Question 14

What is a calorimeter used for?

Answer 14

A calorimeter is used to measure the heat changes during a chemical reaction.

Question 15

True or False: In an exothermic reaction, the products have higher energy than the reactants.

Answer 15

False

Question 16

What is the enthalpy change when water freezes?

Answer 16

It is an exothermic process with a negative enthalpy change.

Question 17

Identify the type of reaction: A + B → C + heat.

Answer 17

Exothermic reaction

Question 18

What is meant by the term ‘heat of reaction’?

Answer 18

The heat of reaction is the change in enthalpy that occurs during a chemical reaction.

Question 19

Fill in the blank: The process of breaking bonds requires ____ energy.

Answer 19

input

Question 20

What does a positive ΔH indicate?

Answer 20

That the reaction is endothermic.

Question 21

Which of the following is an example of an exothermic reaction? A) Photosynthesis B) Respiration C) Electrolysis D) Melting ice

Answer 21

B) Respiration

Question 22

What is the relationship between bond energy and enthalpy change?

Answer 22

The enthalpy change of a reaction can be calculated using the bond energies of the reactants and products.

Question 23

Fill in the blank: In an endothermic reaction, the energy of the products is ____ than that of the reactants.

Answer 23

higher

Question 24

What is the purpose of using standard conditions (298 K and 1 atm) in thermochemistry?

Answer 24

To provide a consistent basis for comparing enthalpy changes.

Question 25

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 ?

Answer 25

Bond dissociation energy

Question 26

True or False: The heat released in an exothermic reaction is equal to the heat absorbed in the reverse endothermic reaction.

Answer 26

True

Question 27

What is the significance of a negative ΔH in a chemical reaction?

Answer 27

It indicates that the reaction releases energy to the surroundings.

Question 28

What does the term 'enthalpy of solution' refer to?

Answer 28

The enthalpy change when one mole of a solute dissolves in a solvent.

Question 29

Fill in the blank: The process of dissolving table salt in water is generally _____.

Answer 29

endothermic

Question 30

Explain why experimental value differs from data book value?

Answer 30

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.

Question 31

What are the standard conditions used?

Answer 31

Pressure = 100 kilopascals Temperature= 298K

Question 32

Formula to calculate : A) standard enthalpy of formation B) standard enthalpy of combustion

Answer 32

A) ΔH = ΣΔHf(products) – ΣΔHf(reactants) B) ΔH = ΣΔHc(reactants) – ΣΔHc(products)

Question 33

Required Practical 2 – Measuring Enthalpy Changes

Answer 33

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

Question 34

Common Sources of Error:

Answer 34

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

Question 35

Improvements:

Answer 35

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

Question 36

Why is the enthalpy change calculated lower than the data book value for this experiment ?

Answer 36

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