energetics

Question 1

What is thermochemistry?

Answer 1

The study of heat changes during chemical reactions.

Question 2

What happens when chemical bonds break?

Answer 2

Energy is required to break bonds.

Question 3

What occurs when chemical bonds form?

Answer 3

Energy is released to form bonds.

Question 4

What is the energy change in an exothermic reaction?

Answer 4

Energy change is negative.

Question 5

What is the energy change in an endothermic reaction?

Answer 5

Energy change is positive.

Question 6

What typically happens to the temperature in an exothermic reaction?

Answer 6

Temperature usually increases.

Question 7

What typically happens to the temperature in an endothermic reaction?

Answer 7

Temperature usually decreases.

Question 8

What is enthalpy (ΔH)?

Answer 8

The measure of the heat content of a substance measured in KJmol-1.

Question 9

What is molar enthalpy change?

Answer 9

The energy change per mole of substance.

Question 10

What is the standard pressure for measuring enthalpy changes?

Answer 10

100 kPa.

Question 11

What is the standard temperature for measuring enthalpy changes?

Answer 11

298K (around 25°C).

Question 12

What is the enthalpy of formation (ΔHfθ)?

Answer 12

The enthalpy change when one mole of a substance is formed from its elements in their standard states.

Question 13

What is the enthalpy of combustion (ΔHCθ)?

Answer 13

The enthalpy change when one mole of a substance is completely burned in oxygen.

Question 14

What is the enthalpy of neutralization (ΔHneutθ)?

Answer 14

The enthalpy change when one mole of water is formed in a reaction between an acid and an alkali.

Question 15

What is calorimetry?

Answer 15

A method to calculate the energy stored in a substance by measuring heat exchange during a reaction.

Question 16

What is the equation for calculating energy change?

Answer 16

q = mcΔt.

Question 17

In calorimetry, what is the specific heat capacity of water?

Answer 17

4.18 J g-1 °C-1.

Question 18

What should be assumed about the density of solutions in calorimetry?

Answer 18

Assume all have the same density as water, 1 cm3 = 1 g.

Question 19

What is Hess’s Law?

Answer 19

The overall enthalpy change for a chemical reaction is independent of the route taken.

Question 20

What is the enthalpy change for a reaction calculated using Hess’s Law?

Answer 20

ΔH3 = ΔH1 + ΔH2.

Question 21

What is the bond dissociation enthalpy?

Answer 21

The enthalpy change when one mole of covalent bonds is broken in the gaseous state.

Question 22

What must be done to calculate the enthalpy change per mole?

Answer 22

Use the equation ΔH = q/n.

Question 23

What is the relationship between the energy of products and reactants in an endothermic reaction?

Answer 23

Energy of the products is higher than the energy of the reactants.

Question 24

What is the relationship between the energy of products and reactants in an exothermic reaction?

Answer 24

Energy of the reactants is higher than the energy of the products.

Question 25

What happens to the temperature of surroundings in an exothermic reaction?

Answer 25

Temperature of the surroundings increases.

Question 26

What happens to the temperature of surroundings in an endothermic reaction?

Answer 26

Temperature of the surroundings decreases.

Question 27

What is the significance of standard states in enthalpy measurements?

Answer 27

They ensure that reactions have gone to completion before measuring energy change.

Question 28

What are the assumptions made in simple calorimetry?

Answer 28

* The liquid in the reaction mixture is mostly water * The liquid containing the fuel has a specific heat capacity of 4.2 J K-1 g-1 * The reaction mixture has a density of 1 g cm-3 * All heat energy is absorbed by the water.

Question 29

What is bond dissociation enthalpy?

Answer 29

The enthalpy change when one mole of covalent bonds is broken with all species in the gaseous state. ## Footnote This definition highlights the specific conditions under which bond dissociation enthalpy is measured.

Question 30

What is mean bond enthalpy?

Answer 30

The average enthalpy change when one mole of a specified covalent bond is broken, averaged out across the range of compounds. ## Footnote Mean bond enthalpy provides a generalized value for bond strengths across different molecules.

Question 31

How do you calculate the total enthalpy change using mean bond enthalpies?

Answer 31

𝛴𝜟 breaking - 𝛴𝜟 making = 𝛴𝜟 total. ## Footnote This equation summarizes the process of calculating the enthalpy change for a reaction based on bond energies.

Question 32

What type of reaction has a net positive enthalpy change?

Answer 32

Endothermic reactions. ## Footnote Endothermic reactions absorb heat, resulting in an increase in enthalpy.

Question 33

What type of reaction has a net negative enthalpy change?

Answer 33

Exothermic reactions. ## Footnote Exothermic reactions release heat, leading to a decrease in enthalpy.

Question 34

What process is associated with breaking bonds?

Answer 34

Endothermic process. ## Footnote Breaking bonds requires energy input, hence it is endothermic.

Question 35

What process is associated with the formation of bonds?

Answer 35

Exothermic process. ## Footnote The formation of bonds releases energy, making it exothermic.

Question 36

Can bond enthalpies be used with substances in states other than gaseous?

Answer 36

No, this method works only with gaseous substances. ## Footnote Bond dissociation energies are specifically measured for gaseous molecules.

Question 37

What can knowing bond enthalpies help identify?

Answer 37

Mistakes when calculating the enthalpy change of a reaction. ## Footnote Understanding bond enthalpies can improve accuracy in thermodynamic calculations.

Question 38

What is the relationship between bond enthalpy and data book averages?

Answer 38

Bond enthalpy values differ from the averages listed in data books. ## Footnote Data book values are often averages that may not reflect specific bond strengths in a given context.

Question 39

Fill in the blank: Breaking bonds is an _______ process.

Answer 39

endothermic. ## Footnote This highlights the energy requirement for bond dissociation.

Question 40

Fill in the blank: The formation of bonds is an _______ process.

Answer 40

exothermic. ## Footnote This emphasizes the energy release during bond formation.