2.8 Energetics

Question 1

What happens in an exothermic change?

Answer 1

Energy is transferred from the system to the surroundings, resulting in an observable increase in temperature. AH is negative.

Question 2

What are examples of exothermic processes?

Answer 2

Burning and respiration.

Question 3

What happens in an endothermic change?

Answer 3

Energy is transferred from the surroundings to the system, resulting in an observable decrease in temperature. AH is positive.

Question 4

What is an example of an endothermic process?

Answer 4

Photosynthesis.

Question 5

What is an endothermic reaction?

Answer 5

An endothermic reaction is a reaction in which the enthalpy of the products is greater than the enthalpy of the reactants.

Question 6

What is an exothermic reaction?

Answer 6

An exothermic reaction is a reaction in which the enthalpy of the products is less than the enthalpy of the reactants.

Question 7

What is the equation to calculate the energy released or absorbed (q)?

Answer 7

The energy released or absorbed, q, is calculated using the equation: q = mcΔT.

Question 8

What does ‘q’ represent in the equation q = mcΔT?

Answer 8

‘q’ is the change in heat energy, measured in joules, J.

Question 9

What does ‘m’ represent in the equation q = mcΔT?

Answer 9

‘m’ is the mass of the reaction mixture, measured in grams, g.

Question 10

What does ‘c’ represent in the equation q = mcΔT?

Answer 10

‘c’ is the specific heat capacity of the solution, measured in joules per gram per Kelvin, J g⁻¹ K⁻¹.

Question 11

What does ‘ΔT’ represent in the equation q = mcΔT?

Answer 11

‘ΔT’ is the temperature change, measured in °C.

Question 12

What is specific heat capacity?

Answer 12

The specific heat capacity of a substance is the heat required to increase the temperature of 1 g of water by 1 K.

Question 13

What is the purpose of polystyrene cups in calorimetry?

Answer 13

Polystyrene cups are used as calorimeters because they are excellent insulators and have a low specific heat capacity.

Question 14

What is the equation to calculate the energy released or absorbed (q)?

Answer 14

The energy released or absorbed, q, is calculated using the equation: q = mcΔT.

Question 15

What does ‘q’ represent in the equation q = mcΔT?

Answer 15

‘q’ is the change in heat energy, measured in joules, J.

Question 16

What does ‘m’ represent in the equation q = mcΔT?

Answer 16

‘m’ is the mass of the reaction mixture, measured in grams, g.

Question 17

What does ‘c’ represent in the equation q = mcΔT?

Answer 17

‘c’ is the specific heat capacity of the solution, measured in joules per gram per Kelvin, J g⁻¹ K⁻¹.

Question 18

What does ‘ΔT’ represent in the equation q = mcΔT?

Answer 18

‘ΔT’ is the temperature change, measured in °C.

Question 19

What is specific heat capacity?

Answer 19

The specific heat capacity of a substance is the heat required to increase the temperature of 1 g of water by 1 K.

Question 20

What are standard conditions?

Answer 20

Standard conditions are 298 K and 100 kPa.

Question 21

What is a standard enthalpy change?

Answer 21

A standard enthalpy change is the change in heat energy at constant pressure, measured at standard conditions.

Question 22

What is the standard enthalpy of formation?

Answer 22

The standard enthalpy of formation is defined as the enthalpy change when one mole of a compound is formed from its elements under standard conditions.

Question 23

What is the standard enthalpy of formation of all elements in their standard states?

Answer 23

By definition, it is zero.

Question 24

What is the standard enthalpy of combustion (ΔH°)?

Answer 24

It is defined as the enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions.

Question 25

What is the value of standard enthalpies of combustion?

Answer 25

Standard enthalpies of combustion always have negative values.

Question 26

What is the standard enthalpy of neutralisation?

Answer 26

The enthalpy change when one mole of water is produced in a neutralisation reaction under standard conditions.

Question 27

What does the symbol ΔH° represent?

Answer 27

It represents the standard enthalpy change.

Question 28

What happens to the enthalpy change when a weak acid is used?

Answer 28

The enthalpy change has a less exothermic value. ## Footnote This is because energy is required to fully ionise the weak acid which would have otherwise been released as heat.

Question 29

Why does the reaction between a weak base and a strong acid have a less exothermic value?

Answer 29

It requires extra energy to ensure the base is ionised. ## Footnote An example is the reaction between ammonia (a weak base) and hydrochloric acid (a strong acid).

Question 30

What is Hess's Law?

Answer 30

Hess's Law states that the enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.

Question 31

What does the principle of conservation of energy state?

Answer 31

The principle of conservation of energy states that energy cannot be created or destroyed but it can change from one form to another.

Question 32

Why are enthalpy changes sometimes calculated using Hess's Law?

Answer 32

Enthalpy changes are calculated using Hess's Law when the enthalpy change cannot be directly measured, such as when the activation energy is very high or the reaction rate is very slow.

Question 33

What are enthalpy cycles used for?

Answer 33

Enthalpy cycles are used to calculate enthalpy changes based on measurable enthalpy changes, such as enthalpies of formation or combustion.

Question 34

What are bond enthalpies?

Answer 34

Bond enthalpies are defined as the energy required to break one mole of a given bond averaged over many compounds.

Question 35

How can enthalpy changes for reactions involving covalent molecules be determined?

Answer 35

Enthalpy changes can be determined by considering bond enthalpies.

Question 36

What is the process of a chemical reaction in terms of bond enthalpy?

Answer 36

A chemical reaction involves breaking all the reactant bonds (endothermic) followed by forming products as atoms recombine (exothermic).

Question 37

What is the formula for calculating the enthalpy change (ΔH) for a reaction?

Answer 37

ΔH = [ bond enthalpies of reactants ] - [ bond enthalpies of products ]

Question 38

What is a tip regarding ionic bonds?

Answer 38

The breaking and making of ionic bonds involves a more complicated sequence of energetic processes and thus cannot be considered in this way.

Question 39

What is the average bond enthalpy?

Answer 39

The average bond enthalpy is the energy required to break one mole of a given bond averaged over many compounds.