3.1.4 - Energetics

Question 1

What is the VALUE for the enthalpy change of formation of an element?

Answer 1

0 kJ mol^-1

Question 2

How do you calculate the energy change in a reaction?

Answer 2

q = mcΔt

energy change = mass of solution x heat capacity x temperature change

Question 3

How do we calculate enthalpy change of a reaction?

Answer 3

1. q=mcΔt (c= 4.18, m = mass of solution)
2. Work out moles
3. Convert q to kJ
4. -q/n = enthalpy change

Question 4

Define mean bond enthalpy

Answer 4

The enthalpy needed to break a covalent bond, averaged over different molecules.

Question 5

State the meaning of the term mean bond enthalpy for a bond: MS

Answer 5

1. The enthalpy change to break 1 mole of covalent bonds
2. Arranged over a range of compounds/molecules

Question 6

When calculating enthalpy change during the reaction of two solutions, what moles do you use?

Answer 6

The moles of the limiting reagent.

Might have to use the stoichiometry.

Question 7

What type of process is bond making?

Answer 7

Exothermic process (-)

Question 8

Suggest how, without a change to apparatus, to reduce the percentage uncertainty in the temperature change?

Answer 8

Increase the concentration of solution.

Question 9

What is enthalpy change?

Answer 9

Enthalpy change is the amount of heat energy taken in or released during any change in a system at constant pressure.

Question 10

What distinguishes an exothermic change from an endothermic change?

Answer 10

In an exothermic change, energy is transferred from the system to the surroundings and ΔH is negative; in an endothermic change, energy is absorbed from the surroundings by the system and ΔH is positive.

Question 11

What are standard conditions for reporting enthalpy changes?

Answer 11

Standard conditions are 100 kPa pressure, 298 K temperature, and solutions at 1 mol dm^-3 concentration.

Question 12

Define the standard enthalpy change of formation.

Answer 12

It is the enthalpy change when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

Question 13

What is the standard enthalpy change of combustion?

Answer 13

It is the enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions, with all reactants and products in their standard states.

Question 14

State Hess’s Law.

Answer 14

Hess’s Law states that the total enthalpy change for a reaction is the same, no matter the route by which the chemical change occurs.

Question 15

How is enthalpy change measured experimentally using calorimetry?

Answer 15

Measure the temperature change in a calibrated calorimeter and use it to calculate the heat absorbed or released, adjusting for mass, specific heat, and temperature change.

Question 16

What are some common errors in measuring enthalpy changes using calorimetry?

Answer 16

Errors include heat loss to the surroundings, incomplete combustion, evaporation of fuels, and assuming all solutions have the specific heat capacity of water.

Question 17

Define exothermic and endothermic reactions.

Answer 17

In an exothermic reaction, energy is transferred from the system to the surroundings, and the ΔH is negative. In an endothermic reaction, energy is transferred from the surroundings to the system, requiring heat energy, and the ΔH is positive.

Question 18

How is Hess’s Law applied using standard enthalpies of formation?

Answer 18

The enthalpy change for a reaction can be calculated using ΔH reaction = Σ ΔfH° products - Σ ΔfH° reactants, summing the standard enthalpies of formation of the products and subtracting those of the reactants.

Question 19

Describe the calorimetric method for measuring the enthalpy change of a reaction.

Answer 19

In the calorimetric method, reactants are mixed in an insulated container to minimize heat loss. Temperatures are measured before and after the reaction at regular intervals. The initial temperature change is extrapolated to determine the exact temperature change at the moment of mixing.

Question 20

Define the mean bond enthalpy and its significance in calculating reaction enthalpies.

Answer 20

Mean bond enthalpy is the average energy required to break one mole of a specified type of bond in a gaseous molecule. It’s significant in calculating reaction enthalpies because it allows estimation of ΔH using the formula ΔH = Σ bond enthalpies broken - Σ bond enthalpies formed, though it is less accurate than methods using formation or combustion enthalpies.

Question 21

Calculate the enthalpy change for the combustion of propan-1-ol given the mean bond enthalpies.

Answer 21

To calculate the enthalpy of combustion, use the bond enthalpies provided: ΔH = Σ bond energies broken - Σ bond energies made. Include all relevant C-H, C-C, O=O bonds broken, and CO2, H2O bonds formed. The calculation will involve subtracting the total energy released by bond formation from the total energy absorbed by bond breaking.

Question 22

How is the enthalpy change of neutralization determined experimentally?

Answer 22

The enthalpy change of neutralization is determined by mixing an acid and a base in a calorimeter, measuring the temperature change of the mixture, and calculating the heat released per mole of water formed, using Q = m × cp × ΔT.

Question 23

Explain why calorimetry experiments often yield less accurate enthalpy values than those calculated from standard enthalpy changes.

Answer 23

Experimental calorimetry often yields less accurate results due to heat losses to the surroundings, incomplete reactions, assumptions about solution properties (such as density and specific heat capacity), and measurement errors in temperature and mass.

Question 24

Describe how Hess’s Law is applied using enthalpy changes of formation and combustion to find the enthalpy change of a reaction.

Answer 24

To apply Hess’s Law using enthalpy changes of formation and combustion, write a Hess cycle linking the target reaction to formation and combustion reactions. Calculate the ΔH of the target reaction by summing the enthalpy changes for formation or combustion of reactants and subtracting those of the products.

Question 25

What precautions should be taken when conducting a calorimetry experiment to ensure accurate results?

Answer 25

Precautions include ensuring thorough mixing of reactants, minimizing heat loss by using insulated containers, accurately measuring the masses and temperatures of reactants, and accounting for the specific heat capacity.

Question 26

Describe the significance of activation energy in a chemical reaction.

Answer 26

Activation energy is the minimum energy that reactant molecules must possess for a reaction to occur. It influences the rate of reaction; higher activation energies generally lead to slower reactions at a given temperature.

Question 27

What values of ΔH does bond breaking have?

Answer 27

Positive (more than 0)

Question 28

Is bond making endo or exothermic?

Answer 28

Exothermic

Question 29

Why may a mean bond enthalpy in a data book be different from the mean bond enthalpy in a specific molecule?

Answer 29

1. The mean bond enthalpy in the data book is the average for a bigger range of molecules
2. than just the specific molecule.

Question 30

Why do you stir the solution during a calorimetry experiment for a solution?

Answer 30

To ensure it is evenly heated.

Question 31

Why is a C-I bond weaker than a C-Cl bond?

Answer 31

The shared electrons have a weaker force of attraction to the iodine nucleus vs chlorine nucleus due to the larger size of the iodine atom. Note higher shielding effectively cancels the higher charge of the iodine nucleus.