Energetics

Question 1

Define enthalpy change (ΔH)

Answer 1

Enthalpy change is the heat energy transferred during a chemical reaction at constant pressure.

Question 2

State the units of enthalpy change.

Answer 2

kJ mol⁻¹
J= kJ /1000

Question 3

What are the standard conditions for measuring standard enthalpy changes (ΔH°)?

Answer 3

298 K (25°C), 100 kPa, and substances in their standard states.

Question 4

Give the correct notations for the following types of standard enthalpy changes: a) Reaction b) Combustion c) Formation

Answer 4

a) Δ Н°r
b) Δ Н°С
c) Δ H°f

Question 5

What happens in an exothermic reaction?

Answer 5

In an exothermic reaction energy is released and the temperature increases

Question 6

What is the ΔH sign of an exothermic reaction? And give an example of an exothermic reaction

Answer 6

Negative (-)
An example of an exothermic reaction is Combustion

Question 7

What happens during an endothermic reaction?

Answer 7

In an endothermic reaction Energy is absorbed and temperature decreases

Question 8

What is the ΔH sign if an endothermic reaction? State an example of an endothermic reaction

Answer 8

Positive (+)
An example of an endothermic reaction is Photosynthesis

Question 9

Bond Enthalpies

Define bond enthalpy

Answer 9

Bond enthalpy is the energy required to break one mole of bonds in gaseous molecules.

Question 10

Is breaking bonds exothermic or endothermic? State the sign of ΔH.

Answer 10

Breaking bonds is endothermic; ΔH is positive (+).

Question 11

Is forming bonds exothermic or endothermic? State the sign of ΔH.

Answer 11

Forming bonds is exothermic; ΔH is negative (−).

Question 12

Write the formula for calculating overall enthalpy change using bond enthalpies.

Answer 12

ΔH = Bonds broken − Bonds formed

Question 13

Why do we use mean bond enthalpies?

Answer 13

We use mean bond enthalpies because they are averages across compounds from data tables.

Question 14

Calculating Enthalpy Change from Bonds

List the steps to calculate enthalpy change using bond enthalpies.

Answer 14

1. Draw molecules to count all bonds.
   
   2. Use bond enthalpy values and multiply by the number of each bond.
   3. Apply: ΔH = Total bonds broken − Total bonds formed

Question 15

Measuring Enthalpy Changes Experimentally

Why is a polystyrene cup used in solution experiments?

Answer 15

To reduce heat loss.

Question 16

State the equation used to calculate heat energy change. Define all terms.

Answer 16

q = mcΔT
Where:
• m = mass of solution (g)
• c = specific heat capacity (4.18 J g⁻¹ K⁻¹)
• ΔT = temperature change (K or °C)

Question 17

Combustion

Describe the steps to determine enthalpy change of combustion using a calorimeter.

Answer 17

1. Burn fuel to heat a known mass of water.
   
   2. Use q = mcΔT (to find the m= mass of water).
   3. Convert q to kJ.
   4. Find moles of fuel burned: n = mass ÷ Mr
   5. Calculate enthalpy: ΔH = q ÷ n
   6. Remember: ΔH is negative (exothermic).

Question 18

Neutralisation

How do you calculate ΔH for a neutralisation reaction?

Answer 18

1. Use stoichiometry to find moles of water formed.
   
   2. Calculate q = mcΔT.
   3. Divide q by moles of water to find ΔHneut. ΔH= q/n

Question 19

💧 6. Enthalpy Change of Solution/Dissolution

How is enthalpy change of solution determined experimentally?

Answer 19

1. Add known mass of solute to water and measure temperature change.
   
   2. Calculate q = mcΔT.
   3. work out moles of solvent by mass/Mr= moles
      4.Find ΔH per mole of solute dissolved. ΔH=q/n

Question 20

Hess’s Law

State Hess’s Law.

Answer 20

Hess’s law suggests Total enthalpy change is the same regardless of the route taken.

Question 21

What two types of data can be used in Hess cycles?

Answer 21

Enthalpy of combustion values and enthalpy of formation values.

Question 22

Write the equation for Hess’s Law using combustion data.

Answer 22

ΔHreaction = ΣΔHc(reactants) − ΣΔHc(products)

Question 23

Write the equation for Hess’s Law using formation data.

Answer 23

ΔHreaction = ΣΔHf(products) − ΣΔHf(reactants)

Question 24

What is the ΔHf of elements in their standard states?

Answer 24

0

Question 25

Practical Sources of Error List common sources of error in enthalpy experiments.

Answer 25

• Heat loss to surroundings • Incomplete combustion • Evaporation of volatile fuels

Question 26

Key equation summery

Answer 26

ΔH = q / n ΔH = Bonds broken − Bonds formed ΔH = ΣΔHf(products) − ΣΔHf(reactants ΔH = ΣΔHc(reactants) − ΣΔHc(products) Energy change (J)

Question 27

What is the standard enthalpy change of reaction (ΔH°r)?

Answer 27

Enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions. ## Footnote Example: N2(g) + 3H2(g) → 2NH3(g) ΔH°r = -92 kJ mol-1

Question 28

What is the standard enthalpy change of formation (ΔH°f)?

Answer 28

Enthalpy change when 1 mole of a compound forms from elements under standard conditions. ## Footnote Example: C(s) + O2(g) → CO2(g) ΔH°f = -394 kJ mol-1

Question 29

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

Answer 29

Enthalpy change when 1 mole of a substance combusts fully in oxygen under standard conditions. ## Footnote Example: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔH°c = -890 kJ mol-1

Question 30

What is the standard enthalpy change of neutralisation (ΔH°neut)?

Answer 30

Enthalpy change when an acid and alkali neutralise under standard conditions, to form 1 mole of water. ## Footnote Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(g) ΔH°neut = -57 kJ mol-1

Question 31

Explain why values from mean bond enthalpy calculations differ from those determined using hess’s law

Answer 31

• Mean bond enthalpies are not specific to a single molecule. • They are averaged from many compounds/different molecules whereas Hess’s Law use experimental data exact to the compounds, Their physical states and Conditions (298 K, 100 kPa)