Energetics

Question 1

Endothermic Reaction

Answer 1

• Reactions that take in energy from the surroundings
• Temperature Decrease
• Bonds are broken
• Positive Enthalpy Value

Question 2

Exothermic Reaction

Answer 2

• Reactions that release energy to the surroundings
• Temperature increase
• Bonds are formed
• Negative Enthalpy Value

Question 3

Examples of Endothermic Reactions

Answer 3

• Thermal Decomposition (most of the time)
• Photosynthesis
• Heating Blue CuSO₄

Question 4

Examples of Exothermic Reactions

Answer 4

• Combustion Reactions
• Neutralisation (Acid + Alkali)
• Adding H₂O to Anhydrous CuSO₄

Question 5

Enthalpy Change

Answer 5

Heat change at constant pressure

Question 6

Standard Conditions

Answer 6

• Pressure - 100kPa
• Temperature - 298K
• Standard state of element found at room temperature and room pressure

Question 7

Standard Enthalpy of Formation

Answer 7

The enthalpy change when 1 mole of a substance is formed from its elements in their standard states under standard conditions

Question 8

Standard Enthalpy of Combustion

Answer 8

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in their standard states

Question 9

Difference between heat and temperature

Answer 9

• Temperature is average kinetic energy of particles in a system
• Heat is the total energy of all the particles in a system

Question 10

What 3 things do you need to know to measure the enthalpy change of a substance?

Answer 10

• Mass of the substance being heated/cooled/reacted
• Temperature Change
• Specific Heat Capacity of the substance being heated/cooled/reacted

Question 11

Specific Heat Capacity (SHC) Equation

Answer 11

q = mcΔT
- Heat (J) = mass (g) x SHC (Jg-1K-1) x temperature change (K or degrees celcius)

Question 12

Convert Q into enthalpy change

Answer 12

• Change to KJ (divide by 1000)
• Divide by the number of moles of the substance used to heat the water or to react to get the enthalpy change in KJ-1

Question 13

Hess’s Law

Answer 13

The enthalpy change of a chemical reaction is the same regardless of the route taken

Question 14

Bond Enthalpy

Answer 14

The energy required to break a particular covalent bond in 1 mole of a molecule in gaseous state

Question 15

Mean Bond Enthalpy

Answer 15

The average bond enthalpy for a given type of bond taken from a range of compounds

Question 16

Why might the value calculated using bond enthalpies be different to the value calculated using Hess’s Law?

Answer 16

• Hess’s Law values are more accurate as they use actual values of enthalpy of formation and combustion
• Bond enthalpy calculations use a mean rather than the actual value

Question 17

What way do the arrows point in Enthalpy of Formation calculations?

Answer 17

Up

Question 18

What way do the arrows point in Enthalpy of Combustion calculations?

Answer 18

Down