Exothermic and Endothermic Reactions

Question 1

Define exothermic

Answer 1

Energy released (exo > exit)

The reactants have more energy than products and the temperature of the surroundings increases

Question 2

Define endothermic

Answer 2

Energy absorbed (endo > enter)

The products have more energy than reactants and the temperature of the surroundings decreases

Question 3

Recall the law of conservation of energy

Answer 3

Mass cannot be created nor destroyed

Question 4

Describe the energy required to break and reform bonds

Answer 4

Breaking bonds requires energy > endothermic

Forming bonds is the opposite of breaking them so it is assumed that forming bonds releases energy > exothermic

Question 5

Define heat

Answer 5

A form of energy

Question 6

Define temperature

Answer 6

An indicator of change in energy or a measure of kinetic energy in only positive values

Scientists use kelvin (K) and linear relationships are represented with the Celsius scale

• Temperature (℃) = temperature (K) – 273
• Temperature (K) = 273 + temperature (℃)

Question 7

Define enthalpy

Answer 7

The measure of energy in a thermodynamic system

Given by the symbol H

Question 8

Describe enthalpy change

Answer 8

A change in energy/release of energy

Given by the symbol ΔH

Question 9

Recall how enthalpy change is calculated

Answer 9

Enthalpy change is determined by subtracting the enthalpy of the reactants from the enthalpy of the products

ΔH = H[products] − H[reactants]

A positive ΔH means more enthalpy in the products > endothermic
A negative ΔH means more enthalpy in the reactants > exothermic

Question 10

Describe a thermochemical equation

Answer 10

A chemical equation that includes the enthalpy change of the reaction

Question 11

Describe the stability of reactants and products

Answer 11

The products of the system are more stable than the reactants

Question 12

Describe and explain bond enthalpies

Answer 12

The energy associated with the bond

Amount of energy required to break one mole of the stated bond to give separated atoms (kJ/mol)

Question 13

Recall how average bond enthalpy is calculated

Answer 13

The sum of bonds broken and bonds formed in a reaction

For example: [using page 8 of the data booklet]
1 × H-H → +436
1 × Cl-Cl → +243
2 × H-Cl → 2 × -431 [negative as forming bonds is exothermic]
= -182kJ ∴ exothermic reaction

Question 14

Define complete combustion

Answer 14

Takes place in the presence of a sufficient amount of oxygen

For example: C + O₂ → CO₂

Question 15

Define incomplete combustion

Answer 15

Takes place when there is an insufficient amount of oxygen supply and is easier than complete combustion

For example:
C + 1/2[O₂] → CO
Then using CO:
CO + 1/2[O₂] → CO₂

Question 16

Recall Hess’s law

Answer 16

The amount of heat energy released or absorbed in a chemical reaction is constant, irrespective of the number of steps or the kind of steps by which the reaction is carried out, provided that the same reactants and products are involved

Question 17

Define latent heat

Answer 17

The energy required to change the state of a specific quantity of a substance