Exothermic & Endothermic Reactions - O Level Chemistry Review

Comprehensive flashcards covering the definitions, examples, energy profiles, and bond energy calculations for exothermic and endothermic reactions based on the Cambridge O Level Chemistry syllabus.

In chemical reactions, what distinguishes the 'system' from the 'surroundings'?

The 'system' is what happens in the chemical reaction itself, while the 'surroundings' is anything other than the chemicals reacting.

What characterizes an exothermic reaction in terms of energy transfer and temperature?

Thermal energy is transferred from the chemical energy store of the system to the surroundings, causing the temperature of the surroundings to increase.

What are three typical examples of exothermic reactions mentioned in the notes?

Combustion, oxidation, and neutralisation reactions.

How is an endothermic reaction defined regarding energy transfer and temperature?

Thermal energy is taken in from the surroundings into the chemical energy store of the system, causing the temperature of the surroundings to decrease.

What are three typical examples of endothermic reactions?

Electrolysis, thermal decomposition reactions, and the first stages of photosynthesis.

What is the mnemonic provided to remember the direction of heat flow in reactions?

In EXothermic reactions heat EXits the system, and in ENdothermic reactions heat ENters the system.

What is the definition of activation energy ($E_a$)?

The minimum amount of energy that particles must collide with for the collision to be successful and for the particles to react together.

What does the term enthalpy change ($\Delta H$) represent?

The transfer of thermal energy during a reaction.

On a reaction pathway diagram, what does the difference in height between the energy of reactants and products represent?

The overall energy change of a reaction ($\Delta H$).

How do the energy levels of reactants and products compare in an exothermic reaction?

The energy of the product is lower than the reactants because thermal energy has been transferred to the surroundings.

How do the energy levels of reactants and products compare in an endothermic reaction?

The energy of the product is higher than the reactants because thermal energy has been taken in from the surroundings.

Is bond breaking an endothermic or exothermic process, and why?

Bond breaking is always an endothermic process because energy needs to be taken in from the surroundings to break the chemical bonds.

Is bond making (forming) an endothermic or exothermic process, and why?

Bond making is always an exothermic process because energy is transferred to the surroundings as the new bond is formed.

What is the sign of the enthalpy change ($\Delta H$) for an exothermic reaction?

The enthalpy change ($\Delta H$) is negative.

What is the sign of the enthalpy change ($\Delta H$) for an endothermic reaction?

The enthalpy change ($\Delta H$) is positive.

What is the mathematical formula used to calculate enthalpy change ($\Delta H$) from bond energies?

$$\text{Enthalpy change } (\Delta H) = \text{Energy taken in} - \text{Energy given out}$$