5.1

Exothermic and endothermic reactions

12 flashcards to master Exothermic and endothermic reactions

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Definition Flip

What is an exothermic reaction?

Answer Flip

An exothermic reaction transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings.

Example: Combustion of methane (CH₄) releases heat.
Definition Flip

What is an endothermic reaction?

Answer Flip

An endothermic reaction takes in thermal energy from the surroundings, leading to a decrease in the temperature of the surroundings.

Example: The reaction between ethanoic acid and sodium carbonate.
Key Concept Flip

What does a reaction pathway diagram show for an exothermic reaction?

Answer Flip

The reactants have higher energy than the products. The diagram shows a decrease in energy from reactants to products. The energy released is given out to the surroundings.

Key Concept Flip

What does a reaction pathway diagram show for an endothermic reaction?

Answer Flip

The reactants have lower energy than the products. The diagram shows an increase in energy from reactants to products. The energy is absorbed from the surroundings.

Definition Flip

What is enthalpy change (ΔH)?

Answer Flip

Enthalpy change (ΔH) is the transfer of thermal energy during a reaction. It's negative for exothermic reactions and positive for endothermic reactions.

Example: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = -890 kJ/mol (exothermic).
Definition Flip

What is the sign of ΔH for an exothermic reaction?

Answer Flip

ΔH is negative for exothermic reactions because energy is released to the surroundings.

Example: Combustion of fuels.
Definition Flip

What is the sign of ΔH for an endothermic reaction?

Answer Flip

ΔH is positive for endothermic reactions because energy is absorbed from the surroundings.

Example: Thermal decomposition of calcium carbonate.
Definition Flip

Define activation energy (Ea).

Answer Flip

Activation energy (Ea) is the minimum energy that colliding particles must have to react. It is the energy required to break the initial bonds for a reaction to occur.

Key Concept Flip

Is bond breaking an endothermic or exothermic process?

Answer Flip

Bond breaking is an endothermic process. Energy is required to overcome the forces holding the atoms together in the bond. This energy is absorbed.

Key Concept Flip

Is bond making an endothermic or exothermic process?

Answer Flip

Bond making is an exothermic process. Energy is released when new bonds are formed as the atoms are attracted to each other. This energy is released.

Key Concept Flip

How do you explain enthalpy change in terms of bond breaking and bond making?

Answer Flip

ΔH = Energy required for bond breaking - Energy released during bond making. If more energy is released than required, the reaction is exothermic (negative ΔH). If more energy is required than released, the reaction is endothermic (positive ΔH).

Key Concept Flip

Calculate the enthalpy change for H₂(g) + Cl₂(g) → 2HCl(g), given bond energies: H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol, H-Cl = 431 kJ/mol.

Answer Flip

ΔH = [1(H-H) + 1(Cl-Cl)] - [2(H-Cl)] = [436 + 242] - [2(431)] = 678 - 862 = -184 kJ/mol. Therefore, the reaction is exothermic.

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4.2 Electrolysis of specific substances 5.2 Bond energies

Key Questions: Exothermic and endothermic reactions

What is an exothermic reaction?

An exothermic reaction transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings.

Example: Combustion of methane (CH₄) releases heat.
What is an endothermic reaction?

An endothermic reaction takes in thermal energy from the surroundings, leading to a decrease in the temperature of the surroundings.

Example: The reaction between ethanoic acid and sodium carbonate.
What is enthalpy change (ΔH)?

Enthalpy change (ΔH) is the transfer of thermal energy during a reaction. It's negative for exothermic reactions and positive for endothermic reactions.

Example: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = -890 kJ/mol (exothermic).
What is the sign of ΔH for an exothermic reaction?

ΔH is negative for exothermic reactions because energy is released to the surroundings.

Example: Combustion of fuels.
What is the sign of ΔH for an endothermic reaction?

ΔH is positive for endothermic reactions because energy is absorbed from the surroundings.

Example: Thermal decomposition of calcium carbonate.

About Exothermic and endothermic reactions (5.1)

These 12 flashcards cover everything you need to know about Exothermic and endothermic reactions for your Cambridge IGCSE Chemistry (0620) exam. Each card is designed based on the official syllabus requirements.

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