Exothermic And Endothermic Reactions

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Notes

Endothermic & Exothermic Reactions

In **exothermic** reactions, thermal energy is transferred from the **system** (reacting chemicals) to the **surroundings**; the temperature of surroundings increases.
Exothermic reactions have a **negative** enthalpy change (ΔH $< 0)$ because the system loses energy.
Common examples: **combustion**, **oxidation**, **neutralisation**; also hand warmers and self-heating cans.
In **endothermic** reactions, thermal energy is transferred from the surroundings to the system; the temperature of surroundings decreases.
Endothermic reactions have a **positive** enthalpy change (ΔH $> 0)$ because the system gains energy.
Common examples: **electrolysis**, **thermal decomposition**, first stages of **photosynthesis**; also cold packs for sports injuries.
Mnemonic: In **Ex**othermic heat **Ex**its; in **En**dothermic heat **En**ters.

Enthalpy Change & Activation Energy

**Enthalpy change (ΔH)** is the transfer of thermal energy during a reaction: negative for exothermic, positive for endothermic.
**Activation energy (Eₐ)** is the minimum energy required for a successful collision between reactant particles.
Reactions with higher activation energy need more energy to start.
**Reaction pathway diagrams** plot energy (y-axis) against progress of reaction (x-axis).
For exothermic reactions: products have **lower** energy than reactants; ΔH is shown as a downward arrow.
For endothermic reactions: products have **higher** energy than reactants; ΔH is shown as an upward arrow.
Activation energy is the energy difference between reactants and the highest point on the curve.

Bond Breaking & Bond Forming

**Bond breaking** is always **endothermic** (energy absorbed from surroundings).
**Bond making** is always **exothermic** (energy released to surroundings).
If more energy is released in bond making than absorbed in bond breaking, the overall reaction is **exothermic** (ΔH negative).
If more energy is absorbed in bond breaking than released in bond making, the overall reaction is **endothermic** (ΔH positive).
The overall enthalpy change is the difference between the energy needed to break bonds and the energy released when new bonds form.

States of matter: particle spacing and arrangement in solid, liquid, and gas. (For context: energy changes in reactions involve breaking/forming bonds between particles.)

Practice questions

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1.In an exothermic reaction, thermal energy is transferred from the surroundings to the system.
Easy
True or false?
2.Bond breaking is an endothermic process.
Easy
True or false?
3.Which of the following is an example of an endothermic reaction?
Easy
- ACombustion
- BNeutralisation
- CThermal decomposition
- DOxidation
4.State the meaning of the symbol ΔH.
Medium
5.Complete the sentence using the words 'system' and 'surroundings'.
Medium
In an endothermic reaction, thermal energy is transferred from $the ____ to$ $the ____$ .
6.Explain why a reaction with a positive ΔH value is endothermic.
Medium
7.Which statement about activation energy is correct?
Hard
- AIt is the energy difference between reactants and products.
- BIt is the minimum energy required for a successful collision.
- CIt is always negative for exothermic reactions.
- DIt is the same as the enthalpy change.
8.In an exothermic reaction, the energy of the products is higher than the energy of the reactants.
Medium
True or false?

Slides

Exothermic & Endothermic Reactions

Chemical reactions involve the transfer of thermal energy between the system (reacting chemicals) and the surroundings.
In exothermic reactions, heat energy is transferred from the system to the surroundings, causing a temperature increase.
In endothermic reactions, heat energy is transferred from the surroundings to the system, causing a temperature decrease.

Which statement describes an exothermic reaction?

Heat energy is transferred from the surroundings to the system.
The temperature of the surroundings decreases.
Heat energy is transferred from the system to the surroundings.
The energy of the system increases.

Examples of Exothermic Reactions

Common exothermic reactions include combustion, oxidation, and neutralisation.
Hand warmers and self-heating cans use exothermic reactions to release heat.

Examples of Endothermic Reactions

Common endothermic reactions include electrolysis, thermal decomposition, and the first stages of photosynthesis.
Cold packs for sports injuries use endothermic reactions to absorb heat.

A student mixes $10 cm^{3}$ of NaHCO₃ with 2 g of citric acid. The temperature drops from 20°C to 16°C. What type of reaction is this?

Exothermic
Endothermic
Neither
Both

Enthalpy Change and Activation Energy

Enthalpy change (ΔH) is the transfer of thermal energy during a reaction.
ΔH is negative for exothermic reactions and positive for endothermic reactions.
Activation energy (Eₐ) is the minimum energy required for a successful collision between particles.

Reaction Pathway Diagrams

Reaction pathway diagrams show energy on the y-axis and progress of reaction on the x-axis.
The difference in energy between reactants and products is ΔH.
The activation energy is the energy difference from reactants to the peak of the curve.

In an exothermic reaction, the enthalpy change ΔH $is ____$ .

Bond Breaking and Bond Forming

Bond breaking is an endothermic process – energy is absorbed.
Bond making is an exothermic process – energy is released.
If more energy is released in bond making than absorbed in bond breaking, the overall reaction is exothermic (ΔH negative).
If more energy is absorbed in bond breaking than released in bond making, the overall reaction is endothermic (ΔH positive).

Which statement is correct?

Bond breaking is exothermic and bond making is endothermic.
Bond breaking is endothermic and bond making is exothermic.
Both bond breaking and bond making are exothermic.
Both bond breaking and bond making are endothermic.

Identifying Reaction Types from Temperature Change

A temperature increase indicates an exothermic reaction.
A temperature decrease indicates an endothermic reaction.
Example: NaOH + HCl gives a temperature rise (exothermic).
Example: NaHCO₃ + citric acid gives a temperature drop (endothermic).

Which of these are true about exothermic reactions?

Heat is transferred from the system to the surroundings.
The enthalpy change ΔH is positive.
The temperature of the surroundings increases.
More energy is absorbed in bond breaking than released in bond making.

Reaction Pathway Diagram for Endothermic Reactions

In endothermic reactions, products have higher energy than reactants.
ΔH is positive (upward arrow).
Activation energy is still the energy from reactants to the peak.

The minimum energy required for a successful collision is called $the ____$ energy.

In an endothermic reaction, the enthalpy change ΔH is:

Negative
Zero
Positive
Variable

Summary

Exothermic: heat exits system, ΔH negative, surroundings get warmer.
Endothermic: heat enters system, ΔH positive, surroundings get cooler.
Bond breaking = endothermic; bond making = exothermic.
Activation energy is the energy barrier to reaction.

Flashcards

What is the definition of an exothermic reaction?

An exothermic reaction transfers thermal energy from the system to the surroundings, resulting in a decrease in the system's energy (negative ΔH) and an increase in the temperature of the surroundings.

What is the definition of an endothermic reaction?

An endothermic reaction transfers thermal energy from the surroundings to the system, resulting in an increase in the system's energy (positive ΔH) and a decrease in the temperature of the surroundings.

Give three examples of exothermic reactions.

Combustion, oxidation, and neutralisation are examples of exothermic reactions.

Give three examples of endothermic reactions.

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

In an exothermic reaction, is the enthalpy change (ΔH) positive or negative?

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

In an endothermic reaction, is the enthalpy change (ΔH) positive or negative?

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

Define activation energy.

Activation energy is the minimum amount of energy required for a collision between reactant particles to result in a successful reaction.

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

The difference in energy between reactants and products represents the overall enthalpy change (ΔH) of the reaction.

On a reaction pathway diagram, how is activation energy shown?

Activation energy is shown as the energy difference between the reactants and the highest point on the curve (the transition state).

Is bond breaking an endothermic or exothermic process?

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

Is bond making an endothermic or exothermic process?

Bond making is always an exothermic process because energy is released to the surroundings when new bonds form.

Why is an exothermic reaction exothermic overall in terms of bond breaking and bond making?

An exothermic reaction occurs when more energy is released during bond making than is absorbed during bond breaking, resulting in a net release of energy.

Why is an endothermic reaction endothermic overall in terms of bond breaking and bond making?

An endothermic reaction occurs when more energy is absorbed during bond breaking than is released during bond making, resulting in a net absorption of energy.

In a reaction pathway diagram for an exothermic reaction, are the products at a higher or lower energy level than the reactants?

The products are at a lower energy level than the reactants, so the arrow representing ΔH points downwards.

In a reaction pathway diagram for an endothermic reaction, are the products at a higher or lower energy level than the reactants?

The products are at a higher energy level than the reactants, so the arrow representing ΔH points upwards.

What is the system in a chemical reaction?

The system is the reacting chemicals themselves.

What are the surroundings in a chemical reaction?

The surroundings are everything other than the reacting chemicals, such as the container and the air.

How can you distinguish an exothermic reaction from an endothermic reaction by temperature change?

An exothermic reaction causes the temperature of the surroundings to increase, while an endothermic reaction causes the temperature of the surroundings to decrease.

What is the mnemonic to remember exothermic and endothermic reactions?

In EXothermic reactions heat Exits the system; in ENdothermic reactions heat ENters the system.

What does the symbol ΔH stand for?

ΔH stands for the enthalpy change, which is the transfer of thermal energy during a reaction.

What is the relationship between bond breaking/forming and the overall energy change of a reaction?

The overall energy change (ΔH) equals the energy required to break bonds in the reactants minus the energy released when new bonds form in the products.

Give a real-world application of an exothermic reaction.

Hand warmers and self-heating cans of food use exothermic reactions to release heat.

Give a real-world application of an endothermic reaction.

Cold packs for sports injuries use endothermic reactions to absorb heat and reduce swelling.

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