Exothermic And Endothermic Reactions

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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 Exothermic heat Exits; in Endothermic heat Enters.

Heat transfer outwards in an exothermic reaction

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.

Reaction pathway diagrams for exothermic and endothermic reactions

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.

Bond breaking is endothermic

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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.
Easy
5.Complete the sentence using the words 'system' and 'surroundings'.
Easy
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?
Medium
- 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.
Easy
True or false?

More questions

9.A student mixes 10 cm³ of NaOH with 10 cm³ of HCl. The initial temperature is 19 °C and the final temperature is 21 °C. What is the temperature change in °C?
Medium
- A3
- B1
- C4
- D2
10.Match each term with its correct description.
Medium
- Exothermic reaction
- Endothermic reaction
- Activation energy
- Heat exits the system
- Heat enters the system
- Minimum energy to start a reaction
11.State two examples of exothermic reactions mentioned in the source.
Easy
12.Complete the sentence about bond breaking and bond forming.
Medium
If more energy is ____ to break bonds than is ____ when new bonds form, the reaction is endothermic.
13.Arrange the following steps in the correct order for an exothermic reaction.
Medium
- Bonds in reactants break (energy absorbed)
- New bonds form in products (energy released)
- Overall energy is released to surroundings
- Reactants have higher energy than products
14.Which of the following are examples of exothermic reactions? (Select all that apply.)
Medium
- ACombustion
- BElectrolysis
- CNeutralisation
- DThermal decomposition
- EOxidation
15.In an endothermic reaction, the temperature of the surroundings increases.
Easy
True or false?
16.Match each reaction type with the correct energy change description.
Easy
- Exothermic
- Endothermic
- Energy of products is lower than reactants
- Energy of products is higher than reactants
17.Which of the following are true about bond breaking and bond forming? (Select all that apply.)
Medium
- ABond breaking releases energy.
- BBond forming releases energy.
- CBond breaking is endothermic.
- DBond forming is endothermic.
18.Arrange the following events in the correct order for an endothermic reaction.
Medium
- Energy is absorbed from surroundings
- Bonds in reactants break
- New bonds form in products
- Products have higher energy than reactants
19.A student mixes 10 cm³ of sodium hydrogencarbonate solution with 2 g of citric acid. The initial temperature is 20 °C and the final temperature is 16 °C. Which statement is correct?
Medium
- AThe reaction is exothermic because the temperature decreased.
- BThe reaction is endothermic because the temperature decreased.
- CThe reaction is exothermic because the temperature increased.
- DThe reaction is endothermic because the temperature increased.
20.Which of the following is a correct statement about the enthalpy change (ΔH) for an exothermic reaction?
Medium
- AΔH is positive because energy is absorbed.
- BΔH is negative because energy is released.
- CΔH is positive because energy is released.
- DΔH is negative because energy is absorbed.
21.In a reaction pathway diagram for an endothermic reaction, which of the following is true?
Medium
- AThe products have lower energy than the reactants.
- BThe activation energy is the difference between reactants and products.
- CThe enthalpy change is positive.
- DThe reaction releases heat to the surroundings.
22.Bond breaking is an exothermic process.
Easy
True or false?
23.Which of the following are examples of endothermic reactions? (Select all that apply.)
Medium
- ACombustion
- BThermal decomposition
- CNeutralisation
- DElectrolysis
- EPhotosynthesis (first stages)
24.Arrange the following steps in the correct order for calculating the enthalpy change of a reaction from bond energies.
Hard
- Calculate total energy released when new bonds form.
- Calculate total energy absorbed to break bonds.
- Subtract the energy absorbed from the energy released.
- Identify all bonds broken and formed.
25.In a reaction pathway diagram for an exothermic reaction, which of the following is true?
Easy
- AThe products have higher energy than the reactants
- BThe activation energy is the energy difference between reactants and products
- CThe enthalpy change (ΔH) is negative
- DThe enthalpy change (ΔH) is positive
26.In an experiment to measure temperature change, a student adds magnesium ribbon to dilute sulfuric acid. The initial temperature is 19 °C and the final temperature is 31 °C. Which of the following is the temperature change?
Easy
- A12 °C decrease
- B12 °C increase
- C50 °C increase
- D50 °C decrease
27.Match each reaction type to the correct description of heat transfer.
Easy
- Exothermic
- Endothermic
- Heat transferred from system to surroundings
- Heat transferred from surroundings to system
28.Arrange the following steps in the correct order for an exothermic reaction to occur.
Medium
- Bonds in reactants break (endothermic)
- New bonds form in products (exothermic)
- Overall heat is released to surroundings
29.Which of the following are exothermic reactions? (Select all that apply.)
Medium
- ACombustion of methane
- BThermal decomposition of calcium carbonate
- CNeutralisation of acid and alkali
- DElectrolysis of water
30.Bond making is an endothermic process.
Easy
True or false?
31.Which of the following are true about activation energy? (Select all that apply.)
Medium
- AIt is the minimum energy required for a successful collision
- BIt is the same for all reactions
- CIt is the energy difference between reactants and the highest point on the reaction pathway diagram
- DIt is always positive
32.Arrange the following steps in the correct order to calculate the enthalpy change of a reaction from bond energies.
Medium
- Identify all bonds broken in reactants
- Identify all bonds formed in products
- Sum the bond energies for bonds broken
- Sum the bond energies for bonds formed
- Subtract total energy released from total energy absorbed
33.A student mixes 10 cm³ of sodium hydroxide solution with 10 cm³ of hydrochloric acid. The initial temperature is 19 °C and the final temperature is 21 °C. Which type of reaction is this?
Easy
- AExothermic
- BEndothermic
- CNeither exothermic nor endothermic
- DCannot be determined
34.In an experiment, 10 cm³ of 0.5 mol/dm³ hydrochloric acid is neutralised by 10 cm³ of 0.5 mol/dm³ sodium hydroxide. The initial temperature of both solutions is 20.0 °C. After mixing, the highest temperature recorded is 23.5 °C. Assuming the specific heat capacity of the solution is 4.2 J/g°C and the density is 1.0 g/cm³, what is the enthalpy change for the neutralisation reaction in kJ/mol?
Hard
- A-29.4 kJ/mol
- B-58.8 kJ/mol
- C-14.7 kJ/mol
- D-117.6 kJ/mol
35.In an experiment, a student adds magnesium ribbon to dilute sulfuric acid in a polystyrene cup and records a temperature rise of 12°C. Which of the following is the most likely reason for using a polystyrene cup?
Easy
- ATo prevent heat loss to the surroundings
- BTo allow the reaction to proceed faster
- CTo prevent the acid from spilling
- DTo observe the colour change
36.A student performs an experiment to determine whether a reaction is exothermic or endothermic. They mix 10 cm³ of 0.5 mol/dm³ hydrochloric acid with 10 cm³ of 0.5 mol/dm³ sodium hydroxide in a polystyrene cup. The initial temperature of both solutions is 20°C. After mixing, the highest temperature recorded is 26°C. The student then calculates the enthalpy change. Which of the following additional pieces of information is essential for the calculation?
Medium
- AThe density of the solutions
- BThe specific heat capacity of the solution
- CThe mass of the polystyrene cup
- DThe volume of the cup
37.In a reaction pathway diagram for an endothermic reaction, which arrow correctly represents the activation energy?
Medium
- AThe arrow from the reactants to the highest point on the curve
- BThe arrow from the products to the highest point on the curve
- CThe arrow from the reactants to the products
- DThe arrow from the highest point to the products
38.In an exothermic reaction, the enthalpy change (ΔH) is positive.
Easy
True or false?
39.Match each term on the left with its correct description on the right.
Medium
- Exothermic reaction
- Endothermic reaction
- Activation energy
- Enthalpy change
- Heat taken in from surroundings
- Heat given out to surroundings
- Minimum energy needed to start a reaction
- Overall heat energy change of a reaction
40.In an experiment to investigate temperature changes, a student places 10 cm³ of sodium hydroxide solution in a polystyrene cup and adds 10 cm³ of hydrochloric acid. The initial temperature is 20°C and the final temperature is 24°C. Which of the following are correct conclusions? (Select all that apply.)
Medium
- AThe reaction is exothermic.
- BThe temperature change is +4°C.
- CThe reaction is endothermic.
- DThe enthalpy change is negative.
41.In a bond energy calculation, the total energy absorbed to break bonds is 500 kJ/mol and the total energy released in forming new bonds is 620 kJ/mol. What is the overall enthalpy change (ΔH) for the reaction?
Medium
42.Complete the sentences about bond breaking and bond forming.
Easy
Bond breaking is an ____ process. Bond forming is an ____ process.
43.In an experiment, 10 cm³ of sulfuric acid reacts with excess magnesium ribbon. The initial temperature is 19 °C and the final temperature is 31 °C. Calculate the temperature change.
Easy
44.The bond energies for H-H, Cl-Cl, and H-Cl are 436 kJ/mol, 243 kJ/mol, and 431 kJ/mol respectively. Calculate the enthalpy change (ΔH) for the reaction: H₂ + Cl₂ → 2HCl.
Hard
45.In an experiment, 10 cm³ of 0.5 mol/dm³ hydrochloric acid is neutralised by 10 cm³ of 0.5 mol/dm³ sodium hydroxide. The initial temperature of both solutions is 20.0 °C. After mixing, the highest temperature recorded is 23.5 °C. Calculate the temperature change.
Easy
46.Complete the sentence about energy changes in reactions.
Easy
In an exothermic reaction, the energy of the products is ____ than the energy of the reactants, and ΔH is ____.
47.State whether bond breaking is endothermic or exothermic.
Easy
48.Explain why the reaction between hydrogen and chlorine is exothermic. Use the following bond energies: H–H = 436 kJ/mol, Cl–Cl = 243 kJ/mol, H–Cl = 431 kJ/mol. In your answer, compare the energy required to break bonds and the energy released when new bonds form.
Hard
49.In a bond energy calculation, the total energy absorbed to break bonds is 500 kJ/mol and the total energy released in forming new bonds is 620 kJ/mol. What is the overall enthalpy change (ΔH) for the reaction?
Medium
- A-120 kJ/mol
- B-132 kJ/mol
- C-108 kJ/mol
- D-96 kJ/mol
50.In an experiment, 10 cm³ of sulfuric acid reacts with excess magnesium ribbon. The initial temperature is 19 °C and the final temperature is 31 °C. Calculate the temperature change.
Easy
- A12 °C
- B14 °C
- C13 °C
- D11 °C
51.The bond energies for H-H, Cl-Cl, and H-Cl are 436 kJ/mol, 243 kJ/mol, and 431 kJ/mol respectively. Calculate the enthalpy change (ΔH) for the reaction: H₂ + Cl₂ → 2HCl.
Medium
- A-165 kJ/mol
- B-147 kJ/mol
- C-183 kJ/mol
- D-201 kJ/mol
52.In an experiment, 10 cm³ of 0.5 mol/dm³ hydrochloric acid is neutralised by 10 cm³ of 0.5 mol/dm³ sodium hydroxide. The initial temperature of both solutions is 20.0 °C. After mixing, the highest temperature recorded is 23.5 °C. Calculate the temperature change.
Medium
- A3.5 °C
- B4.5 °C
- C2.5 °C
- D5.5 °C

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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