Exothermic and endothermic reactions

1. Overview

All chemical reactions involve a transfer of energy between the reaction system and its surroundings. This energy is usually transferred as thermal energy (heat), which determines whether a reaction feels hot or cold. Understanding these energy changes is vital for controlling industrial processes and predicting how substances will react.

---

Key Definitions

• Exothermic Reaction: A reaction that transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings.
• Endothermic Reaction: A reaction that takes in thermal energy from the surroundings, leading to a decrease in the temperature of the surroundings.
• Enthalpy Change ($\Delta H$): The transfer of thermal energy during a reaction (at constant pressure).
• Activation Energy ($E_a$): The minimum energy that colliding particles must have to react.
• Bond Energy: The amount of energy required to break one mole of a specific covalent bond.

---

Core Content

Exothermic Reactions

• Energy is released to the surroundings.
• The temperature of the surroundings increases.
• Examples: Combustion of fuels, respiration, and the reaction between magnesium and hydrochloric acid.
  • Word Equation: Magnesium + hydrochloric acid → magnesium chloride + hydrogen
  • Symbol Equation: $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$

Endothermic Reactions

• Energy is absorbed from the surroundings.
• The temperature of the surroundings decreases.
• Examples: Photosynthesis, thermal decomposition of calcium carbonate, and the reaction between citric acid and sodium hydrogencarbonate.
  • Word Equation: Calcium carbonate → calcium oxide + carbon dioxide
  • Symbol Equation: $CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$

Reaction Pathway Diagrams (Core)

These diagrams show the relative energy levels of reactants and products.

• Exothermic: The products are at a lower energy level than the reactants because energy has been lost to the surroundings.
• Endothermic: The products are at a higher energy level than the reactants because energy has been absorbed.

---

Extended Content (Extended Curriculum Only)

Enthalpy Change ($\Delta H$)

The energy change is expressed as $\Delta H$ (measured in kJ/mol):

• Exothermic: $\Delta H$ is negative ($-$) because the system loses energy.
• Endothermic: $\Delta H$ is positive ($+$) because the system gains energy.

Activation Energy ($E_a$)

Even exothermic reactions require an initial "push" to start. This is the activation energy ($E_a$), represented by the height of the "hump" from the reactant level to the peak of the curve on a diagram.

Detailed Reaction Pathway Diagrams

When drawing these for exams, you must label:

1. Reactants and Products on their respective energy levels.
2. Enthalpy Change ($\Delta H$): The vertical difference between reactants and products.
3. Activation Energy ($E_a$): The arrow from the reactant level to the peak of the curve.

Bond Breaking and Bond Making

• Bond breaking is endothermic: Energy must be taken in to break chemical bonds.
• Bond making is exothermic: Energy is released when new bonds form.
• The overall enthalpy change depends on the balance:
  • If more energy is released making bonds than is taken in breaking them, the reaction is exothermic.
  • If more energy is taken in breaking bonds than is released making them, the reaction is endothermic.

Calculating Enthalpy Change using Bond Energies

Formula: $\Delta H = \text{Energy taken in to break bonds} - \text{Energy released making bonds}$

Worked Example: Calculate the enthalpy change for the reaction: $H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$ Bond energies: H-H = 436 kJ/mol; Cl-Cl = 242 kJ/mol; H-Cl = 431 kJ/mol

1. Energy in (breaking bonds):
   • 1 × (H-H) = 436
   • 1 × (Cl-Cl) = 242
   • Total In = $436 + 242 = 678 \text{ kJ/mol}$
2. Energy out (making bonds):
   • 2 × (H-Cl) = $2 \times 431 = 862 \text{ kJ/mol}$
3. Enthalpy Change ($\Delta H$):
   • $\Delta H = 678 - 862 = -184 \text{ kJ/mol}$
   • Since the value is negative, the reaction is exothermic.

---

Key Equations

• Enthalpy Change: $\Delta H = \text{Energy}{broken} - \text{Energy}{formed}$
  • $\Delta H$: Enthalpy Change (kJ/mol)
  • Positive value ($+$): Endothermic
  • Negative value ($-$): Exothermic

---

Common Mistakes to Avoid

• ❌ Wrong: Thinking that exothermic reactions "take in" heat because they feel hot.
• ✅ Right: Exothermic reactions release heat, which is why the surroundings (and your thermometer) get hotter.
• ❌ Wrong: Drawing the $\Delta H$ arrow from the peak of the graph.
• ✅ Right: $\Delta H$ is only the difference between the reactants and products.
• ❌ Wrong: Forgetting to multiply bond energies by the coefficients in the balanced equation (e.g., $2HCl$ means $2 \times H-Cl$ bonds).

---

Exam Tips

• Command Words: If asked to "Interpret" a diagram, look at whether the product line is higher or lower than the reactant line to identify the reaction type.
• Calculations: Always show your working in three clear steps: (1) Bonds broken, (2) Bonds made, (3) The subtraction. This ensures partial marks if you make a calculator error.
• Signs: Always include the $+$ or $-$ sign in your final answer for $\Delta H$.
• Real-world context: Be prepared to identify reactions in context. Portable hand-warmers use exothermic reactions, while instant ice packs use endothermic reactions.

---

Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [6 marks]

Question:

A student investigates the temperature change when potassium hydroxide pellets are dissolved in water. They add 5.0 g of potassium hydroxide to 50 cm³ of water in an insulated beaker. The initial temperature of the water is 22.0 °C, and the final temperature of the solution is 35.0 °C.

(a) State whether dissolving potassium hydroxide in water is an exothermic or endothermic process. Explain your answer. [2]

(b) Define the term "enthalpy change". [2]

(c) State the sign of the enthalpy change, ΔH, for this reaction. [1]

(d) Sketch a reaction pathway diagram for this reaction, labelling the reactants, products, and the direction of enthalpy change. [1]

Worked Solution:

(a)

1. The process is exothermic. The temperature of the surroundings (the water) increased.
2. Exothermic reactions release heat to the surroundings, causing the temperature to rise. This links the observation to the definition of an exothermic reaction.

How to earn full marks:

• State "exothermic" to get 1 mark.
• Explain that the temperature increased, earning the second mark.

(b)

1. Enthalpy change is the transfer of thermal energy during a reaction. This defines enthalpy change.

How to earn full marks:

• State that enthalpy change is the "transfer of thermal energy" or "heat transfer".
• Mention that it occurs "during a reaction".

(c)

1. Negative. Exothermic reactions have a negative enthalpy change.

How to earn full marks:

• State "negative" or "-" to earn the mark.

(d)

1. 📊A reaction pathway diagram showing reactants (KOH(s) + H2O(l)) at a higher energy level than the products (KOH(aq)). An arrow goes from reactants to products, indicating a decrease in energy. The arrow should be labelled "ΔH".

How to earn full marks:

• Correctly show reactants at a higher energy level than products.
• Label the arrow showing energy change as "ΔH" or "enthalpy change".

Common Pitfall: Make sure you clearly understand the difference between exothermic and endothermic reactions. Relate the temperature change of the surroundings directly to whether heat is being released or absorbed by the reaction.

Exam-Style Question 2 — Short Answer [5 marks]

Question:

Methane ($CH_4$) reacts with oxygen ($O_2$) in a combustion reaction, producing carbon dioxide ($CO_2$) and water ($H_2O$).

(a) State whether bond breaking is an exothermic or endothermic process. [1]

(b) Explain why bond breaking requires energy. [2]

(c) State whether bond making is an exothermic or endothermic process. [1]

(d) In the combustion of methane, more energy is released during bond making than is absorbed during bond breaking. State whether the overall reaction is exothermic or endothermic. [1]

Worked Solution:

(a)

1. Endothermic. Bond breaking requires an input of energy.

How to earn full marks:

• State "endothermic" to get 1 mark.

(b)

1. Energy is required to overcome the attractive forces between atoms in the bond. This explains why energy is needed.
2. Therefore, energy must be supplied to break the bond. This reinforces the explanation.

How to earn full marks:

• Mention that "attractive forces" exist within the bond.
• State that energy is needed to "overcome" or "break" these forces.

(c)

1. Exothermic. Bond making releases energy.

How to earn full marks:

• State "exothermic" to get 1 mark.

(d)

1. Exothermic. More energy released than absorbed means a net release of energy to the surroundings.

How to earn full marks:

• State "exothermic" to get 1 mark.

Common Pitfall: Remember that bond breaking always requires energy (endothermic), and bond making always releases energy (exothermic). The overall reaction's enthalpy change depends on the difference between these two energy changes.

Exam-Style Question 3 — Extended Response [9 marks]

Question:

A student investigates the reaction between hydrochloric acid and sodium hydroxide solution. They mix 50.0 cm³ of 1.0 mol/dm³ hydrochloric acid with 50.0 cm³ of 1.0 mol/dm³ sodium hydroxide solution in an insulated container. The temperature of the solution is recorded every minute for 5 minutes before mixing and 5 minutes after mixing. The results are shown in the table below.

Time (minutes)	Temperature (°C)
-5	21.0
-4	21.0
-3	21.0
-2	21.0
-1	21.0
0	21.0
1	27.5
2	27.0
3	26.5
4	26.0
5	25.5

(a) Plot a graph of temperature (y-axis) against time (x-axis) on the grid provided. Draw two lines of best fit; one for the points before mixing and one for the points after mixing. Extrapolate both lines to the time of mixing (0 minutes). [4]

(b) Use your graph to determine the maximum temperature change, ΔT, for the reaction. Show your working on the graph. [2]

(c) Explain why the temperature decreases after reaching the maximum value. [2]

(d) Suggest one improvement to the experimental procedure to reduce heat loss to the surroundings. [1]

Worked Solution:

(a)

1. 📊A graph with time on the x-axis (from -5 to 5 minutes) and temperature on the y-axis (from 20 to 28 °C). Plot the points from the table. Draw a horizontal line of best fit for the points from -5 to 0 minutes, extending it to 0 minutes. Draw a line of best fit for the points from 1 to 5 minutes, extending it back to 0 minutes. The two lines should intersect the y-axis at different points.

How to earn full marks:

• Correctly label the axes with units.
• Plot all points accurately.
• Draw a correct line of best fit for the points before mixing, extrapolated to 0 minutes.
• Draw a correct line of best fit for the points after mixing, extrapolated to 0 minutes.

(b)

1. Locate the y-intercepts of both lines of best fit. These represent the initial and maximum temperatures.
2. Subtract the initial temperature from the maximum temperature.
3. $\Delta T = 27.8 - 21.0 = \boxed{6.8 \text{ °C}}$ The difference in temperature is the maximum temperature change.

How to earn full marks:

• Correctly read the initial temperature from the graph (approx. 21.0 °C).
• Correctly read the maximum temperature from the graph (approx. 27.8 °C, allow range of 27.6-28.0).
• Calculate the temperature difference correctly $\boxed{6.8 \text{ °C}}$ (allow range of 6.6-7.0).

(c)

1. Heat is lost to the surroundings. This explains the decreasing temperature.
2. The container is not perfectly insulated, so thermal energy escapes. This elaborates on the reason for heat loss.

How to earn full marks:

• State that "heat is lost to the surroundings".
• Explain that the container is not perfectly insulated or give another valid reason for heat loss.

(d)

1. Use a lid on the container. This would reduce heat loss by convection.
2. Use a better insulated container. This would reduce heat loss by conduction.

How to earn full marks:

• Suggest a valid method to reduce heat loss to the surroundings, such as using a lid or better insulation.

Common Pitfall: When extrapolating lines of best fit, make sure they are straight and extend all the way to the y-axis. Read the y-intercept values carefully, and remember to include the correct units in your final answer.

Exam-Style Question 4 — Extended Response [10 marks]

Question:

The reaction between solid ammonium chloride ($NH_4Cl$) and solid barium hydroxide ($Ba(OH)_2$) is an endothermic reaction.

$2NH_4Cl(s) + Ba(OH)_2(s) \rightarrow 2NH_3(g) + BaCl_2(aq) + 2H_2O(l)$

A student carries out the reaction in a small beaker placed on a wet wooden block. They observe that the beaker freezes to the wooden block.

(a) Explain why the beaker freezes to the wooden block. [3]

(b) The following average bond energies are given:

Bond	Average bond energy (kJ/mol)
N-H	390
O-H	460
N-Cl	200
Ba-O	520

(i) Use the bond energies to calculate the energy required to break the bonds in 2 moles of $NH_4Cl$ and 1 mole of $Ba(OH)_2$. [3]

(ii) Explain why it is not possible to calculate the enthalpy change for this reaction using ONLY the bond energies provided. [2]

(iii) State the sign of the enthalpy change, $\Delta H$, for this reaction. [1]

(iv) Define "activation energy". [1]

Worked Solution:

(a)

1. The reaction is endothermic. This identifies the type of reaction.
2. The reaction absorbs thermal energy from the surroundings. This explains the energy transfer.
3. The water on the wooden block loses thermal energy to the reaction, causing it to freeze. This links the energy loss to the freezing of the water.

How to earn full marks:

• State that the reaction is endothermic.
• Explain that the reaction absorbs heat from the surroundings.
• Explain that the water freezes because it loses heat to the reaction.

(b)

(i)

1. Calculate the number of N-H bonds in 2 moles of $NH_4Cl$: $2 \times 4 = 8$ N-H bonds.
2. Calculate the number of N-Cl bonds in 2 moles of $NH_4Cl$: $2 \times 1 = 2$ N-Cl bonds.
3. Calculate the number of Ba-O bonds in 1 mole of $Ba(OH)_2$: $1 \times 2 = 2$ Ba-O bonds.
4. Calculate the number of O-H bonds in 1 mole of $Ba(OH)_2$: $2 \times 1 \times 2 = 4$ O-H bonds.
5. Energy required to break bonds: $(8 \times 390) + (2 \times 200) + (2 \times 520) + (4 \times 460) = 3120 + 400 + 1040 + 1840 = \boxed{6400 \text{ kJ}}$ Summing the energy for each bond.

How to earn full marks:

• Correctly calculate the number of each type of bond present.
• Correctly multiply each bond quantity by the appropriate bond energy.
• Sum all the bond energies correctly to obtain the final answer $\boxed{6400 \text{ kJ}}$.

(ii)

1. The bond energies for the bonds formed in the products ($NH_3$, $BaCl_2$, and $H_2O$) are not provided. This identifies the missing information.
2. To calculate the enthalpy change, we need to know the energy released when the new bonds are formed. This explains why the missing information is crucial.

How to earn full marks:

• State that the bond energies for the products are missing.
• Explain that the energy released during bond formation is needed to calculate the enthalpy change.

(iii)

1. Positive. Endothermic reactions have a positive enthalpy change.

How to earn full marks:

• State "positive" or "+" to earn the mark.

(iv)

1. Activation energy is the minimum energy that colliding particles must have to react. This is the standard definition of activation energy.

How to earn full marks:

• State that activation energy is the "minimum energy".
• Mention that it is required for "colliding particles to react".

Common Pitfall: When calculating bond energies, carefully count the number of each type of bond in the entire balanced equation. Also, remember that you need bond energies for both reactants and products to calculate the overall enthalpy change.