Energy changes and bond energies

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Big idea: energy changes and bond energies

Key concept: Change. Breaking bonds absorbs energy and forming bonds releases energy. The balance determines whether a reaction is exothermic or endothermic.
Related concepts: Models and evidence. Use a scientific explanation to make predictions, then test it against observations.
Global context: Scientific and technical innovation. Energy-change measurements help compare heating fuels and thermal packs.

Exothermic & Endothermic Reactions

Chemical reactions involve transfer of thermal energy between the system (reacting chemicals) and the surroundings.
Exothermic reactions: thermal energy is transferred from the system to the surroundings; temperature of surroundings increases; energy change is negative (ΔH < 0).
Endothermic reactions: thermal energy is transferred from the surroundings to the system; temperature of surroundings decreases; energy change is positive (ΔH > 0).
Common exothermic examples: combustion, oxidation, neutralisation, hand warmers, self-heating cans.
Common endothermic examples: electrolysis, thermal decomposition, first stages of photosynthesis, cold packs.
Mnemonic: In EXothermic reactions heat Exits the system; in ENdothermic reactions heat ENters the system.

Heat transfer outwards in an exothermic reaction

Reaction Pathway Diagrams

Reaction pathway diagrams plot energy (y-axis) against progress of reaction (x-axis).
For exothermic reactions: products have lower energy than reactants; ΔH is negative; arrow points downwards.
For endothermic reactions: products have higher energy than reactants; ΔH is positive; arrow points upwards.
The activation energy (Ea) is the minimum energy required for a successful collision; shown as the 'hump' from reactants to the transition state.
Core candidates interpret diagrams; Extended candidates must draw and label reactants, products, ΔH, and Ea.

Reaction pathway diagrams for exothermic and endothermic reactions

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 needed for a successful collision; varies with reaction.
For a collision to be successful, particles must have energy ≥ Ea and correct orientation.
Reactions with higher Ea require more energy to start.

The reaction pathway diagram for endothermic reactions

Bond Breaking & Bond Forming

Bond breaking is always endothermic (energy taken in).
Bond making is always exothermic (energy released).
If energy released in bond making > energy absorbed in bond breaking → overall exothermic (ΔH negative).
If energy absorbed in bond breaking > energy released in bond making → overall endothermic (ΔH positive).

Bond Energy Calculations

Each bond has a specific bond energy (kJ/mol) – energy to break or energy released when formed.
Method: (1) Write balanced equation. (2) Identify bonds in reactants and products. (3) Sum bond energies for reactants = energy in. (4) Sum bond energies for products = energy out. (5) ΔH = energy in – energy out.
Negative ΔH means exothermic; positive ΔH means endothermic.
Example: H₂ + Cl₂ → 2HCl: energy in = 436+242=678 kJ; energy out = 2×431=862 kJ; ΔH = 678–862 = –184 kJ (exothermic).

Experimental Methods & Observations

Temperature changes are measured using a polystyrene cup (insulator) and a thermometer.
Polystyrene cups reduce heat loss to surroundings, improving accuracy.
Solutions should be left to stand before experiment to reach room temperature (ensure fair test).
Greater mass of solid or larger volume of water affects temperature change; more solid increases change, more water dilutes and reduces change.
Powdered solids dissolve faster than lumps due to larger surface area.
After 1 hour, reaction mixture returns to room temperature (heat lost to surroundings).

Neutralisation Reactions

Neutralisation between an acid and alkali is exothermic (temperature rises).
Example: NaOH + HCl → NaCl + H₂O produces a temperature increase.
The maximum temperature indicates the point of neutralisation (equal moles of acid and alkali).

Think like a scientist

A teacher supervises comparing temperature changes for equal volumes of dilute reactants in an insulated cup.
Comparison: the pair of reactants mixed. Outcome: the maximum temperature change of the mixture.
Control: keep reactant volumes and starting temperatures constant. Explain why this makes the comparison fairer.
Evidence: Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause.
Safety: Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it.
Inquiry task: State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction.

Evaluate the science

Energy-change measurements help compare heating fuels and thermal packs.
Heat loss and the heat absorbed by apparatus make a simple calorimeter an imperfect measure of the reaction's energy change.
Evaluation task: Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.

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1.Which of the following is a correct description of an exothermic reaction?
Easy
- AThermal energy is transferred from the system to the surroundings.
- BThermal energy is transferred from the surroundings to the system.
- CThe temperature of the surroundings decreases.
- DThe energy change of the system is positive.
2.Which of the following is an example of an endothermic reaction?
Easy
- ACombustion
- BNeutralisation
- CThermal decomposition
- DOxidation
3.In a chemical reaction, bond breaking is always an ________ process.
Easy
- Aendothermic
- Bexothermic
- Cneutral
- Dreversible
4.What is the term for the minimum amount of energy required for a successful collision between particles?
Easy
- AEnthalpy change
- BActivation energy
- CBond energy
- DThermal energy
5.A student adds 4 g of solid A to 30 cm³ of water and observes a temperature decrease. What type of reaction is this?
Easy
- AExothermic
- BEndothermic
- CNeutralisation
- DOxidation
6.The reaction pathway diagram for an exothermic reaction shows that the energy of the products is ________ the energy of the reactants.
Easy
- Ahigher than
- Blower than
- Cequal to
- Dunrelated to
7.Given the bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol. What is the enthalpy change for the reaction H₂ + Cl₂ → 2HCl?
Medium
- A+184 kJ/mol
- B-184 kJ/mol
- C+862 kJ/mol
- D-862 kJ/mol
8.Which of the following statements about bond breaking and bond forming is correct?
Easy
- ABond breaking releases energy; bond forming absorbs energy.
- BBond breaking absorbs energy; bond forming releases energy.
- CBoth bond breaking and bond forming release energy.
- DBoth bond breaking and bond forming absorb energy.

Plus de questions

9.In a bond energy calculation, if the total energy absorbed to break bonds is 500 kJ and the total energy released forming new bonds is 600 kJ, what is the enthalpy change and what type of reaction is it?
Medium
- A+100 kJ, endothermic
- B-100 kJ, exothermic
- C+100 kJ, exothermic
- D-100 kJ, endothermic
10.The decomposition of hydrogen bromide: 2HBr → H₂ + Br₂ has an enthalpy change of +103 kJ. Given bond energies: H–Br = 366 kJ/mol, H–H = 436 kJ/mol, what is the bond energy of Br–Br?
Hard
- A193 kJ/mol
- B103 kJ/mol
- C366 kJ/mol
- D436 kJ/mol
11.Which of the following correctly describes the energy change of the system in an endothermic reaction?
Medium
- AThe energy of the system decreases, and ΔH is negative.
- BThe energy of the system increases, and ΔH is positive.
- CThe energy of the system decreases, and ΔH is positive.
- DThe energy of the system increases, and ΔH is negative.
12.In an experiment, 4 g of solid A is added to 30 cm³ of water and the temperature decreases from 20 °C to 16 °C. Which type of reaction is this?
Easy
- AEndothermic, because the temperature decreased
- BExothermic, because the temperature decreased
- CEndothermic, because the temperature increased
- DExothermic, because the temperature increased
13.A student adds 4 g of solid B to 30 cm³ of water and observes a temperature rise from 20 °C to 26 °C. What is the temperature change?
Easy
- A+6 °C
- B-6 °C
- C+4 °C
- D-4 °C
14.In an experiment, 4 g of solid D is added to 30 cm³ of water and the temperature rises from 19 °C to 31 °C. If the experiment is repeated using 60 cm³ of water and the same mass of solid D, what would be the expected temperature change?
Medium
- AHalf of the original temperature change
- BDouble the original temperature change
- CThe same temperature change
- DNo temperature change
15.A student adds 1 g of powdered sodium thiosulphate to 20 cm³ of water and the temperature changes. The experiment is repeated with 2 g, 3 g, 4 g, and 5 g. The results show a linear increase in temperature with mass. What temperature would be expected for 3.5 g?
Hard
- AThe temperature read from the line of best fit at 3.5 g
- BThe average of the temperatures for 3 g and 4 g
- CThe temperature at 3 g plus half the difference between 3 g and 4 g
- DBoth A and C are correct
16.In a neutralisation experiment, 25 cm³ of sodium hydroxide solution is reacted with varying volumes of hydrochloric acid. The temperature change is plotted against acid volume. Two straight lines are drawn through the points; they intersect at 10 cm³ of acid. What volume of acid is needed to neutralise the sodium hydroxide?
Hard
- A10 cm³
- B15 cm³
- C20 cm³
- D25 cm³
17.In a reaction pathway diagram for an exothermic reaction, which statement is correct?
Easy
- AThe products have less energy than the reactants
- BThe products have more energy than the reactants
- CThe activation energy is zero
- DThe enthalpy change is positive
18.Given the bond energies: H–H = 436 kJ/mol, I–I = 151 kJ/mol, H–I = 295 kJ/mol, what is the enthalpy change for the reaction H₂ + I₂ → 2HI?
Medium
- A-3 kJ/mol
- B+3 kJ/mol
- C-184 kJ/mol
- D+184 kJ/mol
19.In an experiment, sodium thiosulphate is powdered before dissolving in water. What is the reason for this?
Easy
- ATo increase the surface area and speed up dissolving
- BTo decrease the surface area and slow down dissolving
- CTo make the temperature change larger
- DTo make the temperature change smaller
20.In an experiment, 30 cm³ of water is placed in a polystyrene cup and its initial temperature is measured. 4 g of solid B is added, stirred, and the final temperature is recorded. The temperature increases by 6 °C. Which of the following is the most likely reason for using a polystyrene cup?
Easy
- ATo reduce heat loss to the surroundings
- BTo allow the reaction to be observed
- CTo prevent the cup from reacting with the chemicals
- DTo increase the rate of reaction
21.A student adds 4 g of solid D to 30 cm³ of water and observes a temperature rise from 19 °C to 31 °C. The experiment is repeated using 60 cm³ of water instead of 30 cm³, keeping the mass of solid D the same. What would be the expected temperature change?
Medium
- A6 °C
- B12 °C
- C24 °C
- D3 °C
22.In a neutralisation experiment, 25 cm³ of sodium hydroxide solution is reacted with varying volumes of hydrochloric acid. The temperature change is plotted against volume of acid added. Two straight lines are drawn through the points and they cross. What does the crossing point represent?
Medium
- AThe volume of acid needed to neutralise the alkali
- BThe maximum temperature reached
- CThe point where the acid is in excess
- DThe point where the reaction becomes endothermic
23.In a reaction pathway diagram, the energy of the reactants is higher than the energy of the products. Which of the following is correct?
Easy
- AThe reaction is exothermic and ΔH is negative
- BThe reaction is exothermic and ΔH is positive
- CThe reaction is endothermic and ΔH is negative
- DThe reaction is endothermic and ΔH is positive
24.A student investigates the reaction between sodium hydroxide and hydrochloric acid. Both solutions are left to stand in the laboratory for about 30 minutes before the experiment. Why is this done?
Medium
- ATo ensure both solutions are at the same temperature
- BTo allow the solutions to react before mixing
- CTo allow any gases to escape
- DTo increase the concentration of the solutions
25.In an experiment to measure temperature changes during a reaction, a student uses a thermometer to stir the mixture. What is the most likely effect of this?
Medium
- AIt ensures even temperature distribution
- BIt increases the rate of reaction
- CIt prevents heat loss
- DIt makes the reaction endothermic
26.Which of the following is an exothermic reaction?
Easy
- ACombustion
- BElectrolysis
- CThermal decomposition
- DPhotosynthesis
27.In an exothermic reaction, thermal energy is transferred from the _____ to the _____.
Easy
- Asystem, surroundings
- Bsurroundings, system
- Csystem, system
- Dsurroundings, surroundings
28.Which of the following is an endothermic process?
Easy
- ABond breaking
- BBond forming
- CNeutralisation
- DCombustion
29.What is the enthalpy change (ΔH) for an exothermic reaction?
Easy
- ANegative
- BPositive
- CZero
- DCannot be determined
30.Bond breaking is always an endothermic process.
Easy
True or false?
31.Complete the sentence using the correct term.
Easy
In exothermic reactions, thermal energy is transferred from the ____ to the ____.
32.Complete the sentences with the correct terms.
Easy
In an exothermic reaction, the enthalpy change ΔH is ____. In an endothermic reaction, ΔH is ____.
33.The reaction N₂ + 3H₂ → 2NH₃ has the following bond energies: N≡N = 945 kJ/mol, H–H = 436 kJ/mol, N–H = 391 kJ/mol. Calculate the enthalpy change for this reaction in kJ/mol. (Enter a negative value if exothermic.)
Medium
34.Bond making is always an endothermic process.
Easy
True or false?
35.In an exothermic reaction, the energy of the products is higher than the energy of the reactants.
Easy
True or false?
36.In an endothermic reaction, the energy change ΔH is positive.
Easy
True or false?
37.In a reaction pathway diagram, the difference in height between the energy of the reactants and the energy of the products represents:
Medium
- AThe activation energy
- BThe enthalpy change
- CThe bond energy
- DThe temperature change
38.Match each reaction type to its correct description.
Medium
- Exothermic reaction
- Endothermic reaction
- Thermal energy is transferred from the system to the surroundings
- Thermal energy is transferred from the surroundings to the system
- The energy of the system increases
39.Arrange the following steps in the correct order for calculating the enthalpy change of a reaction using bond energies.
Medium
- Write a balanced equation
- Add bond energies of all bonds in reactants (energy in)
- Add bond energies of all bonds in products (energy out)
- Calculate ΔH = energy in - energy out
40.A teacher supervises comparing temperature changes for equal volumes of dilute reactants in an insulated cup. What is the independent variable?
Easy
- Athe maximum temperature change of the mixture
- Bthe pair of reactants mixed
- Cthe number of repeated trials
- Dthe scientific explanation written afterwards
41.A teacher supervises comparing temperature changes for equal volumes of dilute reactants in an insulated cup. Which observation or quantity is the dependent variable?
Easy
- Athe pair of reactants mixed
- Bthe title given to the investigation
- Cthe maximum temperature change of the mixture
- Dthe number of references in the report
42.A teacher supervises comparing temperature changes for equal volumes of dilute reactants in an insulated cup. Which decision best controls another factor in this comparison?
Medium
- Achange every condition for each trial
- Buse a different measurement method for every result
- Crecord only results that agree with the prediction
- Dkeep reactant volumes and starting temperatures constant
43.A teacher supervises comparing temperature changes for equal volumes of dilute reactants in an insulated cup. Why should the class repeat measurements or check multiple comparable observations?
Medium
- ATo assess variation and judge how consistent the evidence is
- BTo guarantee that every systematic error disappears
- CTo make an unsupported prediction become correct
- DTo avoid reporting results that disagree with a model
44.A class is evaluating an application of energy changes and bond energies. Which statement identifies a relevant limitation?
Hard
- AAll scientific models reproduce every feature of the real world
- BHeat loss and the heat absorbed by apparatus make a simple calorimeter an imperfect measure of the reaction's energy change.
- COne favourable result proves that a technology has no disadvantages
- DA conclusion needs no evidence when a scientific term is used
45.A teacher supervises comparing temperature changes for equal volumes of dilute reactants in an insulated cup. Which conclusion is most justified by a controlled comparison?
Hard
- AA claim that the result must hold in all possible conditions
- BA claim that alternative explanations never need testing
- CA conclusion restricted to the tested conditions, with uncertainty and limitations stated
- DA claim based only on the most convenient observation
46.A teacher supervises comparing temperature changes for equal volumes of dilute reactants in an insulated cup. Select the two practices that strengthen the validity of this comparison.
Medium
- Akeep reactant volumes and starting temperatures constant
- Bchange the measurement method whenever a result is unexpected
- Crecord the maximum temperature change of the mixture using the same agreed procedure
- Dreport only observations that confirm the prediction
47.When evaluating evidence about energy changes and bond energies, select the two scientifically defensible statements.
Hard
- AOne result proves the explanation in every situation
- BHeat loss and the heat absorbed by apparatus make a simple calorimeter an imperfect measure of the reaction's energy change.
- CA conclusion should state the conditions and uncertainty of the evidence
- DModels never need revision when new evidence appears
48.Design a fair investigation for this question: A teacher supervises comparing temperature changes for equal volumes of dilute reactants in an insulated cup. Identify what to compare, what to record and one control.
Medium
49.Explain one use of the science in energy changes and bond energies and evaluate one limitation of the evidence or application.
Hard
50.A reaction needs 600 kJ/mol to break bonds and releases 750 kJ/mol when new bonds form. What is its overall energy change?
Hard
- A+150 kJ/mol; endothermic
- B-1350 kJ/mol; exothermic
- C+1350 kJ/mol; endothermic
- D-150 kJ/mol; exothermic
51.An exothermic reaction is measured in an open cup. Some energy heats the cup and escapes to the room. How does this affect the temperature-rise estimate of released energy?
Hard
- AIt usually underestimates the magnitude of the energy released by the reaction
- BIt necessarily overestimates the magnitude
- CIt changes the reaction into an endothermic one
- DIt makes every bond energy exactly measurable

Notes de révision

## Big idea: energy changes and bond energies - **Key concept: Change.** Breaking bonds absorbs energy and forming bonds releases energy. The balance determines whether a reaction is exothermic or endothermic. - **Related concepts: Models and evidence.** Use a scientific explanation to make predictions, then test it against observations. - **Global context: Scientific and technical innovation.** Energy-change measurements help compare heating fuels and thermal packs. ## Exothermic & Endothermic Reactions - Chemical reactions involve transfer of thermal energy between the **system** (reacting chemicals) and the **surroundings**. - **Exothermic reactions**: thermal energy is transferred from the system to the surroundings; temperature of surroundings increases; energy change is **negative** (ΔH < 0). - **Endothermic reactions**: thermal energy is transferred from the surroundings to the system; temperature of surroundings decreases; energy change is **positive** (ΔH > 0). - Common exothermic examples: **combustion**, **oxidation**, **neutralisation**, hand warmers, self-heating cans. - Common endothermic examples: **electrolysis**, **thermal decomposition**, first stages of photosynthesis, cold packs. - Mnemonic: In **EX**othermic reactions heat **Exits** the system; in **EN**dothermic reactions heat **EN**ters the system. ## Reaction Pathway Diagrams - Reaction pathway diagrams plot **energy** (y-axis) against **progress of reaction** (x-axis). - For exothermic reactions: products have **lower energy** than reactants; ΔH is negative; arrow points downwards. - For endothermic reactions: products have **higher energy** than reactants; ΔH is positive; arrow points upwards. - The **activation energy (Ea)** is the minimum energy required for a successful collision; shown as the 'hump' from reactants to the transition state. - Core candidates interpret diagrams; Extended candidates must **draw and label** reactants, products, ΔH, and Ea. ## 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 needed for a successful collision; varies with reaction. - For a collision to be successful, particles must have energy ≥ Ea and correct orientation. - Reactions with higher Ea require more energy to start. ## Bond Breaking & Bond Forming - **Bond breaking** is always **endothermic** (energy taken in). - **Bond making** is always **exothermic** (energy released). - If energy released in bond making > energy absorbed in bond breaking → overall **exothermic** (ΔH negative). - If energy absorbed in bond breaking > energy released in bond making → overall **endothermic** (ΔH positive). ## Bond Energy Calculations - Each bond has a specific **bond energy** (kJ/mol) – energy to break or energy released when formed. - Method: (1) Write balanced equation. (2) Identify bonds in reactants and products. (3) Sum bond energies for reactants = **energy in**. (4) Sum bond energies for products = **energy out**. (5) ΔH = energy in – energy out. - Negative ΔH means exothermic; positive ΔH means endothermic. - Example: H₂ + Cl₂ → 2HCl: energy in = 436+242=678 kJ; energy out = 2×431=862 kJ; ΔH = 678–862 = –184 kJ (exothermic). ## Experimental Methods & Observations - Temperature changes are measured using a **polystyrene cup** (insulator) and a **thermometer**. - Polystyrene cups reduce heat loss to surroundings, improving accuracy. - Solutions should be left to stand before experiment to reach **room temperature** (ensure fair test). - Greater mass of solid or larger volume of water affects temperature change; more solid increases change, more water dilutes and reduces change. - Powdered solids dissolve faster than lumps due to larger surface area. - After 1 hour, reaction mixture returns to room temperature (heat lost to surroundings). ## Neutralisation Reactions - Neutralisation between an acid and alkali is **exothermic** (temperature rises). - Example: NaOH + HCl → NaCl + H₂O produces a temperature increase. - The maximum temperature indicates the point of neutralisation (equal moles of acid and alkali). ## Think like a scientist - A teacher supervises comparing temperature changes for equal volumes of dilute reactants in an insulated cup. - **Comparison:** the pair of reactants mixed. **Outcome:** the maximum temperature change of the mixture. - **Control:** keep reactant volumes and starting temperatures constant. Explain why this makes the comparison fairer. - **Evidence:** Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause. - **Safety:** Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it. - **Inquiry task:** State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction. ## Evaluate the science - Energy-change measurements help compare heating fuels and thermal packs. - Heat loss and the heat absorbed by apparatus make a simple calorimeter an imperfect measure of the reaction's energy change. - **Evaluation task:** Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.

Interpreting Reaction Pathway Diagrams: Interpret reaction pathway diagrams to determine relative energies of reactants and products, and identify exothermic or endothermic nature.
Distinguishing Exothermic and Endothermic Reactions: Identify reactions as exothermic or endothermic from temperature change data or energy transfer descriptions.
Bond Energy Calculations: Each bond has a specific **bond energy** (kJ/mol) – energy to break or energy released when formed.
Drawing Reaction Pathway Diagrams: Draw and label reaction pathway diagrams showing reactants, products, enthalpy change, and activation energy for exothermic and endothermic reactions.
Designing scientific inquiries: Plan a fair comparison: A teacher supervises comparing temperature changes for equal volumes of dilute reactants in an insulated cup.
Evaluating evidence and impacts: Energy-change measurements help compare heating fuels and thermal packs. Heat loss and the heat absorbed by apparatus make a simple calorimeter an imperfect measure of the reaction's energy change.
Exothermic & Endothermic Reactions: Chemical reactions involve transfer of thermal energy between the **system** (reacting chemicals) and the **surroundings**.
Reaction Pathway Diagrams: Reaction pathway diagrams plot **energy** (y-axis) against **progress of reaction** (x-axis).
Enthalpy Change & Activation Energy: **Enthalpy change (ΔH)** is the transfer of thermal energy during a reaction; negative for exothermic, positive for endothermic.
Bond Breaking & Bond Forming: **Bond breaking** is always **endothermic** (energy taken in).
Experimental Methods & Observations: Temperature changes are measured using a **polystyrene cup** (insulator) and a **thermometer**.
Neutralisation Reactions: Neutralisation between an acid and alkali is **exothermic** (temperature rises).
system: Chemical reactions involve transfer of thermal energy between the **system** (reacting chemicals) and the **surroundings**.
surroundings: Chemical reactions involve transfer of thermal energy between the **system** (reacting chemicals) and the **surroundings**.
Exothermic reactions: **Exothermic reactions**: thermal energy is transferred from the system to the surroundings; temperature of surroundings increases; energy change is **negative** (ΔH < 0).
negative: **Exothermic reactions**: thermal energy is transferred from the system to the surroundings; temperature of surroundings increases; energy change is **negative** (ΔH < 0).

Cartes mémo

Which of the following is a correct description of an exothermic reaction?

Thermal energy is transferred from the system to the surroundings. In an exothermic reaction, thermal energy is transferred from the system to the surroundings, causing the temperature of the surroundings to increase and the energy change of the system to be negative.

Which of the following is an example of an endothermic reaction?

Thermal decomposition Thermal decomposition is an endothermic reaction because it requires heat to break bonds. Combustion, neutralisation, and oxidation are all exothermic reactions.

In a chemical reaction, bond breaking is always an ________ process.

endothermic Bond breaking requires energy to be taken in from the surroundings, so it is always endothermic.

What is the term for the minimum amount of energy required for a successful collision between particles?

Activation energy The minimum energy needed for a successful collision is called the activation energy (Ea).

A student adds 4 g of solid A to 30 cm³ of water and observes a temperature decrease. What type of reaction is this?

Endothermic A temperature decrease indicates that thermal energy is absorbed from the surroundings, so the reaction is endothermic.

The reaction pathway diagram for an exothermic reaction shows that the energy of the products is ________ the energy of the reactants.

lower than In exothermic reactions, energy is released, so the products have lower energy than the reactants.

Given the bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol. What is the enthalpy change for the reaction H₂ + Cl₂ → 2HCl?

-184 kJ/mol Energy in = 436 + 242 = 678 kJ; energy out = 2 × 431 = 862 kJ; ΔH = 678 - 862 = -184 kJ/mol.

Which of the following statements about bond breaking and bond forming is correct?

Bond breaking absorbs energy; bond forming releases energy. Bond breaking is endothermic (absorbs energy), while bond forming is exothermic (releases energy).

In a bond energy calculation, if the total energy absorbed to break bonds is 500 kJ and the total energy released forming new bonds is 600 kJ, what is the enthalpy change and what type of reaction is it?

-100 kJ, exothermic ΔH = energy in - energy out = 500 - 600 = -100 kJ. A negative ΔH indicates an exothermic reaction.

The decomposition of hydrogen bromide: 2HBr → H₂ + Br₂ has an enthalpy change of +103 kJ. Given bond energies: H–Br = 366 kJ/mol, H–H = 436 kJ/mol, what is the bond energy of Br–Br?

193 kJ/mol Energy in = 2 × 366 = 732 kJ; energy out = 436 + Br–Br; ΔH = +103 = 732 - (436 + Br–Br); solving gives Br–Br = 193 kJ/mol.

Which of the following correctly describes the energy change of the system in an endothermic reaction?

The energy of the system increases, and ΔH is positive. In endothermic reactions, energy is absorbed, so the system's energy increases, and ΔH is positive.

In an experiment, 4 g of solid A is added to 30 cm³ of water and the temperature decreases from 20 °C to 16 °C. Which type of reaction is this?

Endothermic, because the temperature decreased A decrease in temperature means thermal energy is absorbed from the surroundings, so the reaction is endothermic.

A student adds 4 g of solid B to 30 cm³ of water and observes a temperature rise from 20 °C to 26 °C. What is the temperature change?

+6 °C Temperature change = final - initial = 26 - 20 = +6 °C.

In an experiment, 4 g of solid D is added to 30 cm³ of water and the temperature rises from 19 °C to 31 °C. If the experiment is repeated using 60 cm³ of water and the same mass of solid D, what would be the expected temperature change?

Half of the original temperature change Doubling the volume of water doubles the heat capacity, so the same amount of heat released produces half the temperature rise.

A student adds 1 g of powdered sodium thiosulphate to 20 cm³ of water and the temperature changes. The experiment is repeated with 2 g, 3 g, 4 g, and 5 g. The results show a linear increase in temperature with mass. What temperature would be expected for 3.5 g?

Both A and C are correct From a linear graph, you can interpolate by reading the temperature at 3.5 g from the line, which is equivalent to averaging the values at 3 g and 4 g.

In a neutralisation experiment, 25 cm³ of sodium hydroxide solution is reacted with varying volumes of hydrochloric acid. The temperature change is plotted against acid volume. Two straight lines are drawn through the points; they intersect at 10 cm³ of acid. What volume of acid is needed to neutralise the sodium hydroxide?

10 cm³ The intersection of the two lines on the graph indicates the volume of acid at which neutralisation is complete and the temperature change is maximum. That volume is 10 cm³.

How would you design the inquiry for energy changes and bond energies?

A teacher supervises comparing temperature changes for equal volumes of dilute reactants in an insulated cup. Compare the pair of reactants mixed; record the maximum temperature change of the mixture; keep reactant volumes and starting temperatures constant.

What limitation matters when applying energy changes and bond energies?

Heat loss and the heat absorbed by apparatus make a simple calorimeter an imperfect measure of the reaction's energy change.

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