Exothermic & Endothermic Reactions

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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 (Extended)

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 (Extended)

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 breaking is endothermic

Bond Energy Calculations (Extended)

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).

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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 an experiment, 1 g of powdered sodium thiosulphate is added to 20 cm³ of water and the temperature decreases. The experiment is repeated with 2 g, 3 g, 4 g, and 5 g. The results are plotted on a graph of temperature against mass of sodium thiosulphate. Which of the following is the most likely shape of the graph?
Medium
- AA straight line sloping downwards
- BA straight line sloping upwards
- CA curve that levels off
- DA horizontal line
23.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
24.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
25.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
26.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
27.A reaction pathway diagram shows the activation energy (Eₐ) and the enthalpy change (ΔH). Which of the following statements is correct?
Medium
- AEₐ is the energy difference between reactants and products
- BEₐ is the minimum energy required for a successful collision
- CΔH is always positive for exothermic reactions
- DΔH is the energy released during bond breaking
28.Which of the following is an exothermic reaction?
Easy
- ACombustion
- BElectrolysis
- CThermal decomposition
- DPhotosynthesis
29.In an exothermic reaction, thermal energy is transferred from the _____ to the _____.
Easy
- Asystem, surroundings
- Bsurroundings, system
- Csystem, system
- Dsurroundings, surroundings
30.Which of the following is an endothermic process?
Easy
- ABond breaking
- BBond forming
- CNeutralisation
- DCombustion
31.What is the enthalpy change (ΔH) for an exothermic reaction?
Easy
- ANegative
- BPositive
- CZero
- DCannot be determined
32.Bond breaking is always an endothermic process.
Easy
True or false?
33.Bond making is always an endothermic process.
Easy
True or false?
34.In an exothermic reaction, the energy of the products is higher than the energy of the reactants.
Easy
True or false?
35.In an endothermic reaction, the energy change ΔH is positive.
Easy
True or false?
36.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
37.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
38.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
39.Complete the sentence using the correct term.
Easy
In exothermic reactions, thermal energy is transferred from the ____ to the ____.
40.Complete the sentences with the correct terms.
Easy
In an exothermic reaction, the enthalpy change ΔH is ____. In an endothermic reaction, ΔH is ____.
41.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
42.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
- A-75 kJ/mol
- B-93 kJ/mol
- C-84 kJ/mol
- D-102 kJ/mol

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