Exothermic and endothermic reactions

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Energy Transfer in Reactions

Chemical reactions involve energy transfer to or from the surroundings.
Energy is conserved: the total amount of energy in the universe remains the same before and after a reaction.
If a reaction transfers energy to the surroundings, the products have less energy than the reactants by the same amount.
Temperature changes can be measured with a thermometer to determine whether a reaction is exothermic or endothermic.

Heat transfer inwards in an endothermic reaction

Exothermic Reactions

An exothermic reaction transfers energy to the surroundings, causing the temperature of the surroundings to increase.
Examples include combustion, neutralisation, and many oxidation reactions.
Everyday uses include hand warmers and self-heating cans.
In exothermic reactions, the energy released from forming new bonds is greater than the energy needed to break existing bonds.

Heat transfer outwards in an exothermic reaction

Endothermic Reactions

An endothermic reaction takes in energy from the surroundings, causing the temperature of the surroundings to decrease.
Examples include thermal decomposition, electrolysis, the first stages of photosynthesis, and the reaction between citric acid and sodium hydrogencarbonate.
Sports injury cold packs use endothermic reactions to take heat away from the injured area.
In endothermic reactions, the energy needed to break existing bonds is greater than the energy released from forming new bonds.

Diagram showing the transfer of heat energy from the surroundings into an endothermic reaction

Required Practical: Investigating Temperature Changes

Objective: to investigate variables that affect temperature changes in reacting solutions, such as acid plus alkali (neutralisation).
Use a polystyrene cup (calorimeter) to reduce heat loss, a thermometer, and measuring cylinders.
Method: measure initial temperature of acid, add alkali in increments, record highest temperature after each addition.
The temperature rises as more alkali is added, reaches a peak, then falls when one reactant is in excess.
The peak occurs when neutralisation is complete; further addition dilutes the solution and absorbs heat.
Plot a graph of volume added (x-axis) against mean temperature (y-axis) and draw two lines of best fit to find the volume at which neutralisation is complete.

Reaction Profiles

Reaction profiles (energy level diagrams) show the relative energies of reactants and products.
The y-axis shows energy; the x-axis shows progress of the reaction.
The initial rise represents the activation energy (Ea), the minimum energy colliding particles need to react.
In an exothermic reaction, products have lower energy than reactants; the overall energy change is negative (downward arrow).
In an endothermic reaction, products have higher energy than reactants; the overall energy change is positive (upward arrow).
A catalyst lowers the activation energy by providing an alternative pathway, increasing the rate of reaction.

Reaction pathway diagrams for exothermic and endothermic reactions

The Energy Change of Reactions

Bond breaking is endothermic (energy absorbed); bond making is exothermic (energy released).
Overall energy change = energy taken in to break bonds – energy given out when new bonds form.
If more energy is released than absorbed, the reaction is exothermic (negative energy change).
If more energy is absorbed than released, the reaction is endothermic (positive energy change).
Bond energies can be used to calculate the overall energy change: add bond energies of reactants (energy in) and products (energy out), then subtract.

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1.Which statement best describes an exothermic reaction?
Easy
- AIt transfers energy to the surroundings so the temperature of the surroundings increases
- BIt takes in energy from the surroundings so the temperature of the surroundings decreases
- CIt transfers energy from the surroundings so the temperature of the surroundings decreases
- DIt produces energy, which is created during the reaction
2.Which statement best describes an endothermic reaction?
Easy
- AIt takes in energy from the surroundings so the temperature of the surroundings decreases
- BIt transfers energy to the surroundings so the temperature of the surroundings increases
- CIt releases heat energy so the temperature increases
- DIt destroys energy from the surroundings
3.In an exothermic reaction, the temperature of the surroundings increases.
Easy
True or false?
4.Bond breaking releases energy to the surroundings.
Easy
True or false?
5.A reaction is carried out in a test tube. The temperature of the mixture falls from 21 °C to 15 °C. What type of reaction is this?
Medium
- AEndothermic, because energy is taken in from the surroundings
- BExothermic, because energy is transferred to the surroundings
- CExothermic, because the temperature decreased
- DEndothermic, because energy is transferred to the surroundings
6.A student adds magnesium to dilute hydrochloric acid. The mixture becomes warm. Which statement is correct?
Medium
- AThe reaction is exothermic and energy is transferred to the surroundings
- BThe reaction is endothermic and energy is taken in from the surroundings
- CThe reaction is exothermic and energy is taken in from the surroundings
- DThe reaction is endothermic and energy is transferred to the surroundings
7.Which of the following are examples of exothermic reactions? (select all that apply)
Medium
- ACombustion of methane
- BNeutralisation of hydrochloric acid with sodium hydroxide
- CThermal decomposition of calcium carbonate
- DElectrolysis of water
- EOxidation of magnesium
8.Which of the following are examples of endothermic reactions? (select all that apply)
Medium
- AThermal decomposition of calcium carbonate
- BElectrolysis of water
- CCombustion of ethanol
- DReaction of citric acid with sodium hydrogencarbonate
- ENeutralisation of an acid with an alkali

更多題目

9.Draw one line from each term to its correct meaning.
Medium
- Exothermic reaction
- Endothermic reaction
- Activation energy
- Overall energy change
- The minimum energy that colliding particles must have to react
- A reaction that transfers energy to the surroundings so the temperature increases
- The difference in energy between the reactants and the products
- A reaction that takes in energy from the surroundings so the temperature decreases
10.On a reaction profile for an exothermic reaction, which statement is correct?
Medium
- AThe energy of the products is lower than the energy of the reactants
- BThe energy of the products is higher than the energy of the reactants
- CThe energy of the products is the same as the energy of the reactants
- DThe activation energy is the difference between the reactant and product energies
11.On a reaction profile for an endothermic reaction, which statement is correct?
Medium
- AThe energy of the products is higher than the energy of the reactants
- BThe energy of the products is lower than the energy of the reactants
- CThe activation energy is the overall energy change
- DThe reaction transfers energy to the surroundings
12.The overall amount of energy in the universe stays the same after a chemical reaction has taken place.
Easy
True or false?
13.In an exothermic reaction, energy is transferred to the surroundings and the products have more energy than the reactants.
Easy
True or false?
14.A student mixes 25 cm³ of hydrochloric acid with 25 cm³ of sodium hydroxide. The reaction is exothermic. Which temperature change would be expected?
Medium
- AThe temperature of the mixture increases
- BThe temperature of the mixture decreases
- CThe temperature of the mixture stays the same
- DThe temperature first decreases then increases
15.In a bond energy calculation, which equation is correct?
Medium
- AEnergy change = energy taken in − energy given out
- BEnergy change = energy given out − energy taken in
- CEnergy change = energy taken in + energy given out
- DEnergy change = energy given out × energy taken in
16.For a reaction, the energy taken in to break bonds is 678 kJ and the energy given out when new bonds form is 862 kJ. What is the overall energy change and is the reaction exothermic or endothermic?
Medium
- A−184 kJ, exothermic
- B+184 kJ, endothermic
- C−184 kJ, endothermic
- D+184 kJ, exothermic
17.Which statement about bond breaking and bond making is correct?
Medium
- ABond breaking is endothermic and bond making is exothermic
- BBond breaking is exothermic and bond making is endothermic
- CBoth bond breaking and bond making are exothermic
- DBoth bond breaking and bond making are endothermic
18.In an endothermic reaction, which statement is correct?
Medium
- AThe energy needed to break existing bonds is greater than the energy released from forming new bonds
- BThe energy released from forming new bonds is greater than the energy needed to break existing bonds
- CThe energy needed to break existing bonds is equal to the energy released from forming new bonds
- DNo bonds are broken or formed
19.Place the steps of the calorimetry investigation in the correct order.
Medium
- Measure 25.0 cm³ of hydrochloric acid and pour it into the polystyrene cup
- Measure and record the temperature of the acid
- Measure 5.0 cm³ of sodium hydroxide solution using a measuring cylinder
- Add the sodium hydroxide to the cup, fit the lid and stir gently
- Record the highest temperature reached when the thermometer reading stabilises
20.Why is a polystyrene cup used instead of a glass beaker in the temperature change investigation?
Medium
- APolystyrene is a good insulator, reducing heat loss to the surroundings
- BPolystyrene is a good conductor, so heat is transferred quickly
- CPolystyrene reacts with the acid to release heat
- DPolystyrene makes the reaction go faster
21.In the reaction between hydrochloric acid and sodium hydroxide, the temperature rises to a maximum and then falls as more sodium hydroxide is added. Why does the temperature fall after the maximum?
Medium
- AOne reactant is now in excess, so further addition dilutes the solution and absorbs heat
- BThe reaction becomes endothermic after the maximum
- CAll the energy has been used up
- DThe polystyrene cup starts to lose heat to the surroundings only after the maximum
22.Using the table, what is the mean maximum temperature reached when 25 cm³ of hydrochloric acid is added?
Medium
- A32.4 °C
- B32.6 °C
- C32.2 °C
- D32.5 °C
23.The student noticed an anomalous result in the table. Which volume of hydrochloric acid added and which trial contains the anomalous result?
Medium
- A15 cm³, second trial
- B15 cm³, first trial
- C20 cm³, third trial
- D10 cm³, first trial
24.The student concluded the reaction is endothermic. Is this correct, and why?
Medium
- AIncorrect, because the temperature increased, showing energy was transferred to the surroundings
- BCorrect, because the temperature increased, showing energy was taken in
- CIncorrect, because the temperature decreased, showing energy was taken in
- DCorrect, because the temperature decreased, showing energy was transferred to the surroundings
25.What does the height from the reactants' energy level to the peak of the curve represent?
Medium
- AActivation energy
- BOverall energy change
- CEnergy of the products
- DProgress of the reaction
26.If the products' energy level is lower than the reactants' energy level, what type of reaction is shown?
Medium
- AExothermic
- BEndothermic
- CNeutralisation only
- DCannot tell from the profile
27.On a reaction profile for an endothermic reaction, the arrow showing the overall energy change points upwards.
Medium
True or false?
28.Draw one line from each type of reaction to the correct description of its temperature change.
Medium
- Exothermic reaction
- Endothermic reaction
- Neutralisation reaction
- Thermal decomposition reaction
- Temperature of surroundings increases
- Temperature of surroundings decreases
- Always gives energy out
- Takes heat energy in
29.Which of the following statements about energy transfer in reactions are correct? (select all that apply)
Medium
- AIn an exothermic reaction, the heat content of the system falls.
- BIn an endothermic reaction, the temperature of the surroundings falls.
- CEnergy can be created during an exothermic reaction.
- DIn an endothermic reaction, the heat content of the system falls.
- EThe total amount of energy in the universe stays the same after a reaction.
30.Place the steps of the calorimetry investigation into the correct order.
Medium
- Measure 25.0 cm³ of hydrochloric acid and pour it into the polystyrene cup.
- Stand the cup in a beaker to provide support.
- Measure and record the temperature of the acid.
- Add 5.0 cm³ of sodium hydroxide solution, fit the lid and stir gently.
- Record the highest temperature reached when the thermometer reading stabilises.
31.The energy released when new bonds are formed is greater than the energy needed to break existing bonds in an exothermic reaction.
Easy
True or false?
32.Which of the following is the best definition of activation energy?
Medium
- AThe minimum energy that colliding particles must have to react.
- BThe overall energy change of a reaction.
- CThe energy released when bonds are formed.
- DThe energy needed to break all the bonds in the reactants.
33.A reaction profile shows the products at a lower energy level than the reactants. Which statements are correct? (select all that apply)
Hard
- AThe reaction is exothermic.
- BThe overall energy change is negative.
- CThe overall energy change is positive.
- DEnergy is transferred to the surroundings.
- EThe reaction is endothermic.
34.Place the stages of a bond energy calculation into the correct order.
Medium
- Add together the bond energies for all the bonds in the reactants (energy in).
- Add together the bond energies for all the bonds in the products (energy out).
- Subtract the energy out from the energy in to find the overall energy change.
- Decide whether the reaction is exothermic or endothermic from the sign of the energy change.
35.A student adds a spatula of a white solid to water in a polystyrene cup. The temperature of the mixture falls from 20 °C to 14 °C. Which statement is correct?
Medium
- AThe process is endothermic because energy is taken in from the surroundings.
- BThe process is exothermic because energy is transferred to the surroundings.
- CThe process is endothermic because the products have less energy than the reactants.
- DThe process is exothermic because the temperature of the system increases.
36.Which of the following are everyday uses of exothermic reactions? (select all that apply)
Medium
- AHand warmers
- BSelf-heating cans of coffee
- CCold packs for sports injuries
- DBurning fuel for heating
- EElectrolysis of water
37.Draw one line from each term to its correct description.
Medium
- Exothermic reaction
- Endothermic reaction
- Activation energy
- Overall energy change
- Energy is transferred to the surroundings and temperature rises
- Energy is taken in from the surroundings and temperature falls
- Minimum energy colliding particles need to react
- Difference in energy between reactants and products
38.A reaction that takes in thermal energy from the surroundings is called an endothermic reaction.
Easy
True or false?
39.Which statements about bond breaking and bond making are correct? (select all that apply)
Hard
- ABond breaking is always endothermic.
- BBond making is always exothermic.
- CBond breaking releases energy to the surroundings.
- DBond making takes in energy from the surroundings.
- EIn an exothermic reaction, more energy is released making bonds than is taken in breaking bonds.
40.A student dissolves ammonium nitrate in water in a polystyrene cup. The temperature of the solution falls from 22 °C to 16 °C. Which statement about this process is correct?
Medium
- AIt is endothermic because the temperature of the surroundings decreases.
- BIt is exothermic because the temperature of the surroundings decreases.
- CIt is endothermic because the temperature of the surroundings increases.
- DIt is exothermic because the temperature of the surroundings increases.
41.Complete the sentences using the words exothermic or endothermic.
Easy
Thermal decomposition reactions are examples of ____ reactions. Combustion reactions are examples of ____ reactions. The reaction of citric acid with sodium hydrogencarbonate is an ____ reaction.
42.Hydrogen reacts with chlorine: H–H + Cl–Cl → H–Cl + H–Cl. Bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol. Calculate the overall energy change in kJ.
Medium
43.Hydrogen reacts with iodine: H₂ + I₂ → 2HI. Bond energies: H–H = 436 kJ/mol, I–I = 151 kJ/mol, H–I = 295 kJ/mol. Calculate the overall energy change in kJ.
Medium
44.Complete the sentences using the words from the box: endothermic, exothermic, taken in, given out, greater, less.
Medium
The reaction between chlorine and methane is an ____ reaction. This is because the energy ____ forming new bonds is ____ than the energy needed to break existing bonds.
45.What is the name of the minimum amount of energy that colliding particles must have in order to react?
Medium
46.What word is used to describe the reaction when food burns and releases energy?
Medium
47.Complete the sentences about bond breaking and bond making.
Medium
Bond breaking is an ____ process because energy is ____ from the surroundings. Bond making is an ____ process because energy is ____ to the surroundings.
48.Complete the sentences about the reaction profile.
Easy
On a reaction profile, the energy of the reactants and products is shown on the ____ axis. The minimum energy needed for the reaction to start is the ____. The difference in height between the reactants and products is the ____.
49.Hydrogen bromide decomposes: 2HBr → H₂ + Br₂. The overall energy change is +103 kJ. Bond energies: H–Br = 366 kJ/mol, H–H = 436 kJ/mol. Calculate the bond energy of the Br–Br bond in kJ/mol.
Hard
50.What word is used to describe a reaction that transfers energy to the surroundings?
Easy
51.Hydrogen and chlorine react to form hydrogen chloride: H–H + Cl–Cl → H–Cl + H–Cl. Bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol. Calculate the overall energy change in kJ.
Hard
52.Hydrogen reacts with iodine to form hydrogen iodide: H₂ + I₂ → 2HI. Bond energies: H–H = 436 kJ/mol, I–I = 151 kJ/mol, H–I = 295 kJ/mol. Calculate the overall energy change in kJ.
Hard
53.The student made the following conclusion: 'The reaction between hydrochloric acid and sodium hydroxide is an endothermic reaction.' State whether the student is correct or incorrect and give a reason for your answer referring to the results and energy transfer.
Hard
54.Explain, in terms of bonds broken and bonds formed, why the reaction between chlorine and methane is exothermic.
Hard
55.Give one everyday example of an exothermic reaction and one everyday example of an endothermic reaction, stating which type each is.
Medium
56.Explain, in terms of bonds broken and bonds formed, why the reaction between hydrogen and chlorine is exothermic.
Hard
57.A student investigates the temperature change when sodium hydroxide solution is added to hydrochloric acid. Explain why the temperature rises to a maximum and then falls as more sodium hydroxide is added.
Medium
58.Hydrogen reacts with chlorine: H–H + Cl–Cl → H–Cl + H–Cl. Bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol. Calculate the overall energy change in kJ.
Medium
- A-202 kJ
- B-166 kJ
- C-148 kJ
- D-184 kJ
59.Hydrogen reacts with iodine: H₂ + I₂ → 2HI. Bond energies: H–H = 436 kJ/mol, I–I = 151 kJ/mol, H–I = 295 kJ/mol. Calculate the overall energy change in kJ.
Medium
- A-4 kJ
- B-1 kJ
- C-2 kJ
- D-3 kJ
60.Hydrogen bromide decomposes: 2HBr → H₂ + Br₂. The overall energy change is +103 kJ. Bond energies: H–Br = 366 kJ/mol, H–H = 436 kJ/mol. Calculate the bond energy of the Br–Br bond in kJ/mol.
Hard
- A231 kJ/mol
- B174 kJ/mol
- C212 kJ/mol
- D193 kJ/mol
61.Hydrogen and chlorine react to form hydrogen chloride: H–H + Cl–Cl → H–Cl + H–Cl. Bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol. Calculate the overall energy change in kJ.
Hard
- A-202 kJ
- B-166 kJ
- C-148 kJ
- D-184 kJ
62.Hydrogen reacts with iodine to form hydrogen iodide: H₂ + I₂ → 2HI. Bond energies: H–H = 436 kJ/mol, I–I = 151 kJ/mol, H–I = 295 kJ/mol. Calculate the overall energy change in kJ.
Hard
- A-4 kJ
- B-1 kJ
- C-2 kJ
- D-3 kJ

字卡

What is an exothermic reaction?

A reaction that transfers energy to the surroundings, causing the temperature of the surroundings to increase.

What is an endothermic reaction?

A reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease.

Give two examples of exothermic reactions.

Combustion and neutralisation.

Give two examples of endothermic reactions.

Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate.

What is meant by the law of conservation of energy?

Energy cannot be created or destroyed, only transferred. The total amount of energy in the universe remains the same.

In an exothermic reaction, how does the energy of the products compare to the energy of the reactants?

The products have less energy than the reactants, because energy is transferred to the surroundings.

In an endothermic reaction, how does the energy of the products compare to the energy of the reactants?

The products have more energy than the reactants, because energy is taken in from the surroundings.

What is activation energy?

The minimum amount of energy that colliding particles must have in order to react.

On a reaction profile, what does the difference in height between the reactants and products represent?

The overall energy change of the reaction.

How can you tell from a reaction profile whether a reaction is exothermic or endothermic?

If the products are at a lower energy level than the reactants, it is exothermic. If the products are at a higher energy level, it is endothermic.

Why is bond breaking an endothermic process?

Energy must be taken in from the surroundings to break chemical bonds.

Why is bond making an exothermic process?

Energy is released to the surroundings when new chemical bonds are formed.

In an exothermic reaction, how does the energy released from forming new bonds compare to the energy needed to break existing bonds?

The energy released from forming new bonds is greater than the energy needed to break existing bonds.

In an endothermic reaction, how does the energy needed to break existing bonds compare to the energy released from forming new bonds?

The energy needed to break existing bonds is greater than the energy released from forming new bonds.

What equation is used to calculate the overall energy change of a reaction from bond energies?

Energy change = energy taken in to break bonds − energy given out when new bonds form.

In a bond energy calculation, what does a negative overall energy change indicate?

The reaction is exothermic because more energy is released than taken in.

In a bond energy calculation, what does a positive overall energy change indicate?

The reaction is endothermic because more energy is taken in than released.

Why is a polystyrene cup used instead of a glass beaker in temperature change experiments?

Polystyrene is a good insulator, so it reduces heat loss to the surroundings and gives more accurate temperature readings.

In the reaction between hydrochloric acid and sodium hydroxide, why does the temperature rise and then fall as more sodium hydroxide is added?

The temperature rises because the neutralisation reaction is exothermic. Once the limiting reactant is used up, further addition of the other reactant dilutes the solution and absorbs heat, so the temperature falls.

Give an everyday use of an exothermic reaction.

Hand warmers or self-heating cans, which release heat to warm you up.

Give an everyday use of an endothermic reaction.

Sports injury cold packs, which take in heat to reduce swelling.

What is a common mistake when describing energy in a chemical reaction?

Saying a reaction 'produces energy' instead of saying it transfers energy to or from the surroundings.

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