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An exothermic reaction transfers energy to the surroundings, often warming the solution, and has a negative reaction energy change. An endothermic reaction takes energy from the surroundings and has a positive change. Measure a temperature change using the same starting conditions and sensible insulation. A warmer beaker is evidence about the surroundings; explain the opposite energy change of the reacting chemicals.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Distinguish exothermic and endothermic changes
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Is bond breaking endothermic?
Yes. Energy is needed to separate bonded atoms.
After watching 2: Where are products on an exothermic profile?
Below reactants. Energy has been released to the surroundings.
What you'll be able to do
An exothermic reaction transfers energy to the surroundings, often warming the solution, and has a negative reaction energy change. An endothermic reaction takes energy from the surroundings and has a positive change. Measure a temperature change using the same starting conditions and sensible insulation. A warmer beaker is evidence about the surroundings; explain the opposite energy change of the reacting chemicals.
A profile shows reactants, products and an energy barrier. Activation energy is the energy difference from reactants to the top of the barrier. The overall change is the difference between product and reactant levels. A catalyst lowers the barrier by offering another pathway but leaves those initial and final levels unchanged. Exothermic profiles end lower than they start, while endothermic profiles end higher.
Breaking bonds requires energy; making bonds releases energy. Add energies for all bonds broken, subtract energies for all bonds formed, and use coefficients from the balanced equation. Average bond energies make an estimate rather than an exact result for every molecular environment. In a hydrogen–oxygen fuel cell, chemical energy is transferred electrically while reactants are supplied; environmental evaluation includes how the hydrogen was produced.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A catalyst lowers a profile’s peak. What happens to overall reaction energy?
The overall energy change is unchanged.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.