Reaction Energy: Exothermic, Endothermic and Catalysts
Watch a chemical reaction as a hill that the particles have to climb. A profile graph shows the energy of the starting chemicals, the energy of the products and the energy barrier between them, with arrows for the energy given out or taken in. Pick a real reaction, such as burning methane, an acid and an alkali neutralising, calcium carbonate breaking down or ammonium nitrate dissolving, and see whether it warms or cools its surroundings and by how many degrees. Beside it, moving particles show how many have enough energy to get over the barrier at the temperature you choose, and pairs of them react as the run goes on. Switch on a catalyst to lower the barrier without changing the energy given out or taken in, and raise the temperature to see more particles succeed.
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What you can adjust
- Reaction
- 0 – 3
- Catalyst
- 0 – 1
- Temperature
- 0 – 1000 °C
Each reaction has its own real energy change per mole: burning methane gives out 890 kJ, an acid and an alkali neutralising give out 57 kJ, calcium carbonate breaking down takes in 178 kJ and ammonium nitrate dissolving takes in 26 kJ. The energy barrier values are simplified teaching values.
A catalyst gives the reaction an easier path with a lower energy barrier. The energy at the start and at the end do not change, so the energy released or taken in stays the same.
How hot the particles are. Hotter particles move faster and more of them have enough energy to get over the barrier. Neutralising and dissolving go at room temperature; burning methane needs about 600 °C and calcium carbonate about 900 °C.
About this simulation
What it shows
A graph of energy against reaction progress shows the starting chemicals, the hill and the products, with arrows for the energy change and the barrier. Beside it, particles react at your temperature, a thermometer shows the surroundings and a second graph shows how many particles can get over the barrier.
The key idea
Breaking bonds takes in energy and making bonds gives it out. If the products have less energy the extra leaves as heat and the surroundings warm up; if more, heat is taken in and they cool. Only particles with enough energy react, so heating helps, and a catalyst lowers the barrier without changing the energy change.
Try this
Run the defaults: the liquid warms by 6.82 °C. Set Catalyst to With catalyst for a faster reaction with the same energy change. Then pick Burning methane and set Temperature to 600.
A simple model: the energy changes are real values per mole, but the barrier heights are simplified teaching values, and the time on the screen is animation time, not a stopwatch. Burning methane and calcium carbonate breaking down hardly go at room temperature, so they need a high temperature. The temperature change is worked out for a stated amount in about 100 g of water with no heat lost to the air. Calcium carbonate is held at the chosen temperature by a burner, so it shows the heat taken in from the burner instead. The shaded part of the energy graph is enlarged when it would be too small to see. A catalyst takes half off the part of the hill above the higher of the two energy levels.