An exothermic reaction releases heat and warms its surroundings. An endothermic reaction takes in heat and cools them. Which one happens depends on the energy of the bonds broken and the bonds formed.
In a reaction, bonds in the reactants are broken and new bonds in the products are formed. Breaking a bond takes in energy. Forming a bond releases energy.
The two amounts are almost never equal. If forming the new bonds releases more energy than breaking the old ones took in, the extra energy leaves as heat. If breaking takes in more than forming releases, the shortfall is taken in as heat from the surroundings.
A thermometer in the mixture, or a hand on the beaker, shows which case it is.
A reaction that releases heat to its surroundings is exothermic. The temperature of the surroundings rises.
Calcium oxide added to water is exothermic; the previous session wrote it with “+ heat” on the product side. Burning is exothermic. Methane in natural gas burns with oxygen to give carbon dioxide and water, and the heat released is used for cooking.
Respiration in your cells is also exothermic overall. Glucose from food reacts with oxygen to form carbon dioxide and water:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Part of the energy released is stored in other molecules in the cell and used for muscle work. The rest is released as heat, which keeps your body at about 37°C.
Which way does heat flow? Out of the reaction mixture: exothermic, and the surroundings become warmer. Into the reaction mixture: endothermic, and the surroundings become cooler.
During a run, you breathe faster and feel hot. Explain both using respiration.
Running muscles need energy at a higher rate. Respiration in muscle cells speeds up, using more glucose and more oxygen per minute. Breathing faster supplies the extra oxygen and removes the extra carbon dioxide.
Respiration is exothermic. A faster rate releases more heat per minute, so your body temperature tends to rise and you feel hot.
A reaction that takes in heat from its surroundings is endothermic. The temperature of the surroundings falls.
Mixing solid barium hydroxide with solid ammonium chloride in a test tube is endothermic. The tube becomes cold enough to feel clearly. Heat flows from the glass, and from your hand, into the reacting mixture.
Most decomposition reactions are endothermic, as the previous session showed. Thermal decomposition needs heat supplied. Electrolysis needs electrical energy supplied.
Someone says: “An endothermic reaction feels cold, so no energy is involved.” What is wrong with this?
Energy is involved; it flows in the opposite direction. The reaction takes heat in from its surroundings, including the glass and your hand.
Your hand feels cold because it is losing heat to the tube. A cold beaker is a sign of heat flowing into the reaction, not a sign that there is no energy change.
Measure the temperature of the surroundings, or of the mixture, before and after.
1An exothermic reaction is one that…
2Sort each reaction by the direction of heat flow.
3Why is respiration exothermic?
4A reaction that takes in heat from its surroundings, leaving them cooler, is ____.
5A test tube becomes cold while a reaction happens inside it. The reaction is…
6In terms of bonds, why is a reaction exothermic?
7A reaction warms its surroundings. Which pairing is correct?
Next: reactions in which one metal displaces another from its compound.
Noescio