Grades 9 and 10 - Chemistry - High School - Endothermic and Exothermic Reactions — Flashcards

Where the energy transferred in a reaction is heat, you can detect the type of reaction (endothermic or exothermic) by using a thermometer or your skin. Heat energy always flows from higher temperatures to lower temperatures. When you carry out an experiment in the lab at room temperature and the reaction vessel (beaker, conical flask or whatever) feels colder, then you have an endothermic reaction. Heat energy will flow from the warmer surrounings into the reaction mixture - it is taking in energy. If the vessel feels warmer, the reaction must be exothermic as the heat energy will flow from the reaction mixture into the surroundings. In practice, it isn't a good idea to just feel the temperature of a reaction vessel as you could end up with a hot or cold burn! It is better to use a thermometer

But sometimes, temperature can be misleading. When carrying out an electrolysis reaction, the temperature of the electrolyte will sometimes rise. So it must be exothermic? Wrong! Electrolysis reactions are endothermic as they absorb electrical energy. The rise in temperature is actually caused by the heating effect of the current.

Examples of exothermic and endothermic reactions: Good examples of exothermic reactions are burning (some of the enrgy is released into the surroundings as light), the neutralization of a strong acid by a strong alkali and the reaction between calcium oxide and water to make calcium hydroxide (quicklime reacting to form slaked lime). Apart from electrolysis, good examples of endothermic reactions are any thermal decomposition (such as the decomposition of limestone in the blast furnace) and the reaction between ethanoic acid and sodium carbonate.

In a chemical reaction, you always end up with the same numbers and types of atom as you started with, but joined together in a different combination. For example, the reaction between hydrogen and oxygen to give water. To start with, the hydrogen atoms are joined to each other in pairs, as are the oxygen atoms. At the end of the reaction, two hydrogen atoms are joined to one oxygen atom in the compound water. Clearly, to get to this state of affairs, the bond between the hydrogen atoms must have been broken and so must the bonds between the oxygen atoms. Only when that has occured can the bonds of the water molecule form.

To break the bonds between the hydrogen atoms will require an input of energy to make it happen. The same goes for the bonds between the original oxygen molecules. But when the hydrogen atoms join with oxygen atoms, it happens spontaneously and energy is released as the bonds of the water molecule form. Since the bonds that have been formed are different to the original bonds, it is unlikely that the same amount of energy will be released as was used in the first place. If more is released from bond making than was used for breaking the original bonds, energy will be released into the environment and the reaction is exothermic. if less is released, then the reaction is endothermic.

This can be represented on an energy level diagram. It is just a simple graph that shows the energy changes in a reaction. On the vertical axis, you have the energy and along the horizontal axis you have the progress of the reaction. The diagram itself shows the level of energy in the reactants and products as two short horizontal lines. If the products appear lower than the reactants, energy must have been lost from the system. This energy must have traveled into the surroundings i.e. an exothermic reaction. If the products appear higher than the reactants, the opposite is true