It is useful to place carbon and hydrogen into the reactivity series because these elements can be used to extract metals.
Here is the reactivity series including carbon and hydrogen:
Note that zinc and iron can be displaced from their oxides using carbon but not using hydrogen. However, copper can be extracted using carbon or hydrogen.
Chemical reactions: exothermic and endothermic reactions We have explained that a chemical reaction will not occur until substances (reactants) receive enough energy (activation energy) to break chemical bonds. This allows atoms to redistribute themselves to form new bonds and thus form new substances (products). The activation energy needs to be thought of as a barrier to be overcome. If the bonds between the reactants are strong, greater activation energy is required to initiate a chemical reaction; if the bonds are weak, less activation energy is required.
In the first stage of a chemical reaction, the enthalpy of the reactants increases through some form of energy transfer. Therefore, the total energy of the reactants increases by the amount of the activation energies. At this point, bonds are broken. In the next stage, new bonds are made in the formation of the products. The total energy of the products (i.e. the sum of the enthalpies) may be greater than, or less than, that of the reactants. When the total energy of the products is less than that of the reactants, the chemical reaction is called an ‘exothermic reaction’, and when the total energy of the products is more than that of the reactants, the chemical reaction is called an ‘endothermic reaction’.
To be consistent with the law of conservation of energy, in an exothermic reaction, excess energy is transferred to the surrounding materials that do not take part in the reaction (the surrounding environment): the environment heats up.
In an endothermic reaction, energy is taken from the surrounding environment: the environment (the surrounding substance) cools down.
The different types of chemical reaction are shown in Figure 1. Note that the total energy of the whole system (surrounding environment–reactants–products) remains constant before and after the reaction, whereas this is not true for the total energies of the reactants compared to the products.
Energy diagram for an exothermic reaction
Energy diagram for an endothermic reaction
A good example of an endothermic reaction is the use of an instant icepack. Instant icepacks can be used to treat minor burns as well as sporting injuries, such as sprains. A typical icepack contains the ionic compound ammonium nitrate salt (NH4NO3), which reacts with water. In solution (the ionic solid dissolved in water), the ionic bonds are broken, freeing up ammonium ions (NH4+ ) and nitrate ions (NO3 – ). During the reaction, energy is taken from the surrounding environment (for example, the ankle), thus cooling it down. The equation for the reaction is:
NH4NO3water NH4+ + NO3–
Many foods we eat undergo exothermic reactions and literally warm us up. Glucose (C6H12O6), a type of sugar found in many foods, reacts with oxygen (O2, in the air we breathe) to produce an exothermic reaction, with carbon dioxide (CO2) and water (H2O) as the products. The energy given off assists in the maintenance of a constant internal body temperature, which is important for many processes.
The equation for the reaction is:
C6H12O6 + 6O2 → 6CO2 + 6H2O
It is important to note that, in this reaction, the reactants involve more than two molecules. Every molecule of glucose reacts with six molecules of oxygen. The reaction produces six molecules each of carbon dioxide and water. As an exercise, convince yourself of the law of conservation of atoms by determining that the total number of each type of atom is the same on each side of the chemical equation.