The two questions chemists ask about any chemical reaction they plan to use is, “Will it continue to occur?” and, “How fast will it occur?” In our lab we explored a chemical reaction that continues to occur on its own. This reaction is called spontaneous. Spontaneous reactions are changes that, once begun, continue without an input of energy. Chemists need to be able to predict when reactions can occur spontaneously. There are two sets of theories, thermodynamics which answer the question, “Can a reaction occur spontaneously?” and kinetics which answer the question, “How fast can a reaction occur?” Thermodynamics is the study of how heat and other forms of energy are involved in chemical and physical reactions. Kinetics is the study of reaction rates and how they can be affected by variables such as temperature.
To determine if a change can occur spontaneously there are two factors. The first factor that affects spontaneity is if the change gives off heat when it occurs or absorbs heat energy. The second factor that affects spontaneity is if the change results in particles becoming disordered or less disordered. Two rules apply to these two questions. Lower energies are more stable than higher energies therefore energy changes in chemical reactions tend to occur in ways that allow the substances to end up with lower energy. Changes that release energy tend to be favored. Also everything tens to become more disorganized therefore the changes in which particles become more spread out are favored.
If a change both releases energy and becomes more disordered the change is definitely spontaneous. Vice versa, if the change absorbs energy and becomes more ordered the change is definitely not spontaneous.
The mains points on chemical reactions are:
- When chemical reactions occur, bonds in the reactants break and new bonds form to make products.
- Breaking bonds needs energy input therefore bond-breaking is an endothermic change.
- Forming bonds releases energy therefore bond-forming is an exothermic change.
- The overall change can be endothermic or exothermic depending on whether the total energy input or the total energy output is greater.
Chemists call en energy change that occurs at a constant pressure “change in enthalpy” and the symbol is delta H. A negative delta H means the reaction was exothermic and a positive delta H means the reactions was endothermic. To represent the enthalpy change you can draw an energy diagram. This diagram shows the progress of the reaction from reactants to products on the horizontal axis and shows the change in potential energy that occurs as substances progress from reactants to products on the vertical axis.
The relative locations of the reactant energy and product energy indicate whether the overall enthalpy change is endothermic or exothermic. For an exothermic reaction, if the starting point (reactants) is higher than the ending point (products), the system releases heat energy therefore the delta H is negative. For an endothermic reaction, if the energy of the reactants is lower than that of the products, the system absorbs heat energy therefore the delta H is positive.
Bonds must always be broken in the reactants in order to form new bonds in the products. Some initial amount of energy must be applied to the system for the reaction to begin. This is represented by an initial “bump” in the curve to get from reactants to products. The height of this bump is called the activation energy. The intermediate state that is a combination of reactant and product atoms is called the activation barrier.
To speed up a reaction you can use a catalyst. A catalyst is a substance that speeds up a chemical reaction without being used up itself. It works by providing a lower energy pathway which gives the starting materials enough energy to get over the energy barrier.