Chemical reactions are at the heart of life. But as reactions occur, they do so at rates that vary with the concentrations of reactants. So description of each chemical reaction is complicated by values in flux. (There are other important aspects in our lives that are similarly in flux. The tools chemists have found for understanding reaction rates may be valuable for these other aspects as well).
The chemistry approach is to utilize understanding from theory (of the sub-microscopic) to come to understand the observable (macroscopic).
A RATE describes how fast something changes in time. A chemical reaction rate describes how fast a reaction occurs. But a reaction usually involves several different substances. Typically chemists select one of the substances connected by the reaction to describe the reaction rate.For example, for the idealized reaction where substance A reacts with B to make C
It should be noted that while the concentration of each reactant may vary, there is a unique constant, k, for each reaction which distinguishes whether the reaction tends to be slow (small k) or fast (large k).
Often by determining the chemical equation and measuring the speed in one circumstance, chemists can predict the rate of reaction in other circumstances.
Fine Print: Many reactions occur by mechanisms involving numerous steps. In such cases the concentrations involved in the slowest step governs the overall reaction rate. Sometimes reaction rates are increased with assistance from catalysts causing other factors (such as geometry, surface area, or electron transfer rate) to govern reaction rate. So a rate equation based on a balanced equation should be viewed as an educated first guess that should be verified at several concentrations.
At higher temperature, molecules move faster, so they will arrive at the collision sooner. Thus reactions are faster when the temperature is higher. So our bodies are roused into action against an invading pathogen by a fever.
A reaction’s constant, k, increases with higher temperature.
Not all collisions between molecules cause reactions. At slower speeds, collisions of molecules are gentler resulting in more rebounds and fewer breaking of bonds needed to cause reaction. Below a certain characteristic temperature, the rate of reaction drops rapidly with virtually no reaction only a few degrees cooler. It is a common observance that a match raises temperature enough to ignite a fire; but cooling by blowing or flooding will extinguish the flame. Similarly the shelf live of food is prolonged from destructive spoilage reactions by refrigeration, and virtually preserved by freezing (except that evaporation continues creating freezer burn, the misnomer for freezer dehydration).
Scientists often use the physics concept of ENERGY to help understand and predict the effect of temperature on reaction rates.Energy is defined by its ability to do WORK
Physicists had carefully defined POTENTIAL ENERGY as energy stored in forces such a chemical bonds. Chemists sometimes use the term enthalpy when describing potential energy stored in chemical bonds. It requires a certain amount of energy to pull apart each chemical bond. (We won’t worry here about the precise formula for that relationship.) But we can use a graph of potential energy (such as the reaction forming ammonia to the right→) to visualize the chemical forces and the effect of temperature on reaction rates.
The reactants usually start with some potential energy (such as N2 and H2 on the left) depending on their initial chemical bonds. (The potential energy for common bonds has been measured using reactions which form molecules with each kind of bond. These are available in reference tables of Heats of Formation. But note that if we are interested in the energy involved in the reaction, we may for our convenience choose to set the starting potential energies at zero, as we've done here. Forming a bond usually releases potential energy in the form of heat. The potential energy for molecules with multiple bonds can be summed from the known heats of formation to determine the potential energy of the entire molecule.) Once a bond is formed, the atoms have less potential energy than when they were free and independent! So potential energy must be added (increased) to break a bond.
In a collision causing a reaction, the initial potential energy of the molecule is increased (note peak on graph) as bonds are knocked apart. The energy needed to do this is called ACTIVATION ENERGY. Often this energy comes from the speed of the colliding molecules which depends on temperature. (It could also come from light, higher energy radiation, or from other molecules such as ATP.) If the collision has insufficient energy, the molecules rebound back to the reactant state (i.e., back to the left ← on the graph). But if there is sufficient activation energy to break the bonds (and reach the peak), new bonds may form lowering the potential energy to that of the products (on the right side of the graph →).
The products may have more or less potential energy than the initial reactants. If the potential energy is less (as is the case forming ammonia shown on this graph), the excess (difference) energy is usually released as heat. (Alternatively it may be released as light or sound, or may be passed to other molecules such as ATP.) Such a reaction that releases energy is described as EXOTHERMIC (Greek: heat exits); if the products have greater potential energy, more energy will have entered than left, and the reaction is called ENDOTHERMIC(Greek: heat into). This usually results in the reaction mixture cooling itself.Such heat of reaction, ΔH, can be measured in an insulated reaction vessel using the equation