Effect of Temperature on Rate of Reaction
Till now we know that temperature influences the rate of a reaction. If the temperature increases the rate of reaction increases and if the temperature decreases the rate of reaction decreases. If we took an example that the time taken to melt a metal will be much higher at a lower temperature but it will decrease if we increase the temperature. Reaction kinetics is one of the most advanced studies in the field of physical chemistry, and it deals with the study of chemical reactions or processes. The engineers and scientists get help from the information about these factors affecting the rate of a chemical reaction to economically scale up the reactions to industrial scale in various industries. Therefore it is very important for us to know how the rate of a chemical reaction gets affected.
(image will be uploaded soon)
Effect of Temperature
The rate of a chemical reaction can be affected by one of the parameters that is temperature. We have seen milk boiling on a gas stove. On the flame of the stove, the rate of a specific quantity of milk boils depends. The milk boils in less time if the flame height is maximum, and the milk takes more time to boil if the flame height is minimum. Here the flame height resembles the temperature.
Temperature dependence of the rate constant:
(image will be uploaded soon)
The equation is,
K = Aexp(-Ea /RT)
K = The rate constant of the reaction
A = The Arrhenius Constant
Ea = Activation Energy for the reaction (in Joules mol−1)
R = Universal Gas Constant
T = Temperature in absolute scale (in kelvins)
lnk = -(Ea/R)(1/T) + lnA
The milk attains its boiling point in less time, if the temperature is high, and the milk takes more time to attain its boiling point, if the temperature is low. That is not the only reaction that gets affected by temperature. Most of the chemical reactions in their reaction rate show a change with the varying in temperature. for every 10oC rise in temperature, The rate constant for a chemical reaction gets doubled.
If we see the other effect of temperature, when we cook, if we keep the gas at a low temperature, the food cooks slowly. But if we increase the temperature, to its maximum, the food cooks quickly. Therefore the rate of reaction got increased by the temperature. By the Arrhenius equation, this dependence of rate on temperature can be explained. Let’s learn about and deduce this equation.
Arrhenius Equation
Svante Arrhenius extended the work of J.H Van’t Hoff in 1889 and proposed an equation that relates temperature and the rate constant for a reaction quantitatively. The proposed equation is known as the Arrhenius Equation.
Here is the equation on the temperature dependence of the rate of a chemical reaction which is known as the Arrhenius Equation.
K = Aexp(-Ea /RT)
Where,
K = The rate constant of the reaction
A = The Arrhenius Constant
Ea = Activation Energy for the reaction (in Joules mol-1)
R = Universal Gas Constant
T = Temperature in absolute scale (in kelvins)
We knew that the rates of reaction increase with an increase in temperature but it was not known on how to predict the relation between temperature and reaction rates quantitatively. Arrhenius Equation has enabled us to overcome this problem. It is an empirical relation that is used to model the variation of the rate constant with the temperature, which in turn gives information about the rates of chemical reactions at different temperatures.
Fun Facts
If the temperature increases the rate of reaction increases and if the temperature decreases the rate of reaction decreases.
If we took an example that the time taken to melt a metal will be much higher at a lower temperature but it will decrease if we increase the temperature.
Going back to the rate law equation, it follows that a higher rate of constant results in a higher reaction rate. This makes sense because as temperature increases, molecules move faster and collide more frequently, resulting in an increased fraction of molecules with higher energy than the activation energy.
Due to chemical bonds having been broken and made, they also usually give out or take in energy such as heat or light because. It reacts with oxygen in water or in the air to create a new compound called iron oxide (rust), when iron rusts. No mass is lost or gained in every chemical reaction due to change in temperature.
FAQs on Temperature Dependence on Rate of Reaction
1. What is the basic effect of temperature on the rate of a chemical reaction?
Generally, increasing the temperature increases the rate of a chemical reaction. A common rule of thumb for many reactions is that the rate nearly doubles for every 10°C rise in temperature. Conversely, lowering the temperature will slow down the reaction rate.
2. Why does a higher temperature increase the reaction speed?
A higher temperature increases reaction speed for two main reasons based on the collision theory:
- Increased Collision Frequency: At higher temperatures, molecules have more kinetic energy and move faster. This leads to more frequent collisions between reacting particles per unit of time.
- More Effective Collisions: A greater fraction of the colliding molecules will possess the minimum energy required for a reaction, known as the activation energy. This means more collisions are successful in forming products.
3. What is the Arrhenius equation and what does it explain?
The Arrhenius equation is a mathematical formula that describes the relationship between the temperature and the rate constant (k) of a reaction. The equation is: k = A * e-Ea/RT. It shows precisely how the rate constant, and thus the reaction rate, depends on the absolute temperature (T) and the activation energy (Ea).
4. What is 'activation energy' and how does temperature help molecules overcome it?
Activation energy (Ea) is the minimum amount of energy that reacting molecules must possess to be able to form a product upon collision. Temperature doesn't lower the activation energy, but it gives a larger percentage of the molecules enough kinetic energy to overcome this energy barrier. Think of it as giving more people the strength to jump over a wall; the wall's height (Ea) stays the same, but more people succeed.
5. Can you give a simple example of temperature's effect on reaction rates from daily life?
A perfect example is food spoilage. Food is a complex mix of chemicals that can react and spoil. We store food in a refrigerator to lower the temperature, which significantly slows down the rate of these spoilage reactions, keeping the food fresh for longer. Conversely, cooking food uses high temperatures to speed up chemical reactions that make it edible and kill bacteria.
6. What is the 'temperature coefficient' and what does it tell us?
The temperature coefficient is the ratio of the rate constants of a reaction at two temperatures that differ by 10°C. It is usually represented as the ratio k(T+10) / k(T). For many reactions at room temperature, this value is approximately 2 to 3. It provides a simple way to estimate how much a reaction will speed up with a small increase in temperature.
7. How can the Arrhenius equation be used to create a graph?
By taking the natural logarithm of the Arrhenius equation, we get a linear equation: ln(k) = - (Ea/R) * (1/T) + ln(A). This is in the form of a straight line (y = mx + c). By plotting a graph of ln(k) on the y-axis against 1/T on the x-axis, we get a straight line with a slope equal to -Ea/R. This graphical method is a common way to experimentally determine the activation energy of a reaction.
8. Does increasing the temperature always increase the rate of every reaction?
For nearly all simple chemical reactions, increasing the temperature increases the rate. However, in more complex systems, this might not always appear to be the case. For example, in reactions involving enzymes (biological catalysts), an excessively high temperature can cause the enzyme to denature (change its shape). This destroys its catalytic activity, causing the reaction rate to drop sharply after reaching an optimal temperature.
