IS KINETICS A BRANCH OF CHEMISTRY CONCERNED WITH

IS KINETICS! A BRANCH OF CHEMISTRY CONCERNED WITH THE RATE OF CHEMICAL REACTIONS By: Maria Fernanda Arango and Sara Urbina

WHAT IS KINETICS? It is a branch of chemistry that focuses on the rate of change of the concentration of reactants and products in a chemical reaction.

COLLISION THEORY Explains theory of a reaction taking place due to the particles colliding, Once collided the particles undergo a reaction. The rate of reaction can be made increased by: 1) Increasing the amount of times they collide 2) Increasing hte amount of successful collisons.

WHAT IS ACTIVATION ENERGY? This is the MINIMUM energy required by a particle to produce a successful reaction (All particles have kinetic energy, although the activation energy is the minimum amount reguired. ) Symbol: Ea

Collision theory can be taken a step further by evaluating what affects the rate of reactions.

RATES OF REACTION The rate of a reaction is: how quickly a reaction reaches a certain point. Can be defined as: Decrease in concentration of reactants per unit time (s) or An increase in concentration of the products per unit time (s)

What affects the rate of a reaction and how it is measured? A rate of a reaction is measured by the amount of the product produced in a particular period of time. This is affected by different factors: Temperature Concentration Catalyst Particle size

TEMPERATURE! By increaseing the temperature, you also increase the rate of the reaction. *The animation shows that the hotter the solution the more often particles collide and so the reaction takes place more quickly.

PARTICLE SIZE! If the reactants have a larger surface area (i. e. are a powder). They will react more ofter then if they had a smaller surface area (i. e. a big rock). Meaning that if the surface area is larger the rate of the reaction is faster. Because it is easier for the reactants to collide against each other

IS CONCENTRATION! * As we can see the green particle will collide more easily in the purple solution and the concentration of purple molecules is higher than in the in solution. Meaning that reaction will be faster in the purple solution. The higher the concentration of a reaction the more of that reaction there is and so the more often the particles collide. This also speed up the reaction. If a solution have a highre concetration, it will allow the reaction to take place more quickly.

CATALYST! The graph shows that a catalys lowers the activation energy, meaning that more particles now have enough energy to react. It consequently accelerates the rate of the reaction. *This occures because the catalyst provides the right conditions for the reaction to take place more quickly. By allowing the products and reactants to come together faster etc.

SUMMARY!

MATH STUFF. . The rate of a reaction can be measured though the change in concentration. Rate = change in concentration time Units= moldmˉ³sˉ¹ The [ ] mean it´s CONCENTRATION

EXAMPLE! FIRST! We need a blanaced equation like the one below: The co-efficient tell you how much faster something is being made of used up compared to something else. i. e. Iodine and hydrogen are being used up 2 times as fast as H₂O₂. FILL IN THE GAP! Water is being produced ………………. as. . . , compared to Iodide.

CONTINUED… Let us assume that the reaction went on for two minutes, do you REMEMBER how to find the rate of reaction? ? Pssstttt, if you don´t maybe the next slide can help you!

HOW TO FIND THE RATE OF REACTION WITH MATH! Using the same equation. 1) As water is being produced twice as fast you multiply by a HALF Make sure you are aware this it is two in THIS CASE, although you would multiply by the reciprical of the co-efficient. 2) Write it like this: 3) Given that the concentration is 0. 005 moldmˉ³ and it took 2 minutes can you calculate the rate of the reaction?

Procedure: ANSWER! 2. 083 X 10ˉ²

INTRODUCTION TO EQUILIBRIUM: CHEMICAL CHANGE Some reactions will only go to completion if: A) the activation energy (Ea) is low enough B) the products are more stable than the reactants Some reactions don’t happen at all because either: A) activation energy is too high B) products are less stable than the reactants Sometimes the reactants and products have similar energies, meaning the reaction is reversible; meaning it can occur in either direction – forward or backward. This is indicated by means of a double arrow:

DYNAMIC EQUILIBRIUM For any reversible reaction, at some point the rate of the forward and backward reaction will be equal. This balance between the two opposite reactions is dynamic equilibrium; meaning: A) equilibrium = balance between the forward and backward rate of reaction. B) dynamic = there are changes taking place – the reaction still continues. HOWEVER, the concentrations of reactants and products remains constant – but NOT equal.

CHEMICAL EQUILIBRIUM Chemical equilibrium is the state of dynamic equilibrium that takes place in a closed system when the rate of the forward and backward reaction is equal. A closed system means no atoms of the products or reactants can escape to the outside environment (i. e. a closed vessel).

A chemical reaction is in equilibrium when the concentrations of products and reactants remains constant (but not equal) and the rate in the opposite directions are equal. There is no net change in the quantities of products and reactants. A+B C+D At first, the forward reactions occurs rapidly, but the concentrations of the reactants A and B will fall; decreasing the rate. At the start, the reverse reaction cannot occur. However, as soon as products C and D start to form, the rate of the backward reaction increases. Eventually, both rates will be equal and the concentrations will be maintained constant; meaning there is a state of equilibrium.

EXAMPLE: THE HABER PROCESS N 2 + 3 H 2 2 NH 3 When nitrogen and hydrogen react, only 15% of these reactants is converted to ammonia and as the reaction is reversible, the ammonia can react and turn back into nitrogen and hydrogen. However, the position of equilibrium is shifted to the right by using: -A low temperature (compromise) -A high pressure – but not too expensive

AMMONIA!

CHANGE OF CONCENTRATION WITH TIME ESTABLISHING CHEMICAL EQUILIBRIUM When the reaction starts, the concentration of the reactants will start to decrease, as they are formed into products. Due to this, the concentration of the products will start to increase as more reactants react. Finally, when equilibrium is reached – the forward and backward rates are equal – the concentrations will remain constant. However , as the graph shows , the concentrations of products and reactants are not the same.

RATE OF REACTION WITH TIME ESTABLISHING CHEMICAL EQUILIBRIUM When the chemical reaction is initiated, the rate of the forward reaction will be very high. However, the reverse reaction cannot take place as there are no products present that can be turned to reactants. As the reaction progresses, the rate of the forward and reverse reaction will start to equalize: a) The forward reaction rate will decrease while b) The reverse reaction rate will increase In the end, the forward and reverse reaction will coincide; reaching equilibrium.

If the concentrations are constant, then observable properties such as p. H, colour, density and viscosity (among others) will remain constant. The relative concentrations of reactants and products depends on the relative rates of forward and backward reactions. Even though concentrations are constant, any molecule from either the reactants or products may react. However, the system tries to oppose this change and equilibrium is restored once again.

THE EQUILIBRIUM CONSTANT The rate at which a reaction occurs depends on the concentrations of the chemicals present. In the following reversible reaction: The rate expressions for the forward and backward reactions are: Forward rate = kf. [A] [B] Reverse rate = kr. [C] [D] When equilibrium is reached, these rates are equal, therefore: kf. [A] [B] = kr. [C] [D] this rearranges to give As Kf and Kr are constants at a certain temperature, then their ratio should also be equal to a constant. This is then the equilibrium constant (Kc).

The equilibrium constant is given by the concentration of the products raised to the power of their stoichiometric coefficients divided by the concentrations of the reactants also raised to these powers. For the following reaction: a. A + b. B + c. C +. . . p. P + q. Q + r. R +. . . The equilibrium constant is: Kc has no fixed units and these units must be calculated from the equation of the equilibrium constant, given the fact that the concentrations of the products and reactants have units of mol dm-3.

THE IMPORTANCE OF STATE SYMBOLS When calculating Kc, it is important to take into account state symbols: (aq), (s), (l) or (g), because certain concentrations of chemicals will be omitted depending on the state they’re in. State symbol Omitted? Reason S (solid) Yes As all solids have a constant concentration and a fixed density. l (liquid) Yes The concentration of a pure liquid is constant and it also has a fixed density. aq (aqueous) No Concentrations can vary; therefore, they should be included in the calculation. g (gas) No Always taken into account, as concentration can change during the reaction.

Nevertheless, water is a special case. The concentration of water in a liquid phase is taken as constant and it is omitted for calculations of the equilibrium constant in dilute aqueous solutions. If the reaction is not in aqueous solution, then the concentration of water must be included even if it is in the liquid phase; as the concentration of water can vary. However, the concentration of water must be included if it is in the gas phase. State symbols dictate when to take the products or reactants into account when calculating the equilibrium constant Kc.

LE CHATELIER’S PRINCIPLE If the conditions under which the equilibrium was established are changed, then the rates of the backward and forward reaction will no longer be equivalent. The equilibrium has been disturbed and the concentrations of the species will alter until equilibrium is reached once again. Le Chatelier’s principle is a way of predicting the direction in which the position of equilibrium will change once the conditions have been varied. It manifests: “If a change is made to the conditions of a chemical equilibrium, then the position of equilibrium will readjust so as to minimise the change made. ”

THE EFFECT OF CHANGES IN CONDITIONS ON THE POSITION OF EQUILIBRIUM Change Effect on equilibrium Change in Kc? Increase concentration Shifts to the opposite side No Decrease concentration Shifts to that side No Increase pressure Shifts to the side with the least moles of gas No Decrease pressure Shifts to the side with most No moles of gas Increase temperature Shifts in endothermic direction Yes Decrease temperature Shifts in exothermic direction Yes Add a catalyst No change No

SUMMARY

BIBLIOGRAPHY http: //dl. clackamas. edu/ch 105 -03/dynamic. htm http: //www. chemistryexplained. com/Di-Fa/Equilibrium. html http: //www. gcsescience. com/h 4. htm http: //www. chem 1. com/acad/webtext/chemeq/Eq 01. html#SEC 1 http: //www. chemguide. co. uk/physical/equilibria/haber. html http: //www. chemguide. co. uk/physical/equilibria/kc. html http: //www. chem 1. com/acad/webtext/chemeq/Eq-03. html http: //www. blobs. org/science/article. php? article=42 http: //ibchem. com/IB/ibnotes/brief/kin-sl. htm#rat

Wow! My forehead grew from all that chemisty!