Rate of Reaction and Equilibrium

Objectives Describe how to express the rate of a chemical reaction. Identify four factors that influence the rate of a chemical reaction.

Prior Knowledge When we have a reaction we need use an equation to represent it. A + B  AB……………this is a chemical equation What do we call A and B on the left, the items reacting together? What about AB, the item that is produced?

Real World Reactions 2Na + Cl2  2NaCl 2H2 + O2  2H2O Man + Woman  Baby Try to come up with 1 of your own

Rate Rate is a measure of the speed of any change that occurs within a set amount of time. This is measured using the equation of: (Change in Quantity)/ (Change in Time) = Rate Think back to the penny lab. The penny just didn’t magically turn gold. It didn’t even instantly turn silver. All reactions take time, some are incredibly fast. Others very slow.

Collison Theory Even if enough time is given to a reaction, other factors need to be met. The right angle Enough energy They need to collide Collision Theory is where atoms react to form products when they collide with one another. In other words once we have collisions…a product forms…and when a product forms…we can find the rate of the whole reaction.

We also need to achieve a certain energy level. That effective middle shape isn’t always as easy as just getting the right angle. We also need to achieve a certain energy level. Just like for boiling water We do not simply put a pot of water on the stove and get it to boil instantly. A certain amount of energy is required.

In chemistry we call that required energy the activation energy. Activation energy is the minimum energy that colliding particles must have in order to react. When the activation energy is met, we get an “in between” state. We call this a transition state, where atoms are very unstable as they are in the process of becoming products.

Factors Affecting Reaction Rates Temperature Concentration Particle Size Catalysts

Temperature This is the simplest of the factors that can reaction rate. If we heat up a reaction the particles will begin to move around more, and as they move more there is a greater increase that they will collide. More Collisions  Faster Reaction

Concentration Concentration is a reference to the strength of a solution. Depending on the chemical we are working with something with a concentration of 1M and something of 10M could be drastically different. The strength is measured by how many particles of the solution is within a fixed area. With higher concentrations, higher chance of collisions.

Particle Size Imagine adding sugar into coffee If you add a sugar packet it will dissolve fairly quickly. If you added the same mass of sugar, but as a solid clump, it would take longer for the sugar to dissolve. By increasing the surface area of the sugar, decreasing the particle size, we have more areas for a reaction to take place.

Catalysts A catalyst is something that is used to speed up a reaction but is not consumed in it. Heat is a great example, it speeds up the process in boiling water while at the same time it does not change the water into something new.

Objectives Describe how the amounts of reactants and products change in a chemical system at equilibrium. Identify three stresses that can change the equilibrium position of a chemical system. Explain what the value of Keq indicates about the position of equilibrium.

Prior Knowledge We have already discussed the parts of a chemical equation: Reactants react to produce Products. They are almost all written left to right as in: A + B  AB

Reversible Reactions Reversible reactions are present when the reaction needs to even out. Forward: 2SO2 + O2  2SO3 Reverse: 2SO2 + O2  2SO3 Notice the second reaction has the arrow pointing in the opposite direction. This happens so that the equation can stabilize, or reach equilibrium.

A reversible reaction is one in which the reactants forming products and the products reverting to reactants happens simultaneously.

Chemical Equilibrium The reason we have forward and reverse reactions is so that a reaction can stabilize. Chemical Equilibrium occurs when the rates of both the forward and reverse reactions are equal.

Le Chatelier’s Principle Le Chatelier’s Principle states that if a stress is applied to a system, it will find a way to even itself out. These stresses include: Concentration Temperature Pressure

Concentration The concentration as mentioned before is the strength, or amount of a certain piece of the equation within a certain volume. If we increase the concentration on one side of the equation, Le Chatelier’s principle stated that the other side will gain as well to regain balance.

Temperature Typically when a reaction occurs from left to right heat is released. If we revisit the equation: 2SO2 + O2  2SO3 We can also write it as: 2SO2 + O2  2SO3 + heat This means that heat was released. It is essentially a product. That means just like with the concentration aspect. If we add heat to the equation, the side with heat will increase, and the opposite side will increase to compensate.

Pressure Primarily when dealing with gaseous substances, if we have a system at a normal pressure then the reactants and products will find a normal equilibrium. If we increase the pressure however, then the molecules inside will need to somehow make up for that lost space. They do this by creating more of the products, which will take up less space.

Equilibrium Constants This is where we need to use a little bit of math Keq, The equilibrium constant, is the ratio of product concentrations to reactant concentrations at equilibirum. Lets take the following equation as an example: aA + bB  cC + dD Keq = [C]c x [D]d / [A]a x [B]b When we plug in all the numbers and get the answer: If Keq > 1, the products are favored If Keq < 1, the reactants are favored

Example If we have the following equation: N2O4   2NO2 If we have 0.0045 mol of N2O4, and 0.030 mol of NO2, then what is the Keq, and which way does the reaction favor.