Reaction Rates and Chemical Equilibrium Test on April 15

Reaction Rates To say a reaction is “fast” or “slow” is very vague. We need a way to quantitatively describe reaction rates.

Reaction Rates Reaction rate = change in concentration change in time

[H] = the concentration of hydrogen The concentration is denoted by putting brackets around the element symbol [H] = the concentration of hydrogen The concentration is found by dividing the number of specific particles by the total number of particles [H] = # of hydrogen atoms/total number of particles

Rates and Concentrations Wouldn’t the rate depend on how many moles of a substance there are? Imagine if you can 3 miles per hour. It will obviously take you longer to run 4 miles than it would to run 2 miles. However, your rate will be the same. (3 miles per hour)

Kinetic Molecular Theory Particles are always in constant random motion.

Collision Theory If particles are going to react, they need to collide. Two particles cannot react without touching.

Oxygen Molecule O2 O Hydrogen Molecule H2 H O H O H H

Collision Theory The collision theory can explain why reactions involving higher concentrations proceed faster than those with lower concentrations. The higher concentrations allow for a greater probability that molecules will collide. The collision theory also states that in order to produce a product, reacting substances must not only collide, but collide with the correct orientation and sufficient energy.

Collision Theory Analogy To visualize the collision theory, think of the game where a tennis ball is thrown back and forth between two Velcro paddles. In order for the ball and paddle to react, the ball and paddle must first come into contact with each other. Secondly the ball must come in contact with the Velcro part of the paddle. If it hits the side or back of the paddle, the ball will not stick. Finally, the ball must be thrown hard enough so that it will stick to the Velcro and not fall short.

Activation Energy The minimum amount of energy needed for a reaction to begin is called the activation energy. If you provide only the lowest amount of energy needed, the reaction will be very slow. However, if you provide more than enough energy, the reaction will happen faster.

Factors affecting Reaction Rates There are other ways to increase the reaction rate besides increasing the amount of energy you provide (by increasing the temperature). You could increase the concentrations as discussed earlier or increase the surface area (allowing for more collisions because more molecules are exposed).

Catalysts A final way to increase reaction rate is to provide a catalyst. A catalyst is a substance that increases the rate of reaction without actually being involved in the reaction. The opposite of a catalyst is an inhibitor. An inhibitor slows the reaction rate or might even prevent the reaction from occurring at all.

Arrows in Chemical Equations We often represent a chemical reaction with a chemical equation. In a chemical equation, an arrow separates the reactants and the products. This arrow can be misleading tough, because reactions do not generally go in only one direction.

Reversible Reactions Most reactions are called reversible reactions. This means that these reactions can go both forwards and backwards simultaneously.

Take the reaction between nitrogen and hydrogen. N2 + 3H2  2NH3. From this equation we know that nitrogen and hydrogen react to form ammonium. What we don’t often talk about is the reverse reaction that is happening. At the same time nitrogen and hydrogen are reacting to form ammonium, ammonium is breaking down to form nitrogen and hydrogen.

Reversible Reactions Imagine two children playing. One child is putting toys into a box while the other is taking them out. If the two kids are moving at the same pace, the number of toys in the box will remain the same, only the specific toys will change. The process could go on forever. Same idea.

Chemical Equilibrium Chemical equilibrium occurs when the rate of the forward reaction (in our example nitrogen and hydrogen producing ammonium) is equal to the rate of the reverse reaction (ammonium breaking down to form nitrogen and hydrogen).

La Chatelier’s Principle La Chatelier’s Principle states that if stress is applied to a system that is already in equilibrium, the system will shift to relieve that stress. Suppose we let our example reaction proceed and it reaches chemical equilibrium. Then we add several moles of ammonium to the situation. The system will break down the extra ammonium until a new equilibrium is reached to accommodate for our changes.

Factors affecting Equilibrium Some factors that would affect the chemical equilibrium include a change in concentration of a substance, change in volume, or change in temperature. The effects of these changes depends on the specific reaction.

Questions for Discussion Describe what happens during chemical equilibrium. Do reactions stop happening? If you made a graph of concentration vs. time, what aspect of the graph would you look at to determine the rate?

Answers The forward and reverse reactions happen at the same rates. There are still reactions happening. They do not stop. To determine the rate from any graph, you would look at the slope of the line.