Diamonds are forever, aren’t they? Episode 23 recaps reaction rates and rate laws.
Diamonds are forever, aren’t they? Episode 23 recaps reaction rates and rate laws. We start with the definition of a reaction rate using our stoichiometry sandwiches as an analogy (1:30). Focusing on reactants or products, we can measure the rate of appearance or disappearance (2:20). Using collisions, we discuss the factors that influence the rate of a reaction (3:00): concentration of reactants, the temperature, the surface area, or using a catalyst. Zooming in on the relationship between reaction rate and concentration, the episode recaps rate laws. It describes the setup of a rate law using the proportionality constant and reaction orders (5:19) for a general equation as well as for the formation of hydrogen iodide (6:53).
Question:(8:22) How will the reaction rate change as the reaction progresses?
A. decrease B. increase C. stay constant
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Reaction Rates and Rate Law.
Lets Zoom Out:
Unit 5 - Kinetics
Topic 5.1 and 5.2 - Reaction Rates and Introduction to Rate Law
Big idea - Transformations
The seventh James Bond movie with Sean Connery as James Bond is titled “Diamonds are forever”. As a chemist, though, I have to disagree! Diamonds will, over time, slowly degrade to the more stable carbon allotrope: graphite. WHAT? No worries if you have your pocket money invested in diamonds: It will not happen in your lifetime. Or your children's. Or your grandchildrens… This is an example of a reaction with a very, very slow reaction rate. Other reactions are quite the opposite: super fast. As chemists, we can analyse and influence the rate of reaction. Therefore, today’s episode will recap the reaction rates and rate laws.
Let’s zoom in:
The reaction rate is defined as the rate at which an amount of reactant is converted to products per unit of time. U-huh. What does that mean? Let’s use your sandwich analogy again: 2 bread + 3 ham + 1 cheese yields 1 sandwich. To determine how fast you are preparing sandwiches for the local football team, you can measure how many slices of bread you still have every 10 seconds. Your speed would then be the number of slices you’ve “converted” to sandwiches over the time: You start with 50 slices of bread. After 10 seconds you have 46 slices - your rate is 4 slices/10 seconds. In chemistry, we take similar measurements. We measure the concentration of a substance per unit of time. The change in concentration over the change in time is the reaction rate.
You could either measure the rate of disappearance or the rate of appearance. These are related to one another by the reaction’s stoichiometry: You could measure the slices of bread and determine that the rate at which it disappears is 4 slices/10 seconds. Or you count how many sandwiches you build: 2 slices of bread are needed for 1 sandwich. Therefore, 4 slices of bread build 2 sandwiches in 10 seconds. This is your rate of appearance.The same goes for the other ingredients: If 2 slices of bread are disappearing for each sandwich, using the balanced reaction, so are 3 slices of ham and 1 slice of cheese.
Let’s switch to Chemistry and talk a bit more about how we can speed up the reaction. The factors are: increasing the concentration of reactants, the temperature, the surface area, or using a catalyst. We will focus on the first three and discuss catalysts in episode 24.
These factors can be explained with collisions. Our particles have to collide with the right amount of energy and the right orientation for a chemical reaction to happen. Let’s use a crowded train station as an analogy: The more people there are in the train station, the more collisions are happening. In chemistry terms: the higher the concentration of reactant, the faster the reaction rate. Increasing the temperature also increases the speed of our travelers: They want to get out of the train station and into the air conditioned train. Therefore, they are moving faster and our collisions become more frequent and more violent. In chemistry, it is important to have collisions with a minimum amount of energy. Therefore, increasing the temperature will also increase the rate of reaction. Most collisions will happen in the great hall of the train station: When we have a greater surface area, there are more chances for collisions to happen. In chemistry, we can speed up a reaction by pulverizing our reactants to increase the surface area and provide more opportunities for collisions between particles.
Let’s take a closer look at the rate of reaction and the concentration: We can write a rate law for our reaction. The rate law shows how the rate depends on the reactant concentrations. Let’s use a generic equation: a moles of A react with b moles of B to form C moles of C. A general form of the rate law for this reaction is: the rate is equal to k times the concentration of reactant A raised to the power of m times the concentration of reactant B raised to the power of n. What do all these letters mean? k - and careful, it is a lowercase k - is the proportionality constant. It is a constant that depends on the temperature and therefore adds that component to our reaction. m and n, are reaction orders and therefore are verbalized as “the reaction is mth order with respect to the concentration of A”. It shows us how the rate is affected by the reactants' concentration. If, for example, the reaction is 1st order with respect to A, doubling the concentration of A will also double the rate of the reaction. If a reaction is 2nd order, doubling the concentration of A will quadruple the rate of reaction, because 2 squared is 4. IMPORTANT: m and n, our reaction orders, have to be determined experimentally by comparing the initial rates of a reaction. They are NOT related to the coefficients!
Let’s use a “real” example: one mole of gaseous hydrogen reacts with one mole of gaseous iodine to form 2 moles of gaseous hydrogen iodide. The experimental data shows us that the reaction is 1st order with respect to hydrogen and 1st order with respect to iodine. Therefore, the rate law is: rate equals k times concentration of hydrogen raised to the power of 1 times concentration of iodine raised to the power of 1. It tells us that if I double the concentration of hydrogen at constant concentration of iodine we no longer have 1 mole to the first, but 2 moles to the first. Plugging that in, we can see that the rate of the reaction doubles. What if double BOTH reactant concentrations? Then we have 2 to the 1st times 2 to the 1st - our rate quadruples!
To recap…
The reaction rate is defined as the rate at which an amount of reactant is converted to products per unit of time and expressed in change of concentration over change of time. The factors affecting the reaction rate are increasing the concentration of reactants, the temperature, the surface area, or using a catalyst. To communicate the reaction rates dependence on concentration, we can experimentally determine the rate law. The rate law shows the proportionality constant as well as the reaction orders.
Coming up next on the APsolute RecAP Chemistry Edition: reaction mechanism and rate law
Today’s Question of the day is about reaction rates.
Question: How will the reaction rate change as the reaction progresses?
A. decrease B. increase C. stay constant