Grades 9 and 10 - Chemistry - High School - Rates of Reaction — Flashcards

Many factors affect rates of chemical reactions - pressure of gasses, temperature, surface area of solids, concentration and if there is a catalyst. Anything that will change the probability of particles colliding or change the energy of the collisions will affect the rate of a reaction. This is the last of three high school Chemistry quizzes looking at these factors.

When investigating rates of reaction, it is necessary to make a series of measurements over a period of time, for example, how much hydrogen is produced during the reaction of an acid with zinc. The experiment should be repeated several times and, after discarding any anomalous results, the readings averaged and plotted on a scatter graph with time along the horizontal axis. The line of best fit will usually be a curve, with the steepest gradient at the start indicating the fastest rate of reaction. Where the curve is horizontal, is shows that the reaction had finished. The conditions of the experiment can then be changed and the whole process repeated. Plotting the results on the same graph, using different colors, gives a quick and easy visual interpretation, from which you can write your conclusion.

When working out rates of reaction mathematically, as higher tier candidates are more likely to do, dividing the amount of reactant used (or product formed) by the time taken gives a valid rate. This is effectively the same as working out the gradient of a graph. If 15 cm3 of carbon dioxide were released in the first 20 seconds of a reaction, the rate would be 0.75 cm3/s. Later in the reaction, it may take 45 seconds to produce 10 cm3 of carbon dioxide, in which case the rate would have dropped to 0.22 cm3/s. This slowing down of the rate occurs because the concentration of one or both of the reactants changes during the reaction. Professional scientists usually refer to the initial rate of reaction, in other words the rate at the very start.

How would a decrease in temperature affect the rate of a reaction? What about surface area? Have a go at this quiz and test your knowledge of the factors which affect rates of reaction, such as temperature, surface area or pressure.

Collisions of particles cause reactions. At all temperatures above absolute zero, particles are in motion (the kinetic theory). Therefore, when the particles of two reacting substances are mixed, it is very likely that they will collide with each other. Solids mixed together don't normally react as their particles are fixed in place, however, introduce a gas or liquid and collisions will occur because the particles are free to move around.

If the particles are slow moving, they have less energy, collisions between them will therefore also involve less energy than if they were moving quickly. If particles collide head-on, that will involve more energy than if one collides with the other from behind or if they collide from the side or at an angle. This means that not all collisions between particles will lead to a chemical reaction. If there are high concentrations of particles, there will be more collisions per second which means there will be a greater likelihood of collisions that result in a reaction.

When you studied exothermic and endothermic reactions and energy level diagrams, you will have learned that for a chemical reaction to happen, bonds between atoms must first be broken and then they can re-form. There is also a minimum level of energy required for a reaction, the activation energy. This represents the energy that is required to break the bonds of the original reactants when they collide. Putting together the two ideas of collisions between particles and bond breaking/making gives an understanding of rates of reaction.

The rate of a reaction is a measure of how quickly a chemical reaction progresses. Some reactions can be very slow, for example rusting can take weeks or even years, whilst others, such as neutralization or an explosion are over very rapidly. The rate of reaction is very important in industrial situations; take explosions for instance - these are incredibly fast reactions. An explosive is a solid that turns into a huge amount of very hot gas VERY quickly! This makes them useful for several purposes - most notably in warfare - though a more peaceful use is in the quarrying for resources.

At a quarry, they drill holes with a diameter of a few centimeters down into the rocks. They then drop sticks of explosives into the holes along with a detonator. The space above the explosives is then filled with something like sand or fine gravel. When the explosive is ignited (this is an increase in energy, one of the factors which affects the rate of a reaction), it vapourises in a fraction of a second, turning into a huge quantity of gas. The hole that was drilled is far too narrow for the gas to escape quickly. The trapped gas creates a huge pressure since it is confined in such a small volume and this pressure is enough to fracture the rocks, sending them flying into the air. You may have seen video footage of the effect of this fast reaction during your chemistry lessons. Before explosives, quarrying was a much slower and less spectacular process since the quarry workers used hammers and wedges to split the rocks apart. Where there were no natural cracks, they had to make their own holes by hand before they could drive the wedges into the rocks.

There are many different ways that you can follow the rate of reaction, you will have used some of them and seen others demonstrated during your high school studies. It all comes down to finding ways of measuring how fast the reactants are used up or how quickly the products are made. If one of the products is a gas that can safely be released into the air, life is easy! You can simply let it escape from the reaction vessel and measure how the mass decreases as the reaction proceeds. A better method is to collect the gas and measure how much is produced every few seconds. In a reaction between liquids, if an opaque precipitate or suspension is produced, less and less light will be transmitted through the liquids as the reaction progresses. You could stand the reaction vessel on a piece of paper with a cross on it and time how long it takes for the cross to be obscured. For a more technical solution, use data logging and a light sensor to measure how much light is transmitted through the mixture over the course of the reaction.

When measuring the rate of a reaction, what you can't do is just measure a factor at the start or at the end. This does not work, you need to measure something continuously during the reaction, in other words, your measurements must include time. It is best to start timing at the moment that the reactants are mixed and to continue measuring for as much of the reaction as possible. You will then be able to collect sufficient data for it to be both reliable and valid, from which you can plot one or more graphs to help you to draw your conclusions.