ICT skills. Simulating chemical reactions

INTERPRETING CHEMICAL EQUATIONS

On this first page, we show you two tools that will help you practice balancing chemical reactions and understand the proportion of the reacting substances given the amount of products and reactants at the end of the reaction. In future years, this will help us understand the concept of the limiting reactant.

BALANCING CHEMICAL EQUATIONS

We will use an application available on the Anaya website. The application has two parts: Introduction and Play. • The application lets you balance chemical equations in a visual and intuitive way. It continually updates the number of atoms of each element in the reactants and products. To begin, use the Introduction part: activate the tool in the upper right that lets you visualise the balance of elements. • Note that the number of molecules, or fundamental units, must be a whole number in this application. • Use Play to raise the level of difficulty of the equations to balance. If you do not succeed with a problem, the application will give you the option to guess why and make a new attempt, as shown in the picture:

Once we have worked with the application, we must take a step beyond and see what happens in situations where the fundamental units of the reactants are not in stoichiometric proportions. We will now use an application that complements the previous one. You can download it at https://phet.colorado.edu/en/simulation/ reactants-products-and-leftovers. It begins with an example of 2:1 stoichiometry: making a sandwich requires 2 slices of bread and 1 slice of cheese. It seems simple but this is the idea we will use to predict which reactant will be leftover.

REACTANTS, PRODUCTS AND LEFTOVERS

Understand, think, search…

1 Explain why the the coefficient of the simple or elemental substances should be adjusted last. 2 In the same way that when we have four slices of bread and just one of cheese, the cheese ‘limits’ how many sandwiches we can prepare (just one). In stoichiometry we use the concept of the ‘limiting reactant’, which we will study in more depth in future years. If we take the example of the formation of ammonia and we have two mole of nitrogen (N2) and three of (H2), which is the limiting reactant?

SIMULATING REACTION RATES

In this unit we have studied the factors that affect the rate of chemical reactions. We will use a chemical reaction simulator to see the effects of temperature, reactant concentration, and a catalyst. Now, we will use an application available at: https://phet.colorado.edu/es/simulation/legacy/ reactions-and-rates Download and execute it with Java.

The simulation has three parts: Single collision, Many Collisions, and Rate Experiments. In all three cases the reactions are reversible, that is, once the products form, they interact and produce the original reactants. There are many such chemical reactions. We say that they have reached chemical equilibrium.

SINGLE COLLISION

In this part of the simulator, you can see the effects of the orientation of the two reactant molecules and the collision energy. To see how the relative orientation makes a collision effective or not, you must select Angled shot from the Launcher Options (top right corner) and launch the reactant molecule with enough energy.

The effect of the energy of collision is visible by making shots at greater or lesser energy from the launcher. Note that if the shot energy is less than the activation energy you will not get an effective collision. To visualise the energy diagram of the reaction you must activate the Energy View option. RATE EXPERIMENTS

Here the application lets you change the number of reactant molecules. There are options for controlling the experiment on the right: • Select a reaction. We can use one of the given reactions or design our own. The energy values of the reactants and products are editable in the energy diagram, so we may have endothermic or exothermic reactions. The activation energy is also editable, so we can simulate a catalyst. • Number of molecules at the start. Let us change the number of reactant and product molecules at the start of the reaction. • Initial temperature. • Chart options. It is useful to select the bar chart to have an idea of the amounts of reactant and product and the strip chart to see how the reaction progresses.

Understand, think, search…

1 Do various single collision experiments, changing the energy and launch angle. Draw conclusions about the probability of an effective collision and the factors on which it depends. 2 Do a rate experiment with the first pre-set chemical reaction but changing the number of molecules of both reactants.

Activate the stopwatch and the strip chart to measure the time the reaction takes. Put your results in a table and draw conclusions about the importance of the concentration of reactants. Why do we call chemical equilibrium a dynamic equilibrium? 3 Do the same experiment but this time vary the temperature. Draw conclusions.