In this lesson, we will observe a reaction where bonds are formed called a synthesis reaction. We will again focus on the transfer of energy that occurs by modeling the process of bond formation.

Before we observe a synthesis reaction, let's review the transfer of energy that occurred in lesson 3 when an ionic compound split apart in water.

In that simulation we saw that a water molecule slowed down and seemed to lose kinetic energy when it collided with the ionic compound KI. This collision caused the ionic compound KI to split into a K⁺ cation and an I⁻ anion.

If we put together two concepts from the homework and lesson 3, we can come to a conclusion about where the kinetic energy from the water went. 

• Potential energy can be converted to kinetic energy, and kinetic energy can be converted back to potential energy.
• Energy can be transferred from the system to the surroundings, or from the surroundings into the system.

Recall that the water temperature at the beginning of our simulation experiment contained a certain amount of kinetic energy. That kinetic energy decreased the moment the water collided with the ionic compound and it subsequently broke apart. That kinetic energy must have transferred into the system.

In Lesson 2: Breaking Ionic Bonds Lab Activity, we recorded the temperature of water as different ionic solids were added to the beaker. We observed a temperature decrease for each of the five ionic solids.

The ionic bonds of ionic solids (the system) are broken when placed in water (the surroundings) causing a decrease in the overall temperature of the water. The decrease in temperature is is due to the transfer of kinetic energy from the surroundings to the system in order to break the bonds of the ionic compound. Temperature decreasing when bonds are broken is an endothermic process.

Definitions:

Endothermic - An endothermic process absorbs energy from its surroundings, usually in the form of heat.

Exothermic - An exothermic process releases energy from the system to its surroundings, usually in the form of heat.

Your instructor will conduct an experiment using solid zinc and solid iodine to yield solid zinc iodide as shown in the chemical equation below. This type of reaction is known as a synthesis reaction because bonds are only being formed in the reaction, no bonds are broken.

Zn (s) + I₂ (s) → ZnI₂ (s)          

1. Zinc and iodine will be combined in a 1:1 molar ratio as indicated in the balanced equation above. Corresponding gram amounts are shown in the table below.
2. After weighing out appropriate amounts of zinc and iodine, the two solids will be ground together using a mortar and pestle (your instructor will demonstrate).
3. The contents will then be transferred to a test tube mounted to a ring stand inside the fume hood.
4. A surface temperature probe will then be adhered to the bottom of the test tube to record the initial temperature before any reaction occurs.
5. Exactly 1 mL of distilled water will be dispensed into the pulverized mixture of zinc (s) and iodine (s).
6. The greatest difference in temperature from the starting temperature (of the surroundings) must be recorded so the overall temperature change can be calculated.

Consider the following as you model the interaction occurring in trials 1 and 3 below:

• The drawing space is meant to represent a zoom-in of the test tube, so there is no need to illustrate the test tube itself in your model. Only illustrate the contents inside the test tube.
• Temperature that was measured was of the surroundings, not of the system itself.
• Temperature correlates directly with kinetic energy. If temperature increases, kinetic energy should also increase.
• Follow the law of conservation of energy: the total number of energy boxes (potential energy + kinetic energy) filled in the before portion of your model must match the total number of energy boxes filled in the after portion.