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In this Section we explore two aspects of the rate or speed of a chemical reaction. First, in Experiment 1, we will look at how temperature affects the rate of a reaction. Next, in Experiment 2, we will look at the effect of concentration of reactants on the rate of the same reaction. The reaction we will be interested in is the formation of the black and white imageon the print, in the developer. Later, we will study the rate of other related photographic reactions.

This experiment will focus on answering the question: What role does temperature play in the development of a photographic print? We can answer this question by actually developing a print in solutions with different temperatures.

Next, expose three pieces of photographic paper for identical time intervals. Adjust the temperature of the developer in the inner beaker to exactly 25ºC by adding warm water or ice to the outer beaker. Develop, stop, fix and wash one contact print using your optimum times from Experiment 1. Record the time required for the appearance of the image on the print while it is in the developer solution.

Adjust the temperature of the developer solution to 15°C by adding ice to the outer beaker. Develop, stop, fix, and wash another print using the same times as above. Be certain the temperature does not vary by more than 1°C during the developing stage. Finally, adjust the temperature of the developer to 35°C. Repeat the development process as above.

What is the effect of the temperature change on the time for image formation and on the finished print? Explain. Chemists have a rule of thumb which states that "for every temperature increase of 10°C, the reaction rate doubles." Can you think of some way to test this rule on the reactions involved in development? In exposure?

The rates of most chemical reactions increase if the strength or concentration of the reactants is increased. It is very common to find that the rate of a chemical reaction is proportional to the concentration of the reactants, that is, if the concentration of one reactant is doubled, the reaction takes place twice as rapidly. The reason for this is that as the concentration is increased, the number of encounters per second between reactant molecules also increases. The purpose of this experiment is to determine how the rate of the development process depends on the concentration or strength of different components of the developer solutions. The results will help clarify the nature of the chemical reactions taking place during the development of the print.

In order to measure the effect of reactant concentration on the rate of the development process, it is necessary to have some quantitative measure of reaction rate. This is not very difficult. The faster the reaction, the sooner the print is ready. The development reaction involves the formation of silver metal on the paper surface, so the finished print has just the right amount of metallic silver on the paper. By measuring the time it takes to achieve the same darkness or density you have a measure of the reaction rate. Since the faster reaction, or greater reaction rate, corresponds to the shorter time, the rate is proportional to 1 divided by the time. We will refer to this as 1/t and use seconds as the unit of time. For example, if it takes 12 seconds to reach the desired darkness, the rate will be given by 1/12 sec = 0.83 sec^-1.

Developer Solution	Na₂SO₃ (mL)	NaBr (mL)	H₂O (mL)	metol (g)	NaCO₃ (mL)
1	100	10	70	1.20	20
2	100	10	70	0.80	20
3	100	10	70	0.40	20
4	100	10	70	0.20	20

Use the solution containing 1.20 g metol and the conditions you used to obtain an ideal print in the earlier sections. Measure and record the time required in the developer beaker to get a good print. (This is your standard print.) Repeat this step with new paper, leaving the print in the developer until it has the same darkness as the standard print. Measure and record this time. Be sure to stop, fix, and wash all prints. Calculate and record the average time for this developer solution.

Repeat this procedure with the other developer solutions, doing each in duplicate.

Finally, prepare another developer solution but reduce the concentration of another component in the developer (NaBr, Na₂SO₃, or Na₂CO₃) by a factor of two. (Be sure to increase the volume of added water to compensate for any decrease in solution volume and equilibrate to the same temperature as the previous solutions.) Make two prints using this developer, recording the times required for a good print, as above.

Calculate the value of 1/t(ave) for each solution and make an accurate graph of 1/t(ave) vs. grams metol in the developer. Use graph paper. See your instructor if you have a question about doing this.

Does the rate of development depend on the concentration of metol? Is it proportional to the concentration? What about the other chemical for which you varied the concentration? How does changing its concentration affect the reaction rate? What do the results of these experiments tell us about the molecules which are reacting in the photographic emulsion?