Investigating the effect of changing the concentration of an acid on the rate of diffusion using agar jelly

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Investigating the effect of changing the concentration of an acid on the rate of diffusion using agar jelly - Plan

Aim

  • To find out how changing the concentration of an acid will quantitatively affect the rate of diffusion in agar

Introduction

Background Theory

The reaction in the experiment will be between two reactants- sodium hydroxide (NaOH) mixed into the agar jelly, and hydrochloric acid (HCl) in which the agar segments will be immersed, in each experiment. The acid and alkali react to form a metal salt- sodium chloride (NaCl), as well as water and hydrogen gas, which will be seen as effervescence (bubbles forming and given off). The word and symbol equations below show the reaction more clearly:

sodium hydroxide + hydrochloric acid → sodium chloride + water + hydrogen

The reaction is a neutralisation reaction, because the alkaline agar is being neutralised (i.e. its pH is being brought down to 7- the pH of pure water). An indicator has been incorporated into the agar- phenolphthalein, so I can track the neutralisation (and therefore diffusion) process; phenolphthalein is pink in alkalis, and turns colourless when neutral or acidic. In the experiment I will time how long it takes for the agar segment to turn colourless i.e. to be neutralised. I will then calculate the rate of diffusion from this.

The reaction throughout the agar segments will depend on the collisions between the reactant molecules. The reacting molecules in every reaction must collide with enough energy to react, in order for anything to actually happen. If they do hit each other with enough energy to break their current bonds and form new ones, this is called a successful (or fruitful) collision, and the reaction will continue. If they do not collide with enough energy, this is called an unsuccessful or unfruitful collision and the different molecules just bounce off each other. The image to the right (from ) shows what happens in both types of reaction.

Factors that affect the rate of diffusion

Firstly, the size of the agar piece itself will affect how quickly the acid will diffuse to the centre, near the end of the reaction. If it is bigger i.e. collected with a large cork borer, it will take longer for the acid to reach the centre of the agar piece, to neutralise the last part. The way in which the acid diffuses is called capillary action- the HCl molecules move through the lattice structure of the agar from the outside- where there is an acid/agar surface, to the centre. The net movement of the acid molecules is from the outside of the agar to the centre, but the HCl molecules move in all directions, colliding with the NaOH molecules in the porous agar, continuing the neutralisation process.

The surface area of the agar also affects the rate of diffusion, because there are more pores for the HCl molecules to get into the agar and then move through.

The temperature of the agar can alter the diffusion rate, as at a higher temperature the agar will expand, so the “pores” will be bigger, and more HCl can be absorbed.

The concentration of the hydrochloric acid will affect the rate of diffusion as well, because the concentration gradient of H+ ions between the acid and the agar piece is steep:

The definition of diffusion is “the passive movement of particles of a substance from a region of high concentration (in this case the acid) to an area of low concentration (the agar).” The steeper the concentration gradient, the greater the “driving force” for the net movement of the H+ ions (in the HCl molecules), so the diffusion will happen more quickly.

My chosen variable

I have chosen to investigate how the concentration of the hydrochloric acid alters the rate of diffusion, because it seems the easiest to measure.

In the preliminary experiment, I found that samples taken using a large cork borer took significantly longer to completely neutralise than samples taken with a straw- using 1.0M hydrochloric acid, it took 175 seconds for a small piece to turn completely colourless, while it took a large piece (taken with a cork borer) a very long 707 seconds to turn colourless. The size and surface area of the large agar piece were larger than that of the small piece, so I will not investigate the effects of size or surface area, as the experiments using large pieces will take too long.

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The temperature may be difficult to measure, as the temperature of the acid if heated may be different from the temperature of the agar piece, and at high temperature the neutralisation reaction may be too quick to record.

Prediction

I predict that as the concentration of the acid is doubled, the rate of diffusion will double. I believe this because if the concentration is doubled, there are twice as many HCl molecules per unit volume, so the concentration gradient is doubled (twice as steep). The “driving force” is then twice as strong so diffusion should ...

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