Determine whether the rate of the reaction changes as the concentration is varied, and find out what effect changing the type of acid used (strong or weak) will have on the activation energy.

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THE RATES OF REACTION OF METALS WITH ACID

AIM OF EXPERIMENT 1

To determine whether the rate of the reaction changes as the concentration is varied.

AIM OF EXPERIMENT 2

To find out what effect changing the type of acid used (strong or weak) will have on the activation energy.

 

SCIENTIFIC KNOWLEDGE

Factors, which have an effect on the rate of a reaction

        Temperature – when the temperature is increased the rate of the reaction will decrease. This is because at higher temperatures molecules have more energy to move and collide with enough energy to react, so the reaction will be quicker.

        Concentration- if the concentration is increased then the rate will decrease. This is because there are more molecules within the same volume so they are more likely to collide and react, so the reaction will happen more quickly.

        Surface area – When the surface area of a substance is increased the rate of that reaction will decrease as the molecules of the reacting substance will have a greater surface to react on and so more molecules can collide at the same time, as a result the reaction will be quicker.

        Volume – if the volume is decreased then the rate will decrease this is because there are the same amount of molecules within a smaller space so when the molecules move they are more likely to collide and react, resulting in a faster reaction.

When investigating one of these factors all others must remain constant in order to obtain accurate results.

Activation Energy

This is the minimum amount of kinetic (movement) energy required by molecules in order to overcome the repulsion between them and start reacting. The aim of experiment 2 is to find out the effect of using different types of acid (strong or weak) on the activation energy. When the concentration is increased the rate of a reaction decreases this is because, there are more molecules to reach the required energy and start reacting, so the reaction will be faster.

When the molecules that reach or exceed the activation energy collide and react the rate of the reaction at temperature T for a mole of these molecules can be found from:

ln(rate of reaction) = ln(collision rate) – E   (1/T)

This relationship can be rewritten in a form that is more useful and is called the Arrhenius equation:

ln k = constant – E   (1/T)

Where k is the rate constant of the reaction

 R is the gas constant, 8.31 J k   mol        

 E     is the activation energy of the reaction, in J mol  

T is the temperature in Kelvins.

This is a typical graph plotted using the Arrhenius equation:

The effect of concentration on the rate of a reaction

When two substances A and B react, it is possible to follow the reaction by observing how quickly substance A is used up. For a reaction in solution, the concentration of A could be measured at various times, to see how it changes. In this case the measure of the rate of the reaction would be the rate of change of concentration of A, symbol (r).

The rate of change of concentration of substance A can be found by experiment to follow the expression:

r    = k[A]  [B]  [C]

This is called the rate equation. Substance C is included in this general expression, as well as A and B, because rate equations sometimes include concentrations of substances that do not appear in the equation for the reaction.

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The index ‘a’ is called the order of the reaction in respect to the reactant A; ‘b’ is the order of the reaction with respect to B, and so on. The sum of the indices is called the overall order of the reaction. The orders of the reaction are experimental quantities they cannot be deduced from the chemical equation for the reaction.

The possible orders of a reaction

Consider the rate equation in the form:

r    = k[A]  

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