Chemistry: Unit F322: Chains, Energy and Resources



Chemistry: Unit F322: Chains, Energy and Resources

2. – Rates of Reaction

There are two ways of measuring a rate of reaction:

a) Measuring the rate at which product is produced

b) Measuring the rate at which reactant is used up

This graph shows what happens to the concentrations of products and reagents (reactants) over the course of a chemical reaction.

This graph shows the volume of CO2 produced in a reaction between HCl and CaCO3. The curve has three sections:

Collision Theory

To react, particles must collide in the right orientation, and have enough energy to break bonds (activation energy). If there are more frequent successful collisions, it results in a faster rate of reaction.

There are 4 factors which affect the rate of reaction:

1. Surface Area

2. Temperature

3. Concentration/Pressure

4. Catalyst

1. The effect of surface area on rate of reaction

The greater the surface area, the faster the rate of reaction.

The steeper curve of the reactant with the larger surface area, the powder, shows that the rate of reaction was quicker. This is because the surface area available for reaction is greater, so there are more frequent successful collisions and so the rate of reaction is increased.

2. The effect of temperature on rate of reaction

The greater the temperature, the faster the rate of reaction.

As the temperature increases, the kinetic energy of the molecules increases, The Boltzmann distribution curve is displaced to the right with a lower peak, because the average energy of the particles has increased. The total number of particles in each reaction is the same; this is represented by the area beneath the curve.

A higher temperature increases the rate of because the molecules have more kinetic energy and so move faster. The increased kinetic energy means that there are more frequent successful collisions, because a greater proportion of molecules exceed the activation energy. This increases the rate of reaction.

3. The effect of concentration/pressure on rate of reaction

The greater the concentration/pressure, the faster the rate of reaction.

If the concentration of a reactant solution or the pressure of a gas reactant is increased, there are more molecules present per volume. This means more collisions take place, and more are successful, as a greater number of them exceed the activation energy. This means that rate of reaction is increased. The shape of the curve is the same, because the proportion of molecules exceeding the activation energy is the same, but because at a higher pressure/concentration, the number of molecules in greater, and so more exceed the activation energy.

4. The effect of catalysts on rate of reaction

With a catalyst, the rate of reaction is greater.

A catalyst is a substance which increases the rate of reaction without being involved in the reaction itself. Catalysts speed up the rate of reaction by providing an alternative reaction route with lower activation energy.

Enthalpy Profile Diagrams

In an exothermic reaction, energy is given out to the environment, shown by the negative heat enthalpy and the downwards arrow. This means that the reactants have more energy than the products. The activation energy is the minimum energy required for the reaction to occur.

The blue line shows how the enthalpy profile diagram is affected by a catalyst: the energy barrier is reduced and so the rate of reaction is increased because a greater number of particles have energy that is above the activation level.

In an endothermic reaction, energy is taken in from the environment, shown by the positive heat enthalpy and the upwards arrow. This means that the products have more energy than the reactants. The activation energy is the minimum energy required for the reaction to occur.

The blue line shows how the enthalpy profile diagram is affected by a catalyst: the energy barrier is reduced and so the rate of reaction is increased because a greater number of particles have energy that is above the activation level.

Environmental Catalytic Reactions

Homogeneous Catalysts are ones which are in the same physical state as the reactant. Heterogeneous Catalysts are ones which are in a different physical state to the reactant.

1. Ozone Depletion

CFCs react with ozone molecules in a free radical substitution reaction. The catalyst in the reaction is Cl. free radical. It is a homogeneous catalyst as both the chlorine and the ozone are gaseous.

CFCl3(g) Cl(g) + CFCl2 (g)

Cl(g) + O3 (g) ClO (g) + O2 (g)

ClO (g) + O (g) Cl (g) + O2 (g)

2. Air pollution and the Catalytic Converter

Emissions from cars are some of the biggest air pollutants. The three main polluting gases are:

a) Carbon Monoxide

b) Oxides of Nitrogen

c) Unburnt Hydrocarbons (Volatile Organic Compounds)

Catalytic Converters

A catalytic converter is used to decrease the harmful emissions form an internal combustion engine. A three way catalytic converter uses three metals as heterogeneous catalysts, as they are in a different physical state to the reactants. These metals are platinum, rhodium and palladium. The catalyst provides a surface on which the reaction takes place.

• The CO and NO gas molecules diffuse over the catalytic surface. Some molecules are held on to the metal surface by adsorption.

• Temporary bonds are formed between the catalytic surface and the gas molecules. These bonds hold the gases in the correct orientation so that they can react together.

• After the reaction, the CO2 and N2 products are desorbed from the catalytic surface and diffuse away.

Catalysts and Sustainability

← Catalysts lower temperature requirements because they offer an alternative route with lower activation energy. Less energy is required, so less fossil fuels are burned, less CO2 produced.

← Selective catalysts mean fewer waste products are produced, because only the desired product is formed. This increases atom economy and promotes addition reactions.

← Enzymes are proteins with catalytic properties. They generate specific products and operate close to room temperature and pressure.

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At the start of the reaction, there is a high concentration of reactants. Over the course of the reaction, this concentration decreases as more reactant is turned into product.

At the start of the reaction, there is a low concentration of products. Over the course of the reaction, this concentration increases as more product is formed by reactants

Steep gradient shows that the rate off reaction is fast. This is because there is a high concentration of reactants, which means that successful collisions will be more frequent, and so rate of reaction increases.

The lower gradient means that the rate of reaction is slower. There is now a lower concentration of reactant, because some has been turned into product, so there are fewer successful collisions and therefore a lower rate of reaction.

The gradient here is zero, indicating that the reaction has stopped. All of the reactant has been used up, as it has all been turned into product.

With a catalyst, the activation energy is lower. This means that more of the particles will exceed the activation energy, so there will be more successful collisions, and a faster rate of reaction.

Without a catalyst, only the particles in the yellow area would exceed the activation energy. Now, with a catalyst, all of the particles in the red area will also exceed the new, lower activation energy, so the rate of reaction will be increased.

Reaction Pathway

Enthalpy

EA

-ve

H

Products

Reactants

H

Reactants

EA

Enthalpy

Reaction Pathway

Products

+ve

2CO2

7CO2 + 8H2O

N + 2CO2

2CO + O2

C7H16 + 11O2

2NO + 2CO

Unburnt hydrocarbons are released in car exhausts. They react with nitrogen oxides to form low level ozone, which causes breathing difficulties.

Oxides of Nitrogen are formed under high temperatures in the car engine, when atmospheric nitrogen is oxidised to form nitrogen oxides. They cause the formation of low level ozone and reacts with water to form nitric acid, which falls as acid rain.

Carbon Monoxide is a poisonous gas, formed by the incomplete combustion of hydrocarbons. It binds to the haemoglobin in red blood cells, inhibiting the ability to carry oxygen.

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