Fast and Slow Chemistry Chapter 15

Fast and Slow Chemistry Read page 247 What would happen if this was a slow reaction???

Chemical Energy – What is chemical energy? All substances have chemical energy. The chemical energy of a substance is the sum of its potential energy (store energy) and kinetic energy (energy of movement).

What is chemical energy? These energies result from such things as: ◦ Attractions between electrons and protons ◦ Repulsions between nuclei ◦ Repulsions between electrons ◦ Movement of electrons ◦ Vibrations of and rotations around bonds The chemical energy of a substance is sometimes called its heat content or enthalpy. It is given the symbol H

Energy changes during chemical reactions During a chemical reaction, the atoms in the reactants are arranged into products with different chemical energies.

Energy changes during chemical reactions – Exothermic The total chemical energy of the products is less than the energy of the reactants. Since energy is never lost, the difference in energy between reactants and products is released into the environment. It is often released as heat energy.

Energy changes during chemical reactions – Endothermic The chemical energy of the products is greater than the energy of the reactants. Energy must be absorbed from the environment around the reactants in order for the reaction to occur.

Energy Changes The energy released or absorbed during a chemical reaction is called the heat of reaction. Since the heat of reaction is equal to the difference in enthalpy between the products and the reactants, it is given the symbol ∆H, where: ◦ ∆H = H(products) – H(reactants)

∆H For exothermic reactions, ∆H will be negative For endothermic reactions, ∆H will be positive This depends on whether H(products) is greater than or less than H(reactants). Most reactions we encounter are exothermic.

Thermochemical Equations Thermochemical equations show the energy released or absorbed during a chemical reaction. Energy is measured in Joules (J) or kilojoules (kJ). 6CO 2 (g)+6H 2 O(l) → C 6 H 12 O 6 (aq)+6O 2 (g); ∆H=+2803 kJ mol -1 C 6 H 12 O 6 (aq)+6O 2 (g) → 6CO 2 (g)+6H 2 O(l); ∆H=-2803 kJ mol -1

Activation Energies CH 4 (g)+2O 2 → CO 2 (g)+2H 2 O(g); ∆H=-890 kJ mol -1 This is an exothermic reaction. The energy of the reactants is higher than the energy of the products. Why doesn’t natural gas burst immediately into flame and release energy when it comes into contact with air? To start a gas oven, why must we use a match or a spark?

Activation Energies Well what happens to chemical bonds during a reaction? The bonds between the atoms in reactants must first be broken. ◦ For this to occur energy must be absorbed The new bonds form as the products are created ◦ Energy is released as this happens

Activation Energy The energy required to break the bonds of reactants is called the activation energy. A diagram showing this is called an energy profile.

Your Turn Pg 250 Questions 1 and 2

Making Reactions Go Faster The rate at which chemical reactions occur is an important consideration for industrial chemists and chemical engineers. Some reactions take less than seconds, while others take years. Considerable effort is directed towards maximising reaction rates in industry.

Collision Theory For a chemical reaction to occur, the particles involved must collide with each other with sufficient energy to overcome the activation energy ‘barrier’ The rate of reaction can depend on the number of collisions as well as the energy of the collisions being greater than the activation energy.

Factors that affect rates There are four main ways in which reaction rates can be increases: ◦ Increasing the surface area of solids ◦ Increasing the concentration of reactants in solution (or pressure of gases) ◦ Increasing the temperature ◦ Adding a catalyst Explain to me how these factors can increase the rate of reaction?

Extending Collision Theory

Catalyst Many reactions occur more rapidly in the presence of particular elements or compounds. These substances, known as catalysts, are not consumed during the reactions and therefore do not appear as either reactants or products in reaction equations.

Catalysts Many catalysts have been discovered by simple trial and error.

Catalysts There are two types of catalysts ◦ Homogenous catalysts – these are in the same state as the reactants and products ◦ Heterogeneous catalysts – these are in different states from the reactants. Chemists prefer to use Heterogeneous catalysts as they are more easily separated from the products of a reaction

How do catalysts work Particles tend to adsorb to the surface of the catalyst. Adsorption distorts bonds in the reactants allowing the reaction to proceed more easily than it would if the catalyst was absent. Essentially a catalyst lowers the activation energy required to break the bonds of the reactants. The relative energies of the reactants and products are unaffected by the presence of the catalyst. This means ∆H is not changed.

How do catalysts work

Over to you Read the Extension task on page 257 Over to you Page 258 Question 3 and 4