Fast and slow reactions Key knowledge Collision theory and factors that affect the rate of a reaction including activation energy Energy profile diagrams and the use of H notation

Chemical energy The chemical energy of a substance is the sum of its potential energy (stored energy) and kinetic energy (energy of movement) These energies result from such things as: Attractions between electrons and protons in atoms Repulsions between nuclei Repulsions between electrons Movement of electrons Vibrations of and rotations around bonds Chemical energy of a substance is sometimes called its heat content or enthalpy and is given the symbol H

Energy changes during chemical reactions Exothermic reactions Total chemical energy of the products less than the energy of reactants Since energy is never lost, the products release energy into the environment – often as heat E.g. combustion of petrol Endothermic reactions Chemical energy of the products is greater than the energy of reactants Energy must be absorbed from the environment around the reactants in order for the reaction to occur

H = H(products) – H(reactants) Heat of reaction The energy released or absorbed during a chemical reaction is called heat of reaction The heat of reaction is equal to the difference in enthalpy between products and reactants The heat of reaction is given H (delta H) H = H(products) – H(reactants) Exothermic reactions H(products) is less than H(reactants) H will be negative, H0 Endothermic reactions H(products) is greater than H(reactants) H will be positive, H0

Enthalpy

Energy profiles You need to recall what happens to chemical bonds during the course of a reaction The bonds between atoms in the reactants must first be broken. For this to occur, energy must be absorbed Then new bonds form as the products are created Energy required to break the bonds of reactants so that a reaction can proceed is called activation energy

Profile of an exothermic reaction

Profile of an endothermic reaction

Thermochemical equations Thermochemical equations show the energy released or absorbed during a chemical reaction Energy is measured in joules (J) or kilojoules (kJ). The H, has units J mol-1 or kJ mol-1 \ Energy corresponds to the mole amounts specified by the equation

H calculations

Energy calculations

The collision theory For chemical reaction to occur, particles involved must collide with each other with sufficient energy to overcome the activation energy ‘barrier’ to the reaction It makes sense the greater the number of collisions, the greater the rate of reaction

Factors that affect rates Four main ways Increasing the surface area of solids Increasing the concentration of reactants in solution (or pressure of gaseous reactants) Increasing the temperature Adding a catalyst

Increasing surface area In a solid, only those particles that are at the surface can be involved in the reaction. Crushing a solid into smaller parts means that more particles are present at the surface. As a consequence of the greater number of exposed particles, the frequency of collision between the particles and reactants particle increase, and so the reaction occurs more rapidly Example Sherbet

Increasing concentration More molecules or ions dissolved in solution occur faster if the concentration of the dissolved particles is increased. With more particles moving randomly in a given volume of solution, the frequency of collisions is increased.

Increasing the temperature As temperature increases, the average speed and average kinetic energy of the particles increases as well Hence, as temperature increases so does the rate of reaction The effect of temperature on reaction rate cannot be simply explained by the increased frequency of collisions A temperature increase of just 10C causes the rate of many reactions to double, but collisions have increased about one-fiftieth of the amount

Distribution of energies of particles at particular temperatures

The distribution of energies of particles at two temperatures The distribution of energies of particles at two temperatures. Raising the temperature increases the proportion of particles with higher energies.

The energies of particles at two temperatures The energies of particles at two temperatures. EA represents the activation energy for the reaction. The shaded area under the graph represents the proportion of particles with sufficient energy to react.

Catalyst Many reactions occur more rapidly in the presence of particular elements or compounds These substances are catalyst, they are not consumed during the reactions and therefore do not appear in reaction equations

Industrial catalyst uses

Types of catalysts Homogeneous catalysts in the same state as the reactants and products e.g. outer atmosphere, CFCs (Chlorine) turns ozone into oxygen gas Heterogeneous catalysts are in different states from the reactants and products e.g. solid catalyst in car exhaust systems

How a catalyst works Iron is a heterogeneous catalyst Nitrogen and hydrogen molecules come into contact with the catalyst The bonds in nitrogen and hydrogen are broken New bonds are created

Energy changes in the uncatalysed and the catalysed reaction of nitrogen and hydrogen to form ammonia.

A higher proportion of particles have sufficient energy to react when a catalyst is added b, compared with no catalyst a.

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