Rate of Chemical Reactions Unit 3 AOS 2

Heat Energy From Chemical Reactions All substances have chemical energy. The chemical energy of a substance results from the combined effects of its Potential (stored) energy - due to attractions & repulsions between p+ and e- within and between atoms. Kinetic energy - due to the movement of atoms & electrons.

The chemical energy of a substance is known as its enthalpy (or heat content) and is a measure of the total energy of a system and is given the symbol H. The enthalpy of different substances varies because of different values for potential energy and kinetic energy.

The change in enthalpy accompanying a chemical reaction is of greater significance as (unlike enthalpy) it can be measured. The change in enthalpy is represented by ΔH, where the symbol, Δ, signifies 'change in'.   For a chemical reaction, the enthalpy change is given by the relationship: ΔH = H(products) - H(reactants) H - Heat (enthalpy) change for a given system. It refers to the heat change per mole(s) as defined by a chemical equation. Units are kJ mol-1 (or kJ/mol) where kJ is the symbol for the unit of energy - kilojoule

Exothermic and Endothermic Reactions All reactions involve (I) the breaking of bonds; and - (II) the formation of new bonds. The breaking of bonds always requires energy and the formation of bonds always releases energy. The energy required to break bonds in the reactants is called the activation energy, Ea.

a) Exothermic Reactions A reaction with a negative ΔH is said to be exothermic and energy flows from the system into the surroundings, often as heat energy. The combustion of methane, that releases energy, is an example of an exothermic reaction. ΔH = -890 kJmol-1

Figure 1: Energy profile for an exothermic reaction. Energy to break bonds Energy released when new bonds form Ea ΔH As energy is released by the system the temperature of the surroundings increases

b) Endothermic Reactions A reaction with a positive ΔH is said to be endothermic and energy flows from the surroundings into the system. The process of photosynthesis, where energy from the sun is absorbed, is an example of an endothermic reaction. ΔH = +2803 kJmol-1

Figure 2: Energy profile for an endothermic reaction. Energy needed to break bonds Energy released when new bonds form Ea ΔH As energy is absorbed by the system the temperature of the surroundings decreases

Exothermic or endothermic? (a) CO2(g) + C(s)  2CO(g); H = + 161 kJ/mol (b) N2(g) + 3H2(g)  2NH3(g); H = - 92 kJ/mol (c) C6H12O6(aq) + 6O2(g)  6CO2(g) + 6H2O(l) (d) C2H4(g) + 3O2(g)  2CO2(g) + 2H2O(g) (e) I2(g)  2I(g) exothermic exothermic exothermic endothermic

Summary of exothermic and endothermic changes Type of reaction Heat energy change   Sign of H Temperature change Exothermic Endothermic

Reaction Rates The rate of a reaction is a measure of how quickly the reactants are converted into products (change in concentration per unit time). For a reaction to occur the particles must collide and their behaviour is governed by the COLLISION THEORY.

Collision Theory The three main points are: Particles must collide before they can react. The more particles there are in a given volume the higher the number of collisions. The collision between particles must be strong enough that sufficient energy is present to break the bonds in the reactants. (Activation energy – Ea) The particles must collide in the correct orientation. If the molecules are not facing the right direction no reaction can occur. For a successful collision all these three factors must happen.

Measuring reaction rates The reaction rate is a measure of the amount of substance either consumed or produced per unit time. Ways in which reaction rate could be measured are: 1. colour change 2. temperature change 3. gas production/loss in reaction mass

There are five factors that affect the rate of a reaction. Changing any of the following and alter the rateof a chemical reaction Concentration of reactants Pressure of gaseous reactants 3. Temperature 4. Surface area 5. Adding a catalyst

Concentration of reactants and pressure of gaseous reactants The more particles there are in a given volume the higher the number of collisions and the greater number of successful collisions. Increasing the pressure of a gas will increase the concentration of the gas particles resulting in more frequent collisions.

Temperature The temperature of a system is a measure of the average kinetic energy of the particles in it. When the temperature of a system is increased then the average kinetic energy of the particles increases. However, the increase in reaction rate as temperature is increased is not simply explained by the higher number of collisions. It has been found that just a small increase in temperature can lead to a relatively large increase in the reaction rate even though there is only a small increase in the number of collisions.

Temperature and KE A small increase in temperature has a significant effect on the proportion of particles with energies greater than the activation energy, Ea, meaning many more particles will have enough energy to overcome the activation energy barrier and collide successfully.

Fraction of molecules with energy E The graph above shows the effect of temperature on the proportion of particles with energy greater than the activation energy, Ea. At the higher temperature (T2), there are a significantly higher proportion of particles with energy greater than the activation energy Ea as indicated by the area under the graph.

Surface area In a reaction involving a solid, breaking the solid into smaller pieces increases its total surface area. The greater the surface area of a solid the higher the number of particles exposed to other reactants. As a result there will be more successful collisions and an increased rate of reaction.

Adding a catalyst A catalyst provides an alternative pathway that has a lower activation energy than the original pathway. However, a catalyst does not affect the ΔH for the reaction. Lowering the activation energy (Ea) , ensures that more collisions will result in product formation.

A catalyst Does not undergo any permanent change during the reaction. Is needed only in small quantities to have its affect. Increases rate of forwards and reverse reaction equally and therefore does not alter position of equilibrium. Can be either heterogeneous-in a different state to reactants/products or homogeneous- is in the same state as reactants and products.

Factors mainly affecting the proportion with required Ea Factors mainly affecting the collision rate Factors mainly affecting the proportion with required Ea 1. Surface area   2. Temperature 3. Concentration 1. Temperature 2. Catalyst