Chemical Energetics (1) PDF

 p t e r : 7 Cha L E M I C A CH C S R G E T I E N E Physical change Physical changes (such as melting or evaporating) do not produce any new chemical substances These changes are often easy to reverse Making a mixture from 2 or more substances...

 p t e r : 7 Cha L E M I C A CH C S R G E T I E N E Physical change Physical changes (such as melting or evaporating) do not produce any new chemical substances These changes are often easy to reverse Making a mixture from 2 or more substances or dissolving a solute in a solvent are examples of physical changes as no new substances are produced and are usually relatively easy to separate. Chemical change During chemical changes (usually referred to as chemical reactions), new chemical substances are formed that have very different properties to the reactants There may be signs that a new substance has formed, such as: A colour change A precipitate being formed Bubbles of gas being produced Most chemical reactions are very difficult to reverse Energy changes also accompany chemical changes and energy can be given out (exothermic) or taken in (endothermic) The majority of chemical reactions are exothermic with only a small number being endothermic Exothermic Reactions In exothermic reactions thermal energy is transferred to the surroundings so the temperature of the surroundings increases This energy is transferred from the chemical energy store of the chemical system to the surroundings and so the energy of the system falls - this means that the energy change is negative The overall transfer is from the system to the surroundings Combustion, oxidation, and neutralisation reactions are typical exothermic reactions Endothermic Reactions In endothermic reactions thermal energy is taken in from the surroundings so the temperature of the surroundings decreases This energy is transferred to the chemical energy store of the chemical system and so the energy of the system increases - this means the energy change is positive The overall transfer is from the surroundings to the system These types of reactions are much less common than the exothermic reactions Electrolysis, thermal decomposition reactions and the first stages of photosynthesis are typical endothermic reactions  Reaction pathway diagrams are graphical representations of the relative energies of the reactants and products in chemical reactions The energy of the reactants and products are displayed on the y-axis and the reaction pathway (a bit like time) is shown on the x-axis The difference in height between the energy of reactants and products represents the overall energy change of a reaction.  For atoms or particles to react with each other in a chemical system they must first of all come into contact with each other in a collision A number of factors come into play when analysing collisions such Enthalpy as energy, orientation, and number of collisions per Change & second (the frequency of collisions) Activation In terms of the energy of the collision, there is a minimum amount of energy required for the collision to be successful, that Energy is for the particles to react together This minimum amount of energy is called the activation energy (Ea) Different reactions have different activation energies, depending on the chemical identities involved  Reactions which have higher activation energies require more energy to start than those with lower activation energies The transfer of thermal energy during a reaction is called the enthalpy change, ΔH, of the reaction. ΔH is either a positive or a negative value depending on whether the reaction is exothermic or endothermic  Bond Breaking & Bond Forming Bond breaking is always an endothermic process as energy needs to be taken in from the surroundings to break the chemical bonds Bond making is always an exothermic process as energy is transferred to the surroundings as the new bond is formed Enthalpy change (ΔH) = Energy taken in - Energy given out 1) Hydrogen and chlorine react to form hydrogen chloride gas: H + Cl ⟶ 2HCl 2 2 The table below shows the bond energies. Calculate the enthalpy change, ΔH, for the reaction and deduce whether it is exothermic or endothermic. 2) Hydrogen bromide decomposes to form hydrogen and bromine: 2HBr ⟶ H + Br 2 2 The table below shows the bond energies. Calculate the enthalpy change, ΔH, for the reaction and deduce whether it is exothermic or endothermic. 1. Bond breaking is an endothermic change and bond forming is an exothermic change. Bond energy is the amount of energy in kJ/mol needed to break one mole of the specified bond. Use the following bond energies to determine whether this reaction is exothermic or endothermic. You must show your reasoning. 2. Ammonia is used to make nitrogen trifluoride, NF3. Nitrogen trifluoride is essential to the electronics industry. It is made by the following reaction. Determine if the above reaction is exothermic or endothermic using the following bond energies and by completing the following table. The first line has been done as an example. Bond energy is the amount of energy, in kJ /mole, needed to break or make one mole of the bond.