C3.2 Energetics

Temperature of reaction mixture increases during a chemical reaction Release energy into the surroundings Examples: Combustion, neutralisation (Combustion: fuel + oxygen -> carbon dioxide + water + energy)

Temperature of the reaction mixture decreases during a chemical reaction Take energy in from the surroundings Examples: reaction between citric acid and sodium hydrogencarbonate Photosynthesis

Chart that shows the energy involved in a chemical reaction

energy transferred from the surroundings to break bonds in the reactants minus the energy transferred to the surroundings when bonds form in the products Basic equation: Energy for Bonds broken - energy for bonds made

Minimum energy needed for a reaction to start Often provided by a flame or by heating, and it breaks bonds in the reactants

In endothermic reaction amount of energy transferred to break bonds > energy transferred to form new bonds Positive energy change Surroundings cool down Upward arrow on the energy profile Example graph: Energy | ∩ products | ||__________ |reactants|↑ energy change |__________| |______________________ Progress of reaction

In exothermic reaction amount of energy transferred to break bonds < energy transferred to form new bonds negative energy change Surroundings warm up Downward arrow on the energy profile Example graph: Energy | reactants ∩ | _________|| |energy ↓| products |change |_________ |______________________ Progress of reaction

1) draw both axis 2) Draw 2 horizontal lines representing the chemical energy stores in the reactants and products. Reactant line lower than product line for endothermic reaction Reactant line higher than product line for exothermic reaction 3) Draw a curve to represent the activation energy and add labels to the diagram

Energy needed to break 1 mol of a particular bond measured in kJ/mol

Example question: nitrogen + hydrogen -> Ammonia H [ | ] 1) N≡N + 3[H-H] -> 2 H-N-H Bonds broken: N≡N , H-H x 3 : energy: 945 + 3(436) = 2253kJ/mol Bonds made: N-H x 6 : energy: 6(391) = 2346kJ/mol 2) Change in energy = bonds broken - bonds made = 2253 - 2346 = -93kJ/mol The negative sign in the answer shows that the reaction is exothermic

Shows the bonds present in a molecule