Exothermic and Endothermic Reactions in Chemistry: Understanding Temperature Changes, Study Guides, Projects, Research of Chemistry

Download Exothermic and Endothermic Reactions in Chemistry: Understanding Temperature Changes and more Study Guides, Projects, Research Chemistry in PDF only on Docsity!  No Brain Too Small  CHEMISTRY  Exothermic or endothermic - and why? • When solid calcium chloride, CaCl2(s), reacts with water, the temperature increases. • When a person sweats, water is lost from the body by evaporation. • Pentane combustion: C5H12(l) + 8O2(g) → 5CO2(g) + 6H2O(l) ΔrHo = −3509 kJ mol–1 • Instant cold packs contain salts such as ammonium nitrate, NH4NO3. When activated, the salt dissolves in water, causing the temperature to decrease. • H2O(ℓ ) → H2O(g) • Glucose is made in plants by photosynthesis: 6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g) ∆rHo = 2803 kJ mol–1 • The equation for hydrating anhydrous copper sulfate is: CuSO4(s) + 5H2O(l) → CuSO4.5H2O(s) ΔrHo = −78.2 kJ mol–1. • Pentane, C5H12, is a liquid at room temperature. It evaporates at 36.1oC. • Hand warmers contain a supersaturated solution of sodium ethanoate which, when activated, crystallises and releases heat. • When solid sodium hydroxide is added to water, the temperature increases. • Freezing of water to form ice is represented by the following equation. H2O(l) → H2O(s) • Dissolving ammonium nitrate in a beaker of water: NH4NO3(s) → NH4 +(aq) + NO3 –(aq) ΔrHo = 25.1 kJ mol–1 • Glucose is an important source of energy in our diet. C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l) ΔrH° = –2820 kJ mol –1 . Exothermic Endothermic Bond breaking is endothermic. Bond making is exothermic. Energy is given OUT to the surroundings Energy is taken IN from the surroundings Calculations: Bond breaking & making type E.g. N2H4(g) + O2(g) → N2(g) + 2H2O(g) How much energy is released by this reaction? You will be given the average bond enthalpies in kJ mol-1 e.g. N-H 391, N-N 158, O=O 498, N≡N 945, O-H 463 Break: 4 x N-H, N-N and O=O 1564 + 158 + 498 = 2220 kJ Make: N≡N and 4 x O-H (remember bond making is exothermic) -945 + -1852 = -2797 kJ Energy released 577 kJ mol-1 (as 2220 + -2797 = - 577) Note: do not say -577 kJ released…. The ‘released’ tells us that it is exothermic. *You can use “Bond breaking – bond making” BUT don’t include the - sign in front of the bond making figure as well! e.g. 2220 – 2797 = -577 ∆H is negative ∆H is positive Products have LESS energy than the reactants Products have MORE energy than the reactants Products are MORE stable than the reactants Products are LESS stable than the reactants Exothermic Endothermic From equation type E.g. Fe2O3(s) + 2Al(s) → 2Fe(s) + Al2O3(s) ΔrHo = –852 kJ mol–1 How much heat energy is produced when 50.0 g of metal oxide is reacted with (excess) aluminium powder, Al(s). M(Fe2O3) = 160 g mol–1. n(Fe2O3) = 50.0/160 = 0.3125 mol 1 mol Fe2O3 releases 852 kJ of energy, so 0.3125 mol would release 0.3125 x 852 = 226 kJ (3 s.f.) Note: do not say -226 kJ released…. The ‘released’ tells us that it is exothermic. Given on data sheet in 2018 𝑛𝑛 = 𝑚𝑚 𝑀𝑀 𝑛𝑛 is the amount (in mol), 𝑚𝑚 is the mass (in g) and 𝑀𝑀 is the molar mass (in g mol-1). Eact -∆H H Reaction progress reactants products Eact +∆H H Reaction progress reactants products