Refresh Which is true for a chemical reaction in which the products have a higher enthalpy than the reactants? Reaction ∆H A. endothermic positive B. endothermic negative C. exothermic positive D. exothermic negative

Lesson 2 Measuring Enthalpy Changes

Lesson 2: Endothermic and Exothermic Objectives: Understand the technique of calorimetry, including the assumptions underpinning it Calculate enthalpy changes from experimental data Complete a calorimetry experiment

Specific heat capacity, C The specific heat capacity of a substance is the amount of energy required to raise the temperature of one gram by one Kelvin. Specific heat capacity is different for different substances: Substance Specific Heat Capacity J K-1 g-1 Water 4.18 Ethanol 2.44 Air 1.00 Iron 0.450 Copper 0.385

Calorimetry Calorimetry is used to measure the amount of heat released/absorbed in a reaction. The reaction is used to heat some water, and the temperature change measured If we know the mass of water used, the specific heat capacity of the water and the temperature change, we can calculate the heat change. If we are being accurate, we should also take into account the heat capacity of the calorimeter itself as this also heats up.

Calculating heat change, ∆H ∆H = -m.C.∆T Units: Joules, J Where: ∆H is the enthalpy change m is the mass of water used in grams (same as volume in cm3) C is the specific heat capacity of water (4.18 JK-1g-1) ∆T is the temperature change – you should use +/- to signify a decrease/increase in temperature ∆H can easily be converted to a molar value by dividing it by the number of moles of reactant Assumptions: m is just the total mass of water used This is valid as the mass of water used is much greater than the mass of any of the other substances C is just the specific heat capacity of water, ignoring the reactants This is valid as the specific heat capacity of water is much higher than most other substances, so they absorb very little of the heat

For example When 200 cm3 of 1.00 mol dm-3 sodium hydroxide solution was added to 200 cm3 of a 0.400 mol dm-3 solution of sulphuric acid, the temperature rose from 24.5oC to 30.0oC and a neutral solution was obtained. Determine the enthalpy change when one mole of sulphuric acid is fully neutralised by sodium hydroxide. Determine ∆H: ∆H = -m.C∆.T ∆H = -(200+200) x 4.18 x (30.0-24.5) = -9,196 J Determine n(H2SO4): n(H2SO4) = conc. x vol. = 0.400 x (200/1000) = 0.0800 mol Determine Molar ∆H: Molar ∆H = ∆H / n(H2SO4) = -9196 / 0.0800 = 114950 J = -115 kJ mol-1

Calorimetry in Practice In this experiment you will determine the enthalpy change for the reaction of magnesium with sulphuric acid. To do this accurately, you will first need to determine the heat capacity of the calorimeter. Follow the instructions on the wikispaces page

∆H = -m.C.∆T Units: Joules, J Key Points ∆H = -m.C.∆T Units: Joules, J Assumptions: Specific heat capacity of solutions is the same as that of water Total mass is the same as the volume of solution used