Energetics

a reaction that absorbs thermal energy from the surroundings, causing the temperature of the surroundings to decrease

More energy is required to break bonds than is released when forming bonds

a reaction that releases thermal energy into the surroundings, causing the temperature of the surroundings to increase

More energy is released forming bonds than is required to break bonds

bond-breaking is an endothermic process

bond-making is an exothermic process

REFER TO ONENOTE

• the amount of heat energy taken in or given out in a chemical reaction • it is the difference in energy from reactants to products

ΔH

kJ/mol

• ΔH is negative for exothermic reactions • ΔH = (bonds broken) − (bonds made) • more energy is released during bond breaking than is required during bond making, so the system loses chemical energy from its bonds as it is transferred as thermal energy into the surroundings (therefore increasing their temperature) • the products end up with less energy than the reactants, so ΔH is negative

• ΔH is positive for endothermic reactions • ΔH = (bonds broken) − (bonds made) • in endothermic reactions, more energy is required during bond breaking than is released during bond making, so the system absorbs chemical energy into its bonds by absorbing thermal energy from the surroundings (therefore decreasing their temperature) • as a result, the products end up with more energy than the reactants, so ΔH is positive

the amount of energy/heat needed to raise the temperature of 1g of substance by 1 degree celsius

4.2J/g/°C

Q = m x c x ΔT (energy heat change = mass x SHC x change in temp)

• its very good at absorbing and releasing heat so when reactions happen in or near it, the heat is transferred to the water and we can measure ΔT to find how much energy was absorbed/released by reaction • water has a well-known SHC of 4.18 J/g/℃ so can be used in Q = m x c x ΔT calcs • many reactions often happen in water to form an aqueous solution as water is a good solvent

• measure 100cm3 of water into a copper calorimeter using a measuring cylinder • measure its initial temperature using a thermometer • weigh a spirit burner containing ethanol with its lid on (prevents evaporation) • clamp calorimeter with thermometer exactly 1cm above spirit burner and cover apparatus using a draught shield and light spirit burner • stop heating when temperature has risen by 40 degrees and then extinguish flame by placing lid on burner • stir water well and measure max final temperature • immediately record final mass of spirit burner w/lid on • using Q = m x c x change in T, find the energy change of the combustion reaction • calculate the moles of ethanol used using moles = mass/Mr (as this is the limiting reagent) • using enthalpy change = -Q/1000n, find the enthalpy change of the reaction

• place a polystyrene cup in a 250cm3 beaker to keep it stable • transfer 25cm3 of hydrochloric acid solution into the polystyrene cup using a measuring cylinder • place thermometer into acid + record initial temperature of solution • carefully weigh out 0.30g of magnesium powder into a weighing boat using a mass balance • add magnesium to acid quickly and stir solution gently with a thermometer • regularly check temperature until it stops rising and note change in temperature • using Q = m x c x change in T, find the energy change of the reaction • calculate the moles of both reactants and determine limiting reagent using mole ratio • using enthalpy change = -Q/1000n, find the enthalpy change of the reaction

• place a polystyrene cup in a 250cm3 beaker to keep it stable • transfer 25cm3 of distilled water into the polystyrene cup using a measuring cylinder • place thermometer into acid record initial temperature • carefully weigh out 1.5g of potassium chloride into a weighing boat using a mass balance • add potassium chloride to water quickly and stir solution gently with a thermometer • regularly check temperature until it stops rising and note change in temperature • using Q = m x c x change in T, find the energy change of the reaction • calculate the moles of both reactants and determine limiting reagent using mole ratio • using enthalpy change = -Q/1000n, find the enthalpy change of the reaction

• place a polystyrene cup in a 250cm3 beaker to keep it stable • transfer 25cm3 of hydrochloric acid solution into the polystyrene cup using a measuring cylinder • place thermometer into acid • carefully measure out 25cm3 of sodium hydroxide solution into the cup using a measuring cylinder • record initial temperature of solution • add sodium hydroxide to acid quickly and stir solution gently with a thermometer • regularly check temperature until it stops rising and note change in temperature • calculate change in temperature and insert into Q = m x c x change in T • calculate the moles of both reactants and determine limiting reagent using mole ratio • insert moles of lining reagent into change in enthalpy = -Q/1000n

ΔH = -Q/1000n

moles = mass/molar mass moles = concentration x volume

1dm3 = 1000cm3

• use Q = m x c x ΔT to find the energy heat change of the reaction • calculate moles of substance causing the chemical change (e.g limiting reagent, alcohol being combusted) using moles = mass/Mr or moles = conc x vol • using the values for Q and moles, calculate ΔH using ΔH = -Q/1000n