Enthalpy

Define enthalpy (H)

• Refers to all the heat energy that is stored in a chemical system at a constant pressure

What is enthalpy change?

• Change in heat energy of a substance at a constant pressure

What happens in an exothermic reaction?

• Energy is transferred from the system to the surroundings

• Products have less energy than reactants

Reaction profile form an exothermic reaction

• Enthalpy change in an exothermic reaction is always negative

• As energy is given out

Define activation energy

• Minimum amount of energy required for a reaction to take place

What happens in an endothermic reaction?

• Energy is transferred from the surroundings to the system

• The products have more energy than the reactants

Reaction profile for an endothermic reaction

• In an endothermic reaction the enthalpy change is positive

What happens to the bonds in the reactant molecules for a chemical reaction to take place?

• Bonds in the reactant molecules are broken

• This requires energy

How are products formed in a chemical reaction?

• Bonds are made

• Energy is released

What are enthalpy changes always measured under?

• Standard conditions

What are the standard conditions?

• Standard temperature = 25 degrees (298K)

• Standard pressure = 100Kpa

• Standard concentration = 1mol dm^-3

Define standard enthalpy change of combustion

• When 1 mole of a substance reacts completely with oxygen

• With all substances in their standard states

Define standard enthalpy change of formation

• Enthalpy change when 1 mole of a compound is formed from its elements under standard conditions.

• All elements being in their standard rates

Measuring the standard enthalpy change of combustion via an experiment

• Use a balance to measure starting mass of a spirit burner and fuel (ethanol)

• Use a thermometer to measure starting temperature of water in the metal calorimeter

• Remove cap of spirit burner and immediately light the wick (do not leave spirit burner uncapped when not lit as fuel will evaporate)

• Stir water with thermometer, to ensure thermal energy is distributed

• After several minutes extinguish the flame by putting the cap back on

• Read temp on thermometer and measure the final mass of the spirit burner

• Starting mass-final mass=total mass combusted

What equations are required to workout enthalpy change

Enthalpy change = q/1000 divided by mole of fuel

Why may published values of enthalpy change be higher than experimental values?

• Spirit burner being left uncapped- fuel will evaporate making it seem like more fuel was burnt

• Heat energy released does not pass into water- some heat energy transferred to calorimeter and air

• Not all the fuel underwent complete combustion-incomplete combustion releases less thermal energy

• Experiment not carried out under standard conditions

Measuring enthalpy change through another experiment (polystyrene cup)

• In this experiment there is a reaction between Mg and CuSO4 (solution) which is exothermic

Mg + CuSO4 → MgSO4 + Cu

• Weigh the Mg powder

• Copper sulphate must be in excess. The equation shows Mg and CuSO4 react in a 1:1 ratio

• For CuSO4 to be in excess we must add less than 0.1 moles of Mg. E.g 0.05 moles

• Measure 100cm3 of CuSO4 in a polystyrene cup which is an insulator

• Place the cup inside a glass beaker so it doesn’t tip over

• Place the thermometer in the solution.

• Need to ensure solution is the same temp as the room, as the experiment must be conducted under standard conditions

• Take temp readings every 30 seconds until there is a constant temprature

• Then add Mg powder and stir it in with thermometer

• Take temp readings every 30 seconds for a period of time like 15 minutes

• Plot a graph with temp on y axis and time on x

Explain the results

• Once temp reaches a maximum it decreases due to thermal energy being lost to surroundings

• These heat losses would have taken place throughout the reaction

• Extrapolate the line to show what the temp would’ve been if there was no cooling

• use equations to workout enthalpy change

Why can we use 100cm3 as the mass of the solution heated (CuSO4) even though its not in grams?

• CuSO4 was dissolved in water making it an aqueous solution

• It can be assumed that 1g = 1cm3 as density of this solution is similar to water

What is Hess’s law

• Enthalpy change is independent of route taken

Enthalpy cycle that uses standard enthalpy change of formation

• Arrows face upwards

Enthalpy cycles using standard enthalpy change of combustion

• Arrows face downwards

Define average bond enthalpy

• Energy required to break 1 mole of a specific bond in a molecule in its gaseous state

Energy needed to break a bond is the same amount of energy…….

• released when forming a bond