3.2.1 Enthalpy change

What is enthalpy

Enthalpy, H, is a measure of the heat energy in a chemical system

• enthalpy is sometimes thought of as the energy stored within bonds. Enthalpy cannot be measures, but enthalpy changes can

What does system refer to, and what does surroundings refer to

The CHEMICALS - The atoms, molecules or ions making up the REACTANTS/PRODUCTS

The surroundings is everything outside the chemicals eg the thermometer, the lab etc

The universe is everything- and includes both system and surroundings

What is an exothermic reaction and draw the diagram

• energy is transferred from the system (chemicals) to the surroundings. The products have less energy than the reactants

• In an exothermic reaction the H change is NEGATIVE

• Common oxidation exothermic processes are the combustion of fuels and the oxidation of carbohydrates eg glucose in respiration

• Chemical system looses energy, surroundings gain energy, temp of surroundings increase

What is an endothermic reaction and give a diagram

Energy is transferred from the surroundings to the system (chemicals)

They require an input of heat energy eg thermal decomposition of calcium carbonate

The products have more heat energy than the reactants

The H change is POSITIVE

• chemical system gains energy, surroundings loose energy, temperature of the surroundings decreases

Define activation energy

The minimum energy required for a reaction to take place

• the energy input required to break bonds acts as a barrier to the reaction

• Also known as the minimum energy which particles need to collide to start a reaction

• On an enthalpy profile diagram, the arrow goes from the dotted line(that extends from the reactants) to the top of the curve

What are standard conditions and standard states

• pressure is 100kPa - very close to the pressure of 1 atm, 101kPa

• Temperature - 298K (25 degrees Celsius)

• Concentration- 1 mol dm^-3 of solution

• Standard state is the physical state of a substance under standard conditions

What is the enthalpy change of reaction

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions

• or the number of moles of reactants as specified in the balanced equation react together

What is the enthalpy change of formation

The enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions

Therefore there should only be 1 MOL of products

Shown as the triangle symbol, little f then big H

What is the enthalpy change of formation of an element

0 kJ mol ^-1

• as this refers to the formation of one mol of an element from its elements under standard- there is clearly no change

What is he enthalpy change of combustion

The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states

Shown by the big triangle, little c and big H

What i the enthalpy change of neutralisation

The energy change that accompanies the reaction of an acid by a base to form one mole of H20 (l) under standard conditions, with all reactants and products in their standard states

• symbol of big triangle, small neut and big H

What is the equation for heat energy given out or absorbed in a reaction, what is the equation for enthalpy change per mole

q=mc delta c

Enthalpy change per mole = q/ number of moles reacting

Define average bond enthalpy

The breaking of 1 mol of bonds in gaseous molecules

• use mean because every single bond in a compound has a slightly different bond energy

• The definition only applies when the substances start and end in the gaseous state

Does bond breaking release energy?

No bond breaking absorbs energy, bond making releases energy

If a section is endothermic, more energy is ‘-‘ when ‘-‘ bonds than is ‘-‘ when ‘- bonds

More energy is absorbed when breaking bonds than is released when making bonds

In an exothermic reaction, more energy is ‘-‘ when ‘-‘ bonds than is ‘-‘ when ‘-‘ bonds

More energy is released when making bonds than is absorbed when breaking bonds

What is the calculation to find enthalpy change using average bond enthalpy

Bond energies broken (in reactants) - bond energies made(in products)

What determines if a reaction is exothermic or endothermic

The difference between the energy required for bond breaking and the energy released by bond making

Hess’s law

States that total enthalpy change for a reaction is independent of the route by which the chemical change takes place

Why is Hess’es law used

To measure enthalpy change for a reaction that cannot be measured directly by experiments, instead alternative reactions are carried out that can be measured experimentally

Eg impossible to add the exact amount of water - and not easy to measure the temperature change of a solid

Describe a calorimetric method

Wshes the equipment (cup, pipettes etc) with the solutions to be used

Dry the cup

Put the polystyrene cup in a beaker - for insulation and support

Measure out desired volumes of solutions with volumetric pipettes to transfer to insulated co

Clamp thermometer in place - submerged in solution

Measure initial temperature of both solutions- every min for 2 mins, allowing both solutions to acclimatise

At 3 min transfer the second reagent to the first cup

If using a solid use the before and after method

Stir mixture - ensure all solution is the same temp

Record temp every mind for several mins

Why stir the solution, why polystyrene cup, why wash equipment with the solutions that are being used

Stir- ensure all of the solution is at the same temperature

Cup - insulation, reduce the temp lost to surroundings

Wash with solutions- prevent dilutions or contamination - may contain water that dilutes the reagent and affect concentration and results

Describes errors in this method

Energy transfer from surroundings (usually loss)

Approx in specific heat capacity of solution - the method assumes all the solutions have the heat capacity of water

Neglecting the specific heat capacity of the calorimeter - we ignore any energy absorbed by the apparatus

Reaction or dissolving may be incomplete or slow

Density of the solution is taken to be the same as water

If the reaction is too slow, what can happen that means the exact temperature rise is difficult to obtain - how can we counteract this

Cooling may occur simultaneously with the reaction

Counteract via taking readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together

Also measure the temperature of reactants before they are added together t