chapter 9 : enthalpy

what is enthalpy and its unit

the measure of heat energy in a chemical system (H)

what is a chemical system

the atoms, molecules or ions making up the chemicals

how is enthalpy measured

since you can’t directly measured it the enthalpy change is measured from the reactants and products

equation for enthalpy change\Delta H

H(products) - H(reactants) = \Delta H

what are exothermic reactions

reactions that give out heat energy from the system to the surroundings

temperature increase

what are endothermic reactions

reactions that absorb heat energy from the surroundings into the system

temperature decrease

when is enthalpy change positive and negative

positive endothermic

negative exothermic

what is activation energy

the minimum energy input required to break bonds for a chemical reaction to occur

draw energy profile diagrams for an endothermic and exothermic reaction

when will a reaction occur

if reactant particles collide in the correct orientation with sufficient activation energy

what are the standard conditions

10 kPa

25 C / 298 K

1 mol dm-3

substances in state under standard conditions

what is the enthalpy change of a reaction

the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions with all reactants and products in their standard states

what is the enthalpy change of neutralisation

enthalpy change that takes place in the reaction of an acid by a base to form 1 mol of H20 (l)

what is the enthalpy change of formation

enthalpy change when one mole of a compound is formed from its elements in their standard states

what is the enthalpy change of combustion

the enthalpy change when one mole of a compound is completely combusted or reacts in excess oxygen

what is calorimetry

the quantitative study method of energy in a chemical reaction

what 3 figures do you need to calculate energy change

mass

specific heat capacity

temperature change

how do you measure mass and temperature to find energy change

mass of surroundings (that are changing temperature) by weighing (g)

temperature change of surroundings by a thermometer

what is the formula for energy change (Q)

Q = mass (of surroundings) x SHC x change in temperature

what is specific heat capacity

energy to raise temperature of 1g of a substance by 1K

describe a way to measure energy change (7)

1. measure volume of water in a beaker

2. record initial temperature of water

3. weigh mass of methanol in spirit burner

4. light burner and place under beaker to heat while stirring water with thermometer

5. record maximum temperature of water after 3 mins

6. reweigh methanol and find change in mass

7. use formula to find energy change

4 reasons why less energy is transferred in a energy change experiment than supposed to

heat loss to surroundings other than water

incomplete combustion of methanol

evaporation of methanol from the wick

non-standard conditions (textbook values are standard)

2 ways to minimise errors in an energy change experiment

input of oxygen gas to avoid incomplete combustion

drought screens to minimise heat loss

how to determine an enthalpy change of reaction

using a polystyrene cup as it’s an insulator

what is the surroundings in a enthalpy change of reaction experiment

the aqueous solution itself

what is average bond enthalpy

energy required to break one mole of a specified type of bond in a gaseous molecule

why are bond enthalpies always positive

bond enthalpy is the energy to break bonds

therefore energy is taken into the system

therefore bond enthalpies are endothermic

so they always have a positive enthalpy value

what is used to calculate average bond enthalpy

actual bond enthalpy in different chemical environments

what determines whether a reaction is endothermic or exothermic

the difference in energy required for breaking bonds and the energy released when a bond is broken

what are 2 limitations of bond enthalpies

the actual bond enthalpies will be slightly different compared to the average bond enthalpies that are used

calculations using average bond enthalpies must be with gaseous molecules so they aren’t standard

what is hess’ law

if a reaction can take place by two routs and the starting and ending conditions are the same the total enthalpy change is the same for each route

what is the formula for enthalpy change of formation and combustion

formation: products - reactants

combustion: reactants - products