enthalpy changes

what is enthalpy?

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

what is enthalpy change?

the difference in enthalpies of the reactants and products (triangle H)

what does the law of conservation of energy state?

energy cannot be created or destroyed, only transferred between a system and the surroundings of a chemical reaction

what are the two types of reactions?

endothermic and exothermic

what are features of an endothermic reaction?

energy is transferred from the surroundings to the system
enthalpy change is positive
the temperature of the surroundings decreases

what are features of an exothermic reaction?

energy is transferred to the surroundings from the system
enthalpy change is negative
the temperature of the surroundings increases

what is activation energy?

the minimum amount of energy required for a reaction to take place (E_a)

what are standard conditions for enthaly change?

pressure of 100kPa
temperature of 298K

what is standard state?

the physical state of a substance under standard conditions

what is standard enthalpy change of 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 (ΔH^θ_r )

what is standard enthalpy change of formation?

the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states (ΔH^θ_f )

what is standard enthalpy change of combustion?

the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states (ΔH^θ_c)

what is the standard enthalpy change of neutralisation?

the energy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions, with all reactants and products in their standard states (ΔH^θ_neut)

what is calorimetry?

the measurement enthalpy changes in chemical reactions

what are three things a simple calorimeter can be made from?

a polystyrene cup, a vacuum flask, or metal can

what is specific heat capacity of a liquid?

the energy needed to increase the temperature of 1g of a substance by 1 degree celsius (c)

what is the specific heat capacity of water?

4.18 Jg^-1 K^-1

what is the equation for energy change?

energy change = mass x specific heat capacity x temperature change
q = mctriangleT

how can enthalpy change of combustion be calculated from energy change?

1. calculate energy change of water in kJ

2. calculate the mol of substance burnt

3. calculate enthalpy change using energy change / mol of substance

what can make experimental enthalpy change of combustion not very accurate?

heat loss to the surroundings
incomplete combustion
fuel evaporation

what is average bond enthalpy?

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

why might an actual bond enthalpy differ from an average bond enthalpy?

average bond enthalpy is a mean calculated from the actual bond enthalpies in different chemical environments (different molecules)
it takes more energy to break a C-H bond in methane than in propane

what is an endothermic reaction in terms of bonds?

energy is required to break bonds
the energy required when breaking existing bonds is greater than the energy released when forming new bonds

what is an exothermic reaction in term of bonds?

energy is released when bonds form
the energy released when forming new bonds is greater than the energy required when breaking existing bonds

how can enthalpy change of reaction be calculated using average bond enthalpies?

ΔH_r^ꝋ = {(bond enthalpies in reactions) - {(bond enthalpies in products)

what does Hess’s Law state?

if a reaction can take place by two routes, and the initial and final conditions are the same, the total enthalpy change is the same for each route

what is the equation for enthalpy change of reaction from formation using Hess’s Law?

products - reactants
1. write equation for reaction
2. write elements with correct number of moles and state symbols underneath
3. draw upwards pointing arrows to each compound\\
4. write values of energy changes on arrows and multiple by number of moles
5. in a cycle, go from reactions to products, changing sign of value if arrow points in opposite direction

what is the equation for enthalpy change of formation from combustion using Hess’s Law?

reactants - products
1. write equation for formation of compound
2. write combustion products below the equation
3. draw downward pointing arrows from each substances to its combustion products
4. write values on arrows and multiple by number of moles
5. in a cycle, go from reactants to products, changing sign of value if arrow points in opposite direction