chapter 4 energetics

why is the amount of energy involved important to know

• measure the energy values of fuels

• calculate the energy requirements for industrial processes

• working out the theoretical amount of energy rquiredto break bonds and make bonds

• to predict weather a reaction occurs or not

thermochemistry

the study of heat changes during chemical reactions

define exothermic

when energy is given out after bonds are broken and formed

define endothermic

when energy is taken in after bonds are broken and formed

features of an exothermic reaction

• give out heat as they proceed

• adding water to anhydrous copper sulfate

• e.g neutralising an acid with an alkali

• e.g combustion

• temperature increases

features of an endothermic reaction

• take in energy from their surrounding as they proceed

• e.g breakdown of calcium carbonate

• heating hydrated copper sulfate

• temperature decreases

• e.g thermal decomposition

what unit is used to measure energy

kilojoules/mol KJ/mol

uses of exothermic reactions

• to produce large heat outputs / release lots of energy

• by burning carbon or natural gas

uses of endothermic reactions

• to treat sports injuries

• using cold packs

symbol equation of reactions in cold packs

NH4NO3(s) +(aq) → NH4NO3(aq)

why is thermochemistry used

to compare the efficiency of different fuels and for finding alternatives

hydrogen as a fuel

• hard to use because its a gas

• highly flammable/ explosive

• hard to store

• stores lots of energy per gram because of its high density

enthalpy change

heat change measure at a constant pressure

what are the standard conditions for measuring enthalpy changes

100KPa

298 Kelvins

when does a reaction end

once the products cool back to the starting temperature of 298k

enthalpy change in exothermic reactions

• less heat because energy is lost

• so delta H is negative

enthalpy change in endothermic reactions

• product has more energy than the starting materials

• delta H is positive

why does pressure affect the amount of energy given out

• energy is required to push away the atmosphere when releasing a gas

• so greater atmospheric pressure = greater energy used for a reaction to occur

how do the physical states of reactants and products affect enthalpy change

• state symbols must always be included

• because the physical states affect the enthalpy change of reaction

examples of why the physical states matter

• the enthalpy change caries based on the physical states

uses of enthalpy level diagrams

• represent enthalpy changes

• to convey the relative enthalpy levels of reactants

enthalpy level diagrams for exothermic reactions

products have less enthalpy than than the reactants

enthalpy level diagrams for endothermic reactions

products have more enthalpy than the reactants

general name for enthalpy change for any reaction

standard molar enthalpy change of reaction

define enthalpy of formation

is the enthalpy change when one mol of a substance is formed from its constituent elements under standard conditions in their standard state

define enthalpy of combustion

enthalpy change when 1 mol of a substance is completely burnt in oxygen under standard conditions in its standard state

define temperature

average kinetic energy of particles in a system

define heat

measure of total energy of all particles present in a given amount of a substance

how does temperature relate to kinetic energy

• as particles move faster the kinetic energy increases

• this increases the temperature

• temperature is independent of the number of particles present

what depends on how much of a substance is present

• heat

• because the energy of each particle is included

why does a bath have more heat than a red hot nail

because there are mor particles in it

factors affecting the enthalpy change of a reaction

• mass of a substance

• temperature change

• specific heat capacity

define specific heat capacity

the amount of heat needed to raise the temperature of a 1 g of a substance by 1 kelvin

equation for enthalpy change

mass X specific heat capacity X temperature change

uses of a calorometer

• find the approximate enthalpy change when a fuel burns

• to compare the enthalpy of combustion with a series of similar compounds

how does a calorimeter work

• burn the fuel to heat a known mass of water

• measure the temperature rise

• assume all the heat goes into the water

how to do calorimetry questions

step 1: find the enthalpy change

step 2 : convert joues into kilojoules

step3: calculate the moles using cXv=m

step 4:use -q/n to find the enthalpy change per mol

feature of a flame calorimeter that reduce heat loss further

• spiral chimney made of copper

• enclosed flame

• fuel burns in pure oxygen

why is it easy to measure heat changes for reactions that occur in solutions

the heat is generate in the solutions themselves

expanded polystyrene is used

neutralisation reactions

acid + alkali → salt + water

displacement reactions

when a more reactive metal displaces a less reactive one from a compound

example of displacement reactions

copper sulfate + zinc

symbol equation for a displacement reaction

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

allowing for heat loss practical

• apparatus are left in the lab so all reach the same temperature as the starting temperature

hess’s law

The total enthalpy change in a chemical reaction is independent of the route taken

thermochemical cycle

ΔHf​(CO2​)=ΔH1​+ΔH2​

direct pathway + indirect pathway = enthalpy change

define enthalpy of formation

• The enthalpy change when one mole of substance is formed from its constituent elements, under standard conditions with all reactants and products in their standard states

• The symbol is ΔH_f^Ꝋ