What Drives Chemical Reactions?

what is the law of conservation?

energy cannot be created or destroyed, only transferred

what is the total chemical energy inside a substance?

enthalpy

what is enthalpy changed represented by?

ΔH

what is an exothermic reaction?

• products have less enthalpy than reactants

• heat given off to the surroundings

• ΔH is negative

what is an endothermic reaction?

• products have more enthalpy than reactants

• heat energy absorbed by system from surroundings

• ΔH is positive

what is the activation energy?

the minimum amount of energy required for reactant molecules to have a successful collision and start the reaction

what is standard enthalpy change?

the heat transferred at constant pressure under standard conditions and states (ΔHº)

what is the standard enthalpy of reaction?

the enthalpy change when the reactants in a stoichiometric equation react to give the products under standard conditions (ΔHº_r)

what is the standard enthalpy of formation?

the enthalpy change when one mole of a compound is formed from its elements under standard conditions (ΔHº_f)

what is the standard enthalpy of combustion?

the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions (ΔHº_c)

what is the standard enthalpy of neutralisation?

the enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions (ΔHº_neut)

what is calorimetry?

a technique used to measure changes in enthalpy of chemical reactions

what is the specific heat capacity?

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

what is specific heat capacity measured in?

J g^-1 K^-1

what is bond enthalpy?

the energy required to break a particular bond

what is average bond energy?

the energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds

how do you calculate the average bond enthalpy?

take the bond enthalpy of the whole molecule and divide it by the number of bonds

how do you calculate the enthalpy change of reaction?

ΔHº_r = enthalpy change for bonds broken + enthalpy change for bonds formed

what is hess’ law?

the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same

what is lattice enthalpy?

the energy change when one mole of an ionic compound is separated into its gaseous ions (ΔHº_latt)

what is the first ionisation energy?

the standard enthalpy change when one mole of gaseous atoms is converted into one mole of gaseous 1+ ions (ΔHº_ie1)

what is the second ionisation energy?

the standard enthalpy change when one mole of gaseous 1+ ions is converted into one mole of gaseous 2+ ions (ΔHº_ie2)

what is the enthalpy of atomisation?

the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions (ΔHº_at)

what is the first electron affinity?

the standard enthalpy change when one mole of gaseous atoms is converted to one mole of gaseous 1- ions (ΔHº_ea1)

what is the second electron affinity?

the standard enthalpy change when one mole of gaseous 1- ions is converted to one mole of gaseous 2- ions (ΔHº_ea2)

what is the word equation for combustion of non-metals?

non-metal + oxygen → non-metal oxide

what is the general word equation for the combustion of suitable metals?

metal + oxygen → metal oxide

what is the word equation for complete combustion?

fuel + oxygen → carbon dioxide + water

what are the word equations for incomplete combustion?

fuel + oxygen → carbon monoxide + water

fuel + oxygen → carbon + water

what happens in terms of combustion when the length of a hydrocarbon chain increases?

• increased carbon content so more carbon dioxide or carbon monoxide/carbon produced

• stronger london dispersion forces so less volatile

• hydrocarbon releases less energy per unit mass of fuel

what is the greenhouse effect?

• sun emits shortwave radiation that reaches the Earth’s surface

• this energy is absorbed and reemitted as longwave IR radiation

• some IR radation escapes to space but some is absorbed by greenhouse gases in the atmosphere

• these gases reradiate the energy in all directions trapping heat

what is the enhanced greenhouse effect?

as a result of human activities there are higher greenhouse gas levels meaning more heat is trapped so the Earth’s temperature increases

what is a renewable resource?

• will not run out in the forseeable future because they can be replaced over a relatively short period of time

• infinite

• sustainable as they can be produced at the same rate or faster than they are being used

what are non renewable resources?

• will run out in the forseeable future as there are limited supplies

• finite

• not sustainable

what are the main three biofuels?

• bioethanol

• biodiesel

• biogas

how is bioethanol formed?

• plants convert carbon dioxide to glucose by photosynthesis

• glucose converted to ethanol by fermentation

• is carbon neutral as the carbon dioxide absorbed by photosynthesis is equal to the carbon dioxide released when the ethanol is burned

how is biodiesel formed?

• made from renewable vegetable oils

• natural trigylceride oils are converted into esters of methanol

• called transesterification

how is biogas formed?

made when organic matter is broken down by microorganisms in the absence of oxygen

advantages of biofuel

• carbon neutral

• renewable and sustainable (if trees/crops are replanted)

• reduce greenhouse gas emissions and pollution

• reduce landfill waste

• generate income in less developed countries

• create jobs

disadvantages of biofuel

• high cost

• land use conflict (may be needed for food or may result in deforestation)

• environmental concerns (often require pesticides/fertilisers, crops can deplete soil nutrients)

• generally have lower specific energy than fossil fuels

what are fuel cells?

an electrochemical cell where a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode

how do fuel cells work?

as the fuel enters the cell it becomes oxidised which sets up a potential difference within the cell

what are the advantages of hydrogen fuel cells?

• water is the only product

• reaction occurs at room temperature and avoids combustion

• no nitrogen oxides produced

disadvantages of hydrogen fuel cells

• hydrogen is highly flammable

• requires heavy thick-walled storage tanks

• high cost

• hydrogen is non-renewable (mostly sourced from fossil fuels)

advantages of methanol fuel cells

• easier to store and transport than hydrogen

• operates at low temperature and pressure

• higher energy density than hydrogen

• can be made from renewable resources

disadvantages of methanol fuel cells

• toxic and highly flammable

• commonly made from non-renewable fossil fuels

• lower voltage and power output per unit mass (compared to hydrogen)

• lower efficiency

• requires expensive catalysts

• produces carbon dioxide

what is entropy?

a measure of the distribution of matter and/or energy in a system

what happens to the entropy when a solid melts to a liquid?

it increases as the particles become more disordered

• in a solid particles can only vibrate around a fixed point

• in a liquid they can move around each other

what are the units of entropy change? (ΔS^⦵)

J K^-1 mol^-1