Energetics 1 Flashcards
Meaning Enthalpy Change
the heat energy change in a reaction at constant pressure & temperature
what are standard conditions
- standard state
- 100 KPa
- 298K Temperature
what are exothermic reactions
when heat energy is released from the system to the surround, increasing the temperature of the surroundings. Enthalpy change is negative as energy is released when bonds are formed
what are endothermic reactions
when heat energy is absorbed from the surroundings to the system, decreasing the temperature of the surroundings. Enthalpy change is positive, as energy is required when bonds are broken
meaning activation energy
the minimum amount of energy needed to begin breaking the reactant bonds, and to start the chemical reaction
why are enthalpy changes in the data booklet often under standard conditions
because enthalpy change is affected by pressure & temperature, they need to stay constant
Meaning Enthalpy of Formation
enthalpy change when 1 mole of substance is formed from its constituent elements with all substances in their standard states
is Enthalpy of Formation exothermic or endothermic
exothermic
Meaning Enthalpy of Combustion
enthalpy change when 1 mole of substance undergoes complete combustion in oxygen with all substances in their standard states
is Enthalpy of Combustion exothermic or endothermic
exothermic
Meaning Enthalpy of Neutralisation
enthalpy change when 1 mole of water is formed in a reaction between an acid & an alkali, under standard conditions
is Enthalpy of Formation exothermic or endothermic
exothermic
Meaning Enthalpy of Reaction
enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions
how to measure enthalpy change of solids/liquids
- mix the reactants together in a polystyrene cup and measure the temperature with a thermometre every 30 seconds until it returns to its initial temperature
how to measure enthalpy change of gases (flammable liquid)
- as the fuel burns, water heats up. By using.a calorimeter, work out the enthalpy change
why is enthalpy change of own experiments not the same as the results shown in the data booklet
- heat may have escaped through the calorimeter
- fuel might not have combusted completely
- the conditions may not have been standard
meaning hess’s law
the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs
which way do the arrows point in combustion
down
which way do the arrows point in formation
up
explain what happens when enthalpy change is positive
more energy is needed to break the bonds than form them
explain what happens when enthalpy change is negative
less energy is needed to break the bonds than form them
meaning bond enthalpy
the amount of energy required to break 1 mole of a type of bond in a molecule in the gas state
meaning mean bond enthalpy (values in the data book)
the energy needed to break 1 mole of bonds in the gas state, averaged over many different compounds
how to work out enthalpy change of a reaction
bond enthalpies of reactants - bond enthalpies of products
