A2 Energetics I Flashcards
Define Enthalpy Change of Reaction
- The enthalpy change that accompanies a reaction in the molar quantaties specified in the chemical equation
- Under standard conditions with all reactants and products in their standard states
Define Enthalpy Change of Formation
- The enthalpy change that takes place when one mole of a substance is formed from it’s elements
- Under standard conditions, with all reactants and products in their standard states
Define 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
Define Enthalpy Change of Neutralisation
- The enthalpy change that accompanies the reaction of an acid and a base to form one mole of liquid water
- Under standard conditions, with all reactants and products in the standard states
What enthalpy do exothermic reactions have
Negative enthalpy
What enthalpy do endothermic reactions have
Positive enthalpy
What enthalpy is heating water, and therefore what is the problem with accurate readings when water is heated
- Heating water is endothermic
- Enthalpy cannot be measured
Draw an enthalpy profile diagram for an exothermic reaction
Draw an enthalpy profile diagram for an endothermic reaction
Define Activation Energy
The minimum energy required to break all the bonds in the reactants, allowing the reaction to take place
What are standard conditions
- 298K
- 100kPa
What is the standard state of a substance
The physical state of a compound under standard conditions
Define Average Bond Enthalpy
The enthalpy change when 1mol of gaseous covalent bonds are broken
Explaination for why reactions are exothermic
- Bond breaking absorbs energy
- Bond making releases energy
- More energy released than absorbed
- (H) negative
Explaination why reactions are endothermic
- Bond breaking absorbs energy
- Bond making releases energy
- More energy absorbed than released
- (H) positive
Bond energy calculation equation
What must always be remembered when structuring bond enthalpy calculations
Always write the total enthalpy of bonds broken and bonds made
Steps to a combustion calorimetry calculation
- Determine energy transferred to water
- Determine moles of fuel used
- Determine enthalpy
Combustion calorimetry equation for energy transferred to water (symbol + word)
Combustion calorimetry equation for moles of fuel used (symbol + word)
Combustion/solution calorimetry equation for determination of enthalpy (symbol + word)
Solution calorimetry equation for moles of fuel used (symbol + word)
What are the reasons as to why calculated enthalpy data is different for both solution and combustion calorimetry AND how to reduce this effect
- Heat loss to surroundings - use bomb calorimeter
- Data book uses standard values - use standard conditions
- Evaporation of water from beaker - place lid over beaker
What are the reasons as to why calculated enthalpy data is different for combustion calorimetry AND how to reduce this effect
- Incomplete combustion of fuel - burn in plentiful oxygen
- Evaporation of alcohol from wick - place cap over wick when not burning
Combustion calorimetry equation for energy transferred to water (symbol + word)
Hess cycle for combustion and equation
Hess cycle for formation and equation
Theoretical Hess cycle for neutralisation
What must always be remembered when carying out Hess cycle calculations
Add/Subtract data values according to arrow direction
What must you always bear in mind when considering solution calorimetry, and molar ratios
Multiply H by however many moles of reagent you calculated in the equation
