Energetics Flashcards
Enthalpy definition
Energy constant at constant pressure
Bond enthalpies
Enthalpy change on breaking or making a bond (per mole)
Delta H calculation
Delta H = En (break broken) - Em (bonds formed)
What does positive delta h mean
Bonds breaking (putting energy in)
Negative delta h
Making bonds (getting energy out)
What does En / Em mean?
Number of moles of each types of bonds
Enthalpy change definition
Change in heat at constant pressure content under standard conditions
Combustion general equation
Fuel(CnHn+2) + O2 -> CO2 + H2O
Thermochemistry definition
Heat chemistry
Endothermic reaction definition
Energy must be put in to break a bond
Exothermic reaction definition
Every given out when bonds are formed
What does delta mean?
Change in
What does endothermic energy change mean?
Energy is absorbed from surroundings by reactants to form products
What are the x + y axis on energy change graphs?
X- reaction progress,t
Y- enthalpy,H
What is the delta h for endothermic enthalpy change?
Positive - gets more energy
Energy change graph for exothermic
Reactants higher enthalpy than products enthalpy
What does exothermic energy change mean?
Energy is released to surroundings by reactants to form products
What is the delta h for exothermic enthalpy change?
Negative
Enthalpy of products is decreased
Energy change graph for endothermic
Products higher enthalpy than reactants enthalpy
What is the standard enthalpy change or formation? Delta f H degrees
Enthalpy change when 1 mole of a substance is formed form it’s constituent elements with all reactants + products in a standard states
Standard enthalpy change of combustion? Delta c H degrees
Enthalpy change when 1 mole of substance is completely burned in oxygen with all reactants + products in standard states under standard conditions
What is the standard enthalpy change of neutralisation? Delta neut H degrees
Enthalpy change when 1 mole of water is formed in a reaction between an acid + alkali under standard conditions
Combustion general equation
CnH2n+2 + O2 -> H2O
How to measure enthalpy change?
1) check significant figures
2) calculate q using q = mc^T
3) calculate moles of whatever is reacting to raise the temp (mass/mr)
4) delta h = q/moles (divide q by 1000 + put - in front before dividing by moles)
6) answer to sf in step 1
7) each cm3 is 1g
Delta h calculation
q/moles in reaction
Measuring enthalpy change equation
quantity of heat = mass x specific heat capacity x change in temp
q = mc^t
Enthalpy change units
Jg-1 k-1
Average bond enthalpies definition
Exact energy needed to break a bond depends on the compound + bond being broken
CH4 -> CH3 + H
Delta h value
+ 425
CH3 -> CH2 + H
Delta h value
+ 470
CH2 -> CH + H
Delta h value
+ 410
CH -> C + H
Delta h value
+ 335
Delta H calculations using average bond enthalpies
Energy to break bond - energy to make bonds
Bond dissociation energy definition
Bond energy for diatomic molecules is breaking the only bond in the substance
What state must the compound be in when calculating bond enthalpies?
Gaseous
What can be included to change the state to a gas?
vaporisation value
Practical aim for calorimetry - alcohols
Measure enthalpy of combustion (delta H c) of 3 alcohols (methanol, ethanol + propan-1-ol)
Results of calorimetry - alcohols
Record as you progress through practical
Hazards of calorimetry - alcohols
- alcohols are flammable
- methanol is toxic
- remember to cap all burners when not using
Calorimetry cup practical method
- take temp for few mins before reaction
- mix/combine reactants
- take emo for another few mins
Which results of the calorimetry cup do we use?
Only use results which are after highest temp once reacted
Or
If temp decreases only use results which are after lowest temp once reacted
What is Hess’ law definition for enthalpy change of formation?
Enthalpy change of any reaction is independent of route taken
Hess’ law definition
If a reaction takes place by more than one route, the initial + final conditions are the same , the total enthalpy change is the same
What is the delta h for reactants?
Delta H f
What is the delta h for reaction?
Delta h
What is the delta h for products?
Delta h f
Enthalpies change of reaction calculation
Products - reactants
What is the definition for Hess’ law enthalpy change of combustion?
- The enthalpy change when 1 mole of substance burns completely in oxygen
- reactants + products in standard state/conditions (100 kPa)
Calculations for Hess’ law enthapies of combustion
Products = reaction - reactants Reaction = reactants - products Reactants = reaction + products
What is bond energy?
The amount of energy needed to break a bond in a gasworks state + form a bond
Is bond breaking exo or endothermic?
Endothermic as it requires energy
What is bond dissociation energy?
Bond energy for diatomic molecules is breaking the only bond in substance
Calculation for Hess’ law for enthalpies of formation
Reactants + reaction = products
Products - reactants = reaction
Products - reaction = reactants
