Chemistry Enthalpy Flashcards
(24 cards)
What is Enthalpy and what is the symbol?
Enthalpy is the amount of heat energy stored in a chemical system (H)
What is the formula including Enthalpy and explain it?
Change in Enthalpy= Enthalpy of products- Enthalpy of reactants
Enthalpy js hard to determine so instead we determine the change in Enthalpy
How can we present Enthalpy ?
Enthalpy profile diagrams
Draw and explain the Enthalpy profile diagram for an exothermic reaction?
Make sure to add chemical formula and include state symbols and balance it
An upwards arrow from reactants to peak of the curve for activation energy (Ea)
Arrow from reactants to products for Enthalpy change the arrow direction determining whether it’s an exothermic or endothermic reaction
State the standard Enthalpy conditions?
Standard pressure 100 Kpa
Standard temp 25 degrees Celsius
Standard concentration 1mol/ Dm cubed
All chemicals must be in standard states
What is standard Enthalpy change of combustion?
Enthalpy change when reacting 2 mole of a a chemical completely with oxygen under standard conditions
What is standard Enthalpy change of neutralisation?
Enthalpy change when producing 1 mole of water in a neutralisation reaction under standard conditions
What is a key point about the standard Enthalpy change of neutralisation?
The key parts of the reaction is the H+ ion and OH- ion so any strong acid and strong alkali will have the same or similar Enthalpy change of -57 kj/mol
What is Standard Enthalpy change of formation?
Enthalpy change when producing 1 mole of. A compound from its elements under standard conditions in there standard states
What is standard Enthalpy change of reaction?
Enthalpy change of a reaction that does not fit into the other categories in the molar quantities of the balanced equation under standard conditions
Describe the apparatus in determining the standard Enthalpy change of combustion?
1st measure initial mass of fuel with cap on using a balance and starting temp of water using thermometer
2nd uncap fuel and immediately light wick and allow to burn for several minutes whilst stiring the metal can calorimeter with water to distribute heat energy evenly
3rd extinguish flame and messure final temp of water and final mass of fuel and calculate the difference
(mass of water is fixed and known)
State the equation for calculating energy change’s and add units?
q =mc ∆t
q= energy change in joules
m= mass of water in grams
c= specific heat capacity j/g/k
∆t= temperature change of water in degrees Celsius
How do u calculate the Enthalpy change of combustion using this equation and the earlier experiment?
First form the experiment before we use the mass of water SHC and temperature change and substitute into the equation to find energy change in joules
Then change to kJ
Then find the amount of moles that were burned using moles =mass over mr
Then divide the energy transferred by amount of moles and add a - or + shaking endothermic or exothermic reaction
Give 2 reasons why other values may be higher the ours?
Because some of our fuel may have evaporated due to uncapped spirit burner so reaction seems less exothermic then it acc is
And not all thermal energy was transferred to water some Amy be transferred to metal can calorimeter
What’s is the process for determining the standard Enthalpy change of reaction?
- To ensure full reaction need limiting reagent, chemical equation says molar ratio is 1:1 so calculate no moles in our solution copper sulphate [0.1] and to ensure mg fully reacts we use 0.05 moles mg => use equation to find mass of this
- Add coSO4 solution in polystyrene cup un beaker with thermometer measure temp every 30s Untill constant temp then add mg keep stirring and add lid and continue measuring temp for 5mins then plot on graph
- Account for heat losses draw a line going back from the cooling part of the curve and uo from the start of the reaction intersection is the max temp
Now what is the calculation part to determine standard Enthalpy change of reaction?
Use equation q =mc delta T
Mass of water=100g
C solution similar to SHC of water 4.18
Delta T= max temp reached
After calculation convert to KJ
And divide by no of moles of mg used
Lastly x by how much moles of mg involved in 1 mole of the equation
And plus or minus for either exothermic or endothermic reaction
What is Hess law?
The total Enthalpy change is the same if the reaction can be carried out by 2 different pathways as long as the starting and final conditions are the same
Why is Hess law useful?
If a reactions conditions cannot be met we can use 2 different reactions pathways and use Hess law to determine the Enthalpy change
How to carry out Enthalpy cycles for standard Enthalpy change of reaction data?
- write all the elements in there standard states and state symbols under reaction and draw arrows pointing down to them from equation
- Write in all the formation data
3.Draw arrows from reactants to products side with arrows pointing in opposite direction reverse signs and finally add all values tgthr
How to construct and Enthalpy cycles for standard using standard Enthalpy change of combustion data?
Basically same thing just instead of elements alone write in the elements combusted and the arrows to them are opposite so from the equation downwards
What is average bond Enthalpy?
Energy needed to break a specific bond in its gaseous state
Why is it called average?
Because the Same type of bond may require a different amount of energy to break in a different molecule so an average is taken
Describe what process breaking and making bonds is?
Breaking = endothermic
Making= exothermic
Is bond Enthalpies the same as standard Enthalpy change?
No because bond Enthalpy is in the gaseous state whilst standard Enthalpy must be in standard states
