Enthalpy Flashcards
What is enthalpy change of reaction?
The enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation under STD conditions
What is enthalpy change of formation?
The enthalpy change when 1 mole of a substance is formed from its elements under STD conditions
What is enthalpy change of combustion?
The enthalpy change when 1 mole of a substance reacts completely with O2 under STD conditions
What is enthalpy change of neutralisation?
The enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water
What are the standard conditions?
100KPa/ 1atm
298K (25degrees)
1moldm*3
STD states - C= solid H= gas H2O= liquid
How do you calculate standard enthalpy of reaction?
Calc amount of heat lost/gained Calc no.of moles q/n /1000 to get kJ Choose correct sign (-/+)
What must you do if the coefficient of the reactant NOT in excess is not one?
Multiply your answer of standard enthalpy by the number
What is the equation for heat energy q (J)?
Mass of water X Specific heat capacity of water
(4.18Jg-1k-1) X Change in temp
What is enthalpy?
The heat that is stored in a chemical system
What is enthalpy change?
The difference in enthalpy between the products and reactants in a reaction
What is exothermic?
Chemical reaction that transfers heat energy from the system to its surroundings
What is endothermic?
Chemical reaction that transfers heat energy from its surroundings into the system
What happens to the temperature in an exothermic reaction?
It increases
What are examples of exothermic reactions?
Combustion
Oxidation
What happens to the temperature in an endothermic reaction?
It decreases
What are examples of endothermic reactions?
Photosynthesis
Thermal decomposition
What does an exothermic reaction profile look like?
Products have a lower enthalpy
What is the sign for an exothermic enthalpy change?
Negative
What does an endothermic reaction look like?
Products have a higher enthalpy
What is the sign for an endothermic enthalpy change?
Positive
What are the units for enthalpy change?
kJmol*-1
What is activation energy?
The minimum energy required to start a reaction by breaking the bonds
What does the reaction profile lines look like when activation energy is included?
They are curved
What goes on the axis of a reaction profile?
X- Progress of reaction
Y- Enthalpy
Which bit on an AE exo reaction profile tells you the AE and enthalpy change?
AE = Reactants line to curve
Enthalpy change = Reactants to products
Which bit on an AE endo reaction profile tells you the AE and enthalpy change?
AE = Reactants to curve
Enthalpy change = Reactants to products
Why would the standard enthalpy change of an element not be zero sometimes?
Because the element isn’t in its STD state
What is average bond enthalpy?
The average enthalpy change when one mole of gaseous covalent bonds are broken
What is bond enthalpy?
The energy required to break a given covalent bond averaged over a range of compounds
What is enthalpy change?
Heat energy change, measured at a constant pressure
How do you calculate bond enthalpies?
Highlight bonds broken + bonds made
Workout the bonds broken + bonds made
Workout enthalpy change = (bonds broken) - (bonds made)
Why would the answer of a bond enthalpy be different to one in the textbook?
Not calculated under STD conditions
Used in calculations as averages
What is Hess’ Law?
If a reaction can take place by more than one route, and the initial and final concentrations are the same, the total enthalpy change is the same regardless of route taken
Why is there no O2 value when calculating standard enthalpy change of combustion?
Since O2 can’t combust with O2
What are the factors that influence rate?
Concentration Pressure (if a gas) Temperature SA (if a solid) Presence of catalyst
When does a reaction take place?
When particles collide
What will happen if there is more successful collisions?
Faster rate of reaction
How do you calculate rate?
Change in conc/ Time
What are the units for rate?
moldm-3/s
How can rate of reaction be calculated from a graph?
From the gradient
dy/dx
What does rate of reaction measure?
How fast a reactant is being used up or how fast a product is being formed
What does it tell you about rate if the graph is steeper?
Faster rate
What is rate of reaction?
The change in concentration of a reactant or product in a given time
How does rate change between the start and the end of the reaction?
Start = fastest rate as each reactant at highest conc Middle = starts to slow as reactant being used up so conc decreases End = zero as reactant used up so conc doesn't change
What does the slope of the graph look like when the reaction has stopped?
It plateaus
What is collision theory?
A theory that states that two reacting particles must collide for a reaction to occur
What happens in most collisions?
Molecules collide but then bounce off and remain unchanged
What makes a collision effective?
Particles collide with correct orientation
Particles have sufficient energy to overcome activation energy barrier of the reaction
Why does increasing the conc increases rate?
Increases no. of particles in same volume
Particles closer together
So more frequent collisions
So more successful collisions
Why does increasing the pressure of a gas increase rate?
Increases conc of molecules As same no.of gas molecules occupy a smaller volume So closer together So collide more frequently So more effective collisions
What methods can be used to measure rate of reaction?
Monitoring the removal (decrease in conc) of a reactant
Following the formation (increase of conc) of a product
What properties can be used to measure rate?
Concentration
Gas volume
Mass of reactants/products
Colour
Why does increasing temperature increase rate?
Particles have more KE
So more frequent collisions
So more collisions have the Ae
If a reaction produces a gas, what two methods can be used to determine rate?
Monitoring the volume of gas produced at regular time intervals using gas collection
Monitoring the loss of mass of reactants using a balance
Why can these two methods (gas) be used?
Vol of gas produced + loss of mass are both proportional to change in conc of a reactant/product
What is breaking bonds?
Endothermic
What is making bonds?
Exothermic
Does bond breaking require or release energy?
Require
Does bond making require or release energy?
Release
What is a catalyst?
A substance that changes the rate of a chemical reaction without undergoing a permanent change. It lowers the activation energy of the reaction by providing an alternative reaction pathway
Is a catalyst used up in a chemical reaction?
NO
What may a catalyst do in a reaction?
May react with a reactant to form an intermediate
OR
Provide a surface on which the reaction can take place
What happens to a catalyst at the end of a reaction?
It is regenerated
What is a homogenous catalyst?
A catalyst that is the same physical state as the reactants
What does a homogeneous catalyst do?
Reacts with reactants to form an intermediate
Intermediate then breaks down to give the product + regenerate catalyst
What are some examples of homogeneous catalysts?
Used to make esters (H2SO4) both liquids
Used for ozone depletion (.Cl radical) both gases
What is a heterogeneous catalyst?
A catalyst that is a different physical state to the reactants
What state are heterogeneous catalysts normally?
Usually solids
What do heterogeneous catalysts normally come into contact with?
Gaseous reactants or solutions in reactants
What does a heterogeneous catalyst do?
Reactant molecules adsorbed onto surface of catalyst , where reaction takes place
Product molecules leave the surface of catalyst by desorption
What is desorption?
The release of an adsorbed substance from a surface
What is adsorption?
The process by which a solid holds molecules of a gas or liquid or solute as thin film
What does a catalytic convertor do?
Oxidises CO to CO2
Reduces NO to N
Why is a catalytic convertor good?
Converts harmful gases into less harmful products
What does a catalytic convertor do to any unburnt hydrocarbons?
Oxidises them into H2O + CO2
Where can a catalytic convertor be found?
Petrol vehicles
What does the catalyst in a catalytic convertor contain?
Platinum
Rhodium
Palladium
How does a catalytic convertor work?
Hot exhaust gases passed over heterogeneous catalyst
What does the catalytic convertor look like and why is this useful?
Honeycomb mesh
Provides large SA for reactions to take place
What are 4 advantages of using catalysts industrially?
Reduce energy requirements so less electricity or fossil fuels used
Reduces temperature needed
Uses less fossil fuels so cuts CO2 emissions
Makes product faster + uses less energy so cuts costs
What is the Boltzmann distribution?
Some molecules move slowly with low energy
Some molecules move fast with high energy
Most molecules have average speed + energy
This spread of molecular energies = Boltzmann distribution
What are the features of the Boltzmann distribution curve?
No molecules have zero energy = curve starts at origin
Area under curve = total no. of molecules
No maximum energy for molecule = curve doesn’t meet X-axis
What does the curve look like of a low temperature on a Boltzmann graph?
Stays the same and to the left
What does the curve look like of a high temperature on a Boltzmann graph?
Lower and to the right
What does the curve of a higher temperature on a Boltzmann graph tell us?
More molecules have energy greater than or equal to Ea
So greater proportion of collisions will lead to an increased rate
Collisions more frequent as molecules have more KE
But increased energy more important than increased frequency
What does the curve look like of the presence of a catalyst on a Boltzmann graph?
Curve doesn’t change
But provides an alternative roue with a lower activation energy
What does the curve of the presence of catalyst on a Boltzmann graph tell us?
Greater proportion of molecules exceed new lower Ea
More molecules now have a greater or equal energy to lower Ea
On collision more molecules react to form a product
So increased rate
What does the curve look like of a higher concentration on a Boltzmann graph?
Peak gets higher as increase no. of particles
Doesn’t shift left or right since amount of energy stays the same
What is on the X-axis of a Boltzmann graph?
Energy
What is on the Y-axis of a Boltzmann graph?
No. of molecules with given energy
