Energetics Unknown Information

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  • Created by: Lotto65
  • Created on: 04-04-18 15:20
If a 100g block of iron and a 50g block of iron have the same temperature, which has a higher internal energy and why?
The 100g block because there is more of it
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If you have a 100g block of iron and a 50g block of iron, which has a higher average kinetic energy of particles?
Neither they both have the same because they are the same temperature!
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An endothermic reaction relates to a decrease in what?
Internal energy of the system
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In an endothermic reaction, what is the heat energy taken in converted into?
Internal energy
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Is it possible to measure the enthalpy of a system?
No only enthalpy change
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Combustion is what sort of chemical reaction?
Exothermic (new products are formed from the combination of a hydrocarbon and oxygen)
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Why is there no scale on an enthalpy level diagram?
No way of measuring initial and final enthalpy of a system
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A negative sign with an enthalpy change means...
A decrease in enthalpy
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Enthalpy of reaction =...
Enthalpy of products - enthalpy of reactants
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What sort of reaction is the reaction between nitrogen and oxygen to form nitrogen (II) oxide?
Endothermic
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Why do we use standard conditions?
To make enthalpy changes comparable and transferable
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What are the units for enthalpy change?
kJmol-1
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If we used half as many moles of reactants as was in the chemical reaction, how would you change the enthalpy change?
Halve it
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Give two methods of measuring enthalpy changes
Specific heat capacity and calorimetry
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When temperature changes are considered, what does 1K =?
1 degrees Celcius
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What are some units for specific heat capacity?
Jg-1K-1 or Jg-1C-1 or kJkg-1K-1 or Jkg-1K-1
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In the equation q=mcdeltaT, what is q?
The heat energy required for the constraints in the equation
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Which substances are more difficult to heat, one with a higher or lower specific heat capacity?
Higher
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How can specific heat capacity be applied to a cooling substance?
When a 1g substance cools by 1K, the same amount of joules are given out as the specific heat capacity
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What is the principle of calorimetry?
The heat given out by a substance can be used to heat another substance of known specific heat capacity and mass so q=mcdeltaT can be used
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What is the specific heat capacity of water?
4.18 Jg-1C-1
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The heat given out in a calorimetry experiment not only heats the water. What else does it heat?
The can containing the water and the surrounding air
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How could you improve the systematic errors in a calorimetry experiment to ensure the water is heated properly?
Draught exclude the can to prevent convection currents. Insulate the can to reduce heat loss. Calculate the specific heat capacity of the can and take that into consideration
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What are other major issues with calorimetry experiments as well as heat loss?
Incomplete combustion which gives out less heat and evaporation of water and alcohol
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What is a bomb calorimeter?
Calorimeter is heavily insulated and flame is ignited electronically with plentiful supply of oxygen
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The enthalpy change of neutralisation is always...
Exothermic
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What does infinite dilution mean?
Any further dilution of the solution produces no further enthalpy change
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The enthalpy change of solution is...
Exothermic or endothermic
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What do we assume 200cm3 has a mass of?
220g - assume density is the same as water where 1cm3 has mass of 1g
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When calculating enthalpy of neutralisation, what major assumption is made other than density of solutions?
The solution has the same specific heat capacity as water because it is composed of mostly water
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Why might the literature values for enthalpy changes be different to experimental values?
Assumptions made about density and specific heat capacity. Heat lost to surroundings and calorimeter. Specific heat capacity of polystyrene cup not taken into account
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If you use half the mass as you did before, how does the temperature change in calorimetry?
It doesn't! You use half the volumes of reactants but only half the reaction mixture is being heated so the temperature change is the same
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If you add a solid to a liquid in an enthalpy change experiment, what do you use as the mass in the q equation?
Just the mass of the liquid because heat is transferred to it
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What do you use mass of in the q equation?
Whatever heat is transferred to
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What is an issue with endothermic reactions and calorimetry?
Vigorous stirring greatly improves the experiment because substances could be slow to dissolve and therefore take in energy from the surroundings in more time so the temperature drop is not as large
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What is another issue with dissolving enthalpy reactions?
Mass of substance dissolved not taken into account when calculating heat energy
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Why do you have to extrapolate graphs in enthalpy reactions?
Reactions do not occur instantaneously so sometimes the reaction mixture heats up but it is also cooling at the same time. Extrapolating calculates an estimate for the temperature change that would have occurred if a reaction was instantaneous
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What do we have to consider with extrapolation?
It is a matter of judgement and could introduce errors
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Why do we use Hess' law?
For some experiments where it is not possible to design them to calculate the enthalpy change
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Why does Hess' law work?
The route between two substances cannot affect the amount of enthalpy those substances have so the enthalpy is constant for a certain set of conditions
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When you add something to both sides of the top equation in a Hess' law cycle, what happens to the enthalpy change?
Does not change if the same thing is added to both sides
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Equations for the enthalpy change of formation show the formation of how much product?
1 mole
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Why is graphite often used as the carbon source for formation reactions?
Most stable form of carbon
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What is the enthalpy change for the formation of a substance in its standard state and why?
Zero because no heat energy is taken in or given out when one mole of an element in its standard state is formed
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What must always be included in enthalpy change equations? Why?
State symbols because the state of substances affects the enthalpy change of the reaction
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What are enthalpy change of formation reactions?
Can be either exothermic or endothermic
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Enthalpy change has the same units as...
Gibbs free energy
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What is the first thing to do before setting up a Hess' law cycle?
Write out the equations if not done for you
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How can a general equation for Hess' law involving enthalpies of formation be given?
The sum of the enthalpies of formation of products - the sum of enthalpies of formation of reactants
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Is 1/2H2 correct in a bond enthalpy equation for HCl?
No H(g) is correct because that represents one mole of hydrogen while 1/2H2 represents half a mole of H2 molecules
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What state do bond enthalpies only apply to?
Gaseous states
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Why does this reaction not represent bond enthalpy? Br2(l) -->2Br(g)
This equation can be split into two stages: One is the enthalpy of vaporisation forming gaseous bromine molecules and the other is the bond enthalpy to produce single atoms of bromine
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The reaction Br2(l) --> 2Br(g) is what sort of reaction?
Atomisation
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Are bond enthalpies exothermic or endothermic?
Endothermic because it is about breaking bonds
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Why might there be some inaccuracies in calculating bond enthalpies?
The values in table for bond enthalpies are averages because e.g. the C-H bond in different compounds has a different bond energy
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When are bond enthalpy calculations most accurate?
When making or breaking a few bonds as possible
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What is a general equation for calculating bond enthalpies?
Sum of bonds broken (reactants) - sum of bonds made (negative sign relates to exothermic bond making reaction already so input all values as positive)
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If you had 4 C-H bonds on one side of the reaction and 4 C-H bonds formed in the products, do you need to worry about these bond enthalpies when calculating the bond enthalpies of a reaction?
No
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Is using the enthalpy change of combustion or formation (experimental data) more or less reliable than using bond enthalpy data for a reaction?
More
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Why is experimental data more reliable than bond enthalpy data?
Experimental data is specific to particular substances while bond enthalpies are averages
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What sort of bond is present in an oxygen molecule?
Double
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Why might a reaction be more exothermic or endothermic?
If there are stronger or more bonds on one side of the equation
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Why are bond enthalpies only related to gases?
In solids and liquids we would then have to think about intermolecular forces and convert solids and liquids to gases before using bond enthalpies
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For calculating the bond enthalpies of liquids, what other enthalpy reaction is needed?
Enthalpy of vaporisation
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In an endothermic energy level diagram, what is a higher/ less stable?
Products
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What are the axes on an energy level diagram?
X axis = Reaction coordinate. Y axis = potential energy
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What is higher/ less stable in an exothermic energy level diagram?
Reactants
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What wavelengths of UV light do oxygen molecules absorb to break their bonds?
Less than 242nm (higher energy)
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What wavelength of UV light do ozone molecules absorb to break the bonds?
Less than 330nm (lower energy)
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Why do oxygen molecules absorb higher energy UV light than ozone molecules?
Double bond between oxygen atoms is stronger than the bond between ozone oxygen atoms
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What spectrum of UV light is needed to break bonds in oxygen molecules?
UV-C
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What spectrum of UV light is needed to break bonds in ozone?
UV-B
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What sort of reaction is standard enthalpy change of atomisation?
Endothermic
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In an equation for enthalpy change of atomisation, what must you ensure is formed?
Only one mole of the gaseous atoms from their element
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What states do ionisation energy and electron affinity involve?
Gases
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What type of reaction are ionisation energies?
Endothermic
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What sort of reaction is first electron affinity? Why?
Exothermic because it is favourable to bring an electron from infinity to where it feels attraction from the nucleus
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What sort of reaction is second electron affinity? Why?
Endothermic because it is unfavourable to add an electron to an ion so more energy must be added
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If lattice enthalpy is defined as the breaking of a lattice, what sort of reaction is it?
Endothermic
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If lattice enthalpy is defined as the making of a lattice, what is it defined as?
Exothermic
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In a Born-Haber cycle, what does an upward arrow suggest?
Endothermic reaction
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In a Born-Haber cycle, what does a downwards arrow suggest?
Exothermic
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In a Born-Haber cycle, what does an upward arrow suggest?
Endothermic reaction
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In a Born-Haber cycle, what does a downwards arrow suggest?
Exothermic
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In Born-Haber cycles, what type of reaction is lattice enthalpy?
Endothermic (broken apart into constituent gaseous ions)
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What goes at the very bottom of a Born-Haber cycle?
The chemical formula for the ionic compound
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What is the order of reactions in a Born-Haber cycle starting at the enthalpy change of formation?
Enthalpy change of formation, atomisation of metal, ionisation energy (ies) of metal, atomisation of non-metal, electron affinity (ies) of non-metal, lattice enthalpy
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Which equations are the only exothermic equations in a Born-Haber cycle?
Standard enthalpy change of formation, standard enthalpy change of 1st electron affinity
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In Born-Haber cycles like the formation of MgCl2, what must be done to the atomisation and first electron affinity calculations for chlorine?
Multiply them by 2 because 2 chlorine atoms involved
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The higher the charge on the ions... Why?
The greater the lattice enthalpy because they attract each other more strongly
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What is proportional to the product of the charges on ions in a lattice?
The force between ions
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The smaller the ions... Why?
The larger the lattice enthalpy because the distance between nuclei is smaller so the stronger the attraction
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What causes a larger effect on lattice enthalpy?
Charge on ions as opposed to size of ions
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A high lattice enthalpy means...
A high melting point
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What are the two stages of dissolving an ionic compound?
1) lattice enthalpy 2) standard enthalpy change of hydration
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Hydration enthalpies are always...
Exothermic
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What is the lattice enthalpy always in the enthalpy of hydration?
Endothermic
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Enthalpy change of solution =...
Lattice enthalpy + enthalpy change of hydration for positive ions + enthalpy change of hydration for negative ions
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What must be done to the enthalpy of hydration for the enthalpy of solution of MgCl2?
Multiply it by 2
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The bigger the ion... (enthalpy change of hydration) Why?
The smaller the enthalpy of hydration because weaker electrostatic attraction between ions and water
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The larger the charge... (hydration enthalpy) Why?
The greater the hydration enthalpy because stronger electrostatic attraction with water molecues
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What is more significant in deciding which ions have a higher hydration enthalpy?
Charge of ions
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Can you calculate entropies from experimental data?
Yes unlike enthalpies
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A decrease in number of moles of gas means...
A decrease in disorder
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To determine if there has been an increase or decrease in level of disorder, what can you look at?
The change in the number of moles of gas
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What if the number of moles of gas is the same on both sides of the equation?
No entropy change
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Entropy change =...
Total standard entropy of products - total standard entropy of reactants
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Does a spontaneous reaction have to happen quickly? Why?
No because it relates to if an outside influence is needed or not not the speed
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What is more useful; low entropy or high entropy? Why?
Low entropy because it is concentrated and can bring about a change
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Where will a system move in terms of entropy?
Low to high entropy
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In an exothermic reaction, what happens to the entropy of the surroundings?
Increases
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In the delta G = Delta H -T delta S equation, what is Delta S?
Entropy change of the universe
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When calculating the temperature at which a reaction will become spontaneous, what must we consider?
The values of enthalpy change and entropy change alter with temperature but to do the calculations, you may continue to use the values at a certain temperature so the calculation is only approximate
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How could you make a reaction spontaneous that isn't?
Electrolysis with a continuous passage of electric current
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Will an exothermic reaction ever be spontaneous?
Yes at a certain temperature
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Reactions with a positive deltaS...
Become more spontaneous as temperature increases
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Reactions with negative deltaS...
Become less spontaneous as temperature increases
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An equilibrium mixture always has a lower Gibbs free energy than what?
Pure reactants or pure products
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The formation of equilibrium is what?
Negative Gibbs free energy
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Equilibrium marks what?
Maximum entropy and minimum value for Gibbs free energy (not change of Gibbs free energy)
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What does Gibbs free energy depend on?
Amount of each substance present
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What is equal at equilibrium?
Gibbs free energy of reactants and products
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What is the value of free energy change at equilibrium?
0 - no desire to move towards reactants or products
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If free energy change is negative, where does equilibrium lie?
Closer to products (reaction is spontaneous and proceeds in forwards direction)
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If free energy change is positive where does equilibrium lie?
Closer to reactants (non-spontaneous and hard to proceed in forwards direction)
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On a composition versus Gibbs free energy graph, what is free energy change?
The difference between the Gibbs free energy of reactants and products
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In a composition versus Gibbs free energy graph showing the minimum closer to products, what is the value of Gibbs free energy change going down each slope towards the minimum point?
Gibbs free energy change is negative on both slopes
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Other cards in this set

Card 2

Front

If you have a 100g block of iron and a 50g block of iron, which has a higher average kinetic energy of particles?

Back

Neither they both have the same because they are the same temperature!

Card 3

Front

An endothermic reaction relates to a decrease in what?

Back

Preview of the front of card 3

Card 4

Front

In an endothermic reaction, what is the heat energy taken in converted into?

Back

Preview of the front of card 4

Card 5

Front

Is it possible to measure the enthalpy of a system?

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Preview of the front of card 5
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