Equilibrium Flashcards by Mona Fazakas

What is equilibrium?

A state of rest or balance due to equal action of opposing forces

When forward and reverse action occur at the same rate

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What is collision reaction theory?

Molecules must be arranged in correct orientation in order to react

There must also be enough energy for the reaction to occur

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What are the four conditions for a system at equilibrium?

Constant macroscopic (observable) properties
Closed system at constant temperature
Forward and reverse reaction rates are equal
Equilibrium can be approached from either direction (forward or reverse)

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What is percent yield?

The amount of product you get at equilibrium compared to the maximum amount you could get

A quantitative reaction has a percent yield of 100%

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What are the steps for calculating percent yield?

Balance equation

Use stoichiometry to determine expected outcome

Calculate percent reaction using out/in formula

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What does a percent yield less than 50% mean?

Reactants are favoured (left shift)

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What does a percent yield greater than 50% mean?

Products favoured (right shift)

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What does a percent yield greater than 99.9% mean?

The reaction is quantitative

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What are the steps for writing an ICE table?

Write the reaction equation
Set up an ICE (Initial, Change, Equilibrium) and fill in what you know
Calculate the change of one of the reagents
Use stoichiometry to calculate change for the other reagents
Complete the table

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What is an equilibrium constant (Kc)?

Kc is a constant value we can assign to reaction to show the relationship between products and reactants

If Kc > 1, then the products are favoured in the equilibrium

If Kc < 1, then the reactants are favoured in the equilibrium

We can ignore the concentrations of pure solids and liquids

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What will change the Kc value of an equilibrium?

The systems temperature (if temperature changes, Kc changes)

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What won’t change the Kc value of an equilibrium?

The reagent concentration

Any catalyst present

The time taken to reach equilibrium

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What is Le Chatelier’s Principle?

When a chemical system at equilibrium is disturbed by a change in property, the system will work to oppose the change until equilibrium is restored

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What happens to a chemical reaction if there is an increased concentration of a substance?

There is a shift away from the substance that is changed because the extra concentration needs to be used up

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What happens to a chemical reaction if there is an decreased concentration of a substance?

There will be a shift towards the substance because you need to produce more of the substance to make up for what was removed

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What happens to a chemical reaction if there is an increase in the pressure of the system?

There is shift towards the side with fewer moles of gas because there’s less volume

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What happens to a chemical reaction if there is a decrease in the pressure of the system?

There is a shift towards the side with more moles of gas because there is an increase in volume

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What happens to a chemical reaction if there is an increase in the temperature of the system?

There is a shift away from the heat/energy because this extra energy needs to be used up

What happens to a chemical reaction if there is a decrease in the temperature of the system?

There is a shift towards the heat/energy because more energy needs to be produced to make up for the loss

What happens to a chemical reaction if there is a catalyst added to the system?

There is no shift because the rates of both the forward and reverse reactions are increased by the same amount.

What is an endothermic reaction?

In an endothermic reaction, heat can be thought of as a “reactant”

Heat enters the system

Has a positive enthalpy (H) value

What is an exothermic reaction?

In an exothermic reaction, heat can be thought of as a “product”

Heat leaves the system

Has a negative enthalpy (H) value

What is the Bronsted-Lowry definition of acids and bases?

An acid is a proton diner in a specific reaction

A base is a proton acceptor in a specific reaction

What does amphiprotic mean?

It is a substance that can act as either an acid or a base

What are strong electrolytes in a complex reaction?

They dissolve and ionize (break down) Include ionic salts, strong acids, and strong bases

What are weak electrolytes in a complex reaction?

They do not dissociate and ionize They are weak acids and weak bases

What are the steps for predicting acid-base reaction?

1. List all entities as they would appear in the solution (include water) 2. Identify each substance as a potential acid or base 3. Use data booklet to determine the strongest acid and base 4. Write the reaction equation showing H+ transfer 5. Predict which side of the equation is favoured

What is the Acid Ionization Constant (Ka)?

It indicates the extent to which an acid will react with water It is the ratio of the dissociated form of the acid to the undissociated form

What are the two calculations you make using Ka?

1. Be able to calculate Ka when given H3O, may need to use an ICE table 2. Be able to calculate H3O when given Ka, check for the rule of 1000

What is the rule of 1000?

If the initial acid is 1000 times greater than Ka then assume initial concentration is the same as equilibrium concentration

How can you convert from Ka to Kb?

Kw = Ka*Kb Kb = Kw/Ka

How do you determine which reaction will be dominant when a substance is amphoteric?

You compare Ka and Kb values The higher K value will tell you which reaction dominants and whether the substance is an acid or a base

What is titration?

The slow addition of a titration (which is a solution of known concentration) to a known volume of another solution of unknown concentration

What is buffering?

Buffering occurs because initially, any acid added immediately react with the excess hydroxide and is converted to water

What is the endpoint?

The endpoint is the observable colour change in a titration?

What is the equivalence point?

Is when the amount of acid and base are precisely chemically equal

What is an indicator?

An indicator is a weak Bronsted-Lowry, conjugate acid base pair that is formed with the dye dissolved in water

What is a weak acid/weak base titration?

They do not have detectable equivalence point, so pH curves are normally not done for those reactions

What is a strong acid/strong base titration?

It has a pH of 7 at the equivalence point There is a wide range of useful indicators

What is a strong acid/weak base titration?

It has a pH < 7 at equivalence point Chose an indicator that changes colour at a lower pH

What is a weak acid/strong base titration?

Has a pH > 7 at equivalence point Chose an indicator that changes colour at a higher pH

What is a polyprotic substance?

An ion that can accept or donate multiple protons For every proton transferred by a polyprotic substance, the strength of the new acid or base entity formed greatly decreases

What are buffers?

Buffers allow for a solution to resist change in pH when a strong acid or base is added Buffers involve a weak conjugate acid-base pair

What is buffering capacity?

It is the limit of the ability of a buffer to maintain pH level