3.2 formulas and key concepts Flashcards
How can you tell if a chemical reaction has taken place?
- Colour change
- Effervescence
- Precipitate formed
- pH change
- Temperature change
What is meant by enthalpy?
The thermal energy stored in a chemical system.
What is meant by an exothermic reaction?
A reaction which more heat is given out than taken in
What is meant by an endothermic reaction?
A reaction which LESS heat if given out than taken in
Is bond BREAKING exothermic or endothermic?
Endothermic
Is bond MAKING exothermic or endothermic
Exothermic
How can you tell if a reaction is endothermic?
If more energy is required to break bonds than is released in making bonds, the reaction is endothermic
How can you tell if a reaction is exothermic?
If more energy is released in making bonds than is required to break the bonds, then the reaction is exothermic
What is meant by activation energy?
The minimum amount of energy required for a reaction to take place
What is meant by bond energy?
The amount of energy that is needed to break a covalent bond
What is meant by average bond enthalpy?
Breaking of 1 mol of bonds
What is meant by enthalpy change?
Heat energy transferred in a reaction of constant pressure
Formula for enthalpy change
Enthalpy change = Enthalpy taken in - Enthalpy taken out
What are the steps for working out enthalpy change? (using bond enthalpies)
1) Write out ALL the bonds in the equation
2) Work out the total energy taken in (bonds broken)
3) Work out the total energy given out (bonds made)
4) Subtract the energy out from energy in to calculate the enthalpy change
What unit is used to show an enthalpy reaction?
△H (kJ/mol)
What is the unit of an exothermic reaction’s enthalpy change
-△H (kJ/mol)
What is the unit of an endothermic reaction’s enthalpy change
△H (kJ/mol)
What are the three main types of enthalpy change?
- Enthalpy change of neutralisation
- Enthalpy change of formation
- Enthalpy change of combustion
Define enthalpy change of neutralisation
The enthalpy change when one mole of water is formed from a neutralisation reaction
Define enthalpy change of combustion
The enthalpy change when one mole of substance(what is being burnt) reacts completely with oxygen
Define enthalpy change of formation
The enthalpy change when one mole of compound is formed from its elements in their standard state
What are the standard conditions?
Pressure - 100kPa (1atm)
Room temperature - 298K (25 degrees celsius)
What formula is associated with enthalpy change?
Q = mc△T
Where
Q = Energy (Joules) m = mass of water/solution (grams)
c = 4.18 △T = Change in temperature
What steps are used to calculate enthalpy change? (using Q=mc△T )
1) Write out the formula required and every piece of key information provided by the text
2) Input numbers into formula
3) Divide by 1000 to convert energy to kJ from J
4) To work out enthalpy change use △H = (Q) / number of moles
What do you do to the calculation if the reaction is exothermic?
change the △H to -△H
What does Hess’s law state?
Hess’s law states that “the enthalpy change for a chemical reaction is independent of the route taken”
What is the enthalpy change of an element?
The enthalpy change of elements are 0, as you can’t form an element from an element
Which way do the arrows go in a combustion cycle?
Arrows go DOWNWARD (△cH)
Which way do the arrows go in a formation cycle?
Arrows go UPWARDS
When calculating Hess’s law what must you do?
Follow the arrows!
NOTE : If the first arrow goes down, follow it
If the 2nd arrow goes down, go AGAINST it (add a minus before the value of the product)
NOTE: If the first arrow goes upwards, go AGAINST it! (add a minus before the value of the reactants)
NOTE: Going against the arrow CHANGES the sign from a negative to a positive
Why is Hess’s law useful?
Hess’s Law is useful because it allows us to calculate enthalpy changes which we can’t carry out experimentally as it is too expensive and highly dangerous
Steps to calculate Hess’ law
1) Write the equation for the reaction whose enthalpy change you are trying to calculate
2) Then add the path for the alternative route around the reaction (with enthalpies you already know)
3) Follow the route to calculate the enthalpy change
Why is it important to control the rate of reaction?
Safety - A reaction that happens too quickly can be dangerous
Cost - It is important to find a rate of reaction that produces your product quickly but is not too expensive to achieve.
What are the 4 main factors that can affect rate of reaction?
Temperature
Catalyst
Surface area
Concentration
What must occur for there to be an increased rate of reaction?
- More frequent collisions - This will increase the speed of particles of have more particles present
- More successful collision - This gives particles more energy or lowers the activation energy
What does the collision theory state?
The collision theory states that for a reaction to occur particles have to COLLIDE
- They must collide WITH ENOUGH ENERGY for a reaction to occur
- They must also collide at the CORRECT ORIENTATION
How does the surface area affect the rate of reaction?
Using several smaller pieces of a substance instead of one large piece increases the surface area. This means more collisions can happen at one moment so the rate of reaction INCREASES
How does concentration affect the rate of reaction?
Using a more concentrated solution means there are more particles reactant in the solution. This means more particles can collide at one moment so the rate of reaction INCREASES
How does temperature affect the rate of reaction?
Using a higher temperature solution means the particles of reactant in the solution have more energy. This means more particles move faster and can collide more often with more force so the rate of reaction increases
How does the use of a catalyst affect the rate of reaction?
Using a catalyst means the particles of reactant in the solution need LESS energy to react. THis means more particles have enough energy to react so the rate of reaction INCREASES
How can we measure the rate of reaction?
- Measuring the time taken to produce gas
- Measuring how long it takes for the product to go cloudy
- Measuring how long it takes for the pH to change
- Measuring how long it takes for colour change to occur
What are two types of catalyst?
Heterogenous catalyst and homogeneous catalyst
What is a heterogeneous catalyst?
This involves the use of a catalyst in a DIFFERENT state from the reactants. Typical examples involve a solid catalyst being used with the reactants being either a gases or liquids
What is a homogeneous catalyst?
This has the catalyst in the SAME state from the reactants. Typically everything will be present as a gas, liquid or solid
How do catalysts work?
- Reactants adsorb onto the surface of the catalyst
- This weakens the bonds of the reactants
- They break and new bonds form
- The products diffuse away from the surface
Why are catalysts important?
Catalysts allow LOWER temperatures to be used so reduces energy consumption. This reduces the amount of fossil fuels that need to be burnt. This results in less CO2 being produced so the reaction becomes MORE sustainable
What are the benefits of catalysts?
Catalysts increase sustainability by:
Lowering temperature
Reducing energy demand
Reduces CO2 emissions
Greater atom economy!
What are the key characteristics of the boltzmann distribution curve?
- No molecules have 0 energy
- ALL molecules possess some energy
- Ea = Activation energy
- Molecules past the point of Ea, possess energies greater than he activation energy and will be involved in a successful collision
What goes on the axis for a Boltzmann distribution curve?
X axis - Energy
Y axis - Number of molecules
What does increasing the temperature do the Boltzmann distribution curve?
- Moves the curve to high energies (NOTE ; This is sideways not upwards- VERY IMPORTANT) This means that the curve gets FLATTER
- Increases the spread (curve gets broader and flatter
- Keeps SAME AREA under the curve
What does using a catalyst do the Boltzmann distribution curve?
When adding a catalyst, you have the SAME graph, it only lowers the activation energy (activation energy goes leftward)
What is a reversible reaction?
A reaction that can be reversed
What is an irreversible reaction?
A reaction that CANNOT be reversed
Which sign is used to represent a reversible reaction? (Give an example)
⇌
e.g. N2 + 3H2 ⇌ 2NH3
Which sign is used to represent a irreversible reaction? (Give an example)
→
e.g. 2Mg + O2→ 2MgO
Using a sign, represent a FORWARD reaction?
→
Using a sign, represent a BACKWARD reaction?
←
Describe what happens in closed system (link it with equilibrium)
In a closed system, a reversible reaction will reach a point of equilibrium where the forwards and backwards reactions are EQUAL. As long as conditions are constant, this equilibrium position won’t change
What is meant by Le Chatelier principle?
If conditions change, the position of equilibrium for a reaction will respond by also changing
What are the conditions that can cause “stress” that forces the other side to change?
- Concentration
- Temperature
- Pressure (gaseous state only)
What does Le Chatelier’s principle mean?
When a change is applied to a system in dynamic equilibrium, the system reacts in such a way as to oppose the effect of the change
Describe concentration’s effect on equilibrium
Talk about the effect of increasing and decreasing the concentration of BOTH the reactants and the products
Increasing concentration of REACTANT - More forward reaction occurs. The equilibrium position moves to the RIGHT
Decreasing concentration of REACTANT - More backward reaction occurs. The equilibrium position moves to the LEFT
Increasing concentration of PRODUCT - More backward reaction occurs. The equilibrium position moves to the LEFT
Decreasing concentration of PRODUCT - More of the forward reaction occurs. The equilibrium positions moves to the RIGHT
Describe the effect of temperature on equilibrium
Talk about the effect of increasing and decreasing the temperature of both an exothermic and an endothermic reaction
FOR EXOTHERMIC
Increasing temperature - More of the backward reaction occurs. The equilibrium position moves to the LEFT
Decreasing temperature - More the forward reaction occurs. The equilibrium position moves to the RIGHT
FOR ENDOTHERMIC
Increasing temperature - More of the forwards reaction occurs. The equilibrium position moves to the RIGHT
Decreasing temperature - More of the backward reaction occurs. The equilibrium position moves to the LEFT
Describe pressure’s effect on equilibrium
Talk about the effect of increasing and decreasing the pressure
Increasing pressure - The reaction responds to oppose this change and so more of the forward reaction occurs. The equilibrium position moves to the RIGHT
Decreasing pressure - The reaction responds to oppose this change and so more of the backward reaction occurs. The equilibrium position moves to the LEFT
V. important knowledge to know about dynamic equilibrium
1) When talking about equilibrium always use the term “RATE”
2) At equilibrium there are NO observable products
3) Adding a catalyst does NOTHING to the equilibrium point
What is Kc and what does it tell us
Kc = The equilibrium constant
It tells us about the position of the equilibrium
What is the formula for Kc?
Kc = [C] to the power of c x [D] to the power of d
ALL DIVIDED by
[A] to the power of a x by [B] to the power of b
Where A = First reactant
B = Second reactant (if there is one)
C = First product
D = Second product ( if there is one)
In the Kc formula what do the lower case letters represent?
The lower case letters represent the number of moles of each product or reactant.
If Kc is much higher than 1 what does it mean?
It means that the equilibrium point lies far to the RIGHT
If Kc is much lower than 1 what does it meant?
It means that the equilibrium point lies far to the LEFT
In an exothermic reaction, increasing temperature shifts equilibrium to the left, but what does it do to Kc
It decreases Kc
