TOPIC 7 - Rates Of Reaction And Energy Processes Flashcards
Rate of reaction
A measurement of how fast the reaction happens.
How can rate of reaction be observed?
By measuring how quickly the reactants are used up/how quickly the products are formed.
Rate of reaction formula
Amount of reaction used up or product formed/
Time
How do you measure precipitation?
- mix 2 reactant solutions and put flask on paper that has a mark on it.
- Observe the mark through the mixture and measure how long it takes for mark to be obscured. Faster disappears= faster reaction.
Why is the observation of precipitation subjective?
As different people may not agree on exactly when mark disappears
How do you measure change in mass in a reaction?
- Measure solution using a mass balance . Quicker the solution drops in mass = faster rate of reaction
- Finished when mass stops changing
Safety precautions when measuring change in mass of a reaction.
Can put in fume cupboard
How do you observe the volume of gas given off?
Use a gas syringe, more gas given off during a set time interval= faster rate of reaction
Is finished when no more gas is produced
Safety precautions when observing volume of gas given off?
Use right size gas syringe
How do you measure how surface area affects rate?
Use gas syringe, stopwatch, marble chips and hydrochloric acid.
Measure gas given off at set time intervals and record in table
Repeat w same volume/concentration of acid as well as same marble mass but w more crushed marble.
Repeat w powder
Reaction to test for temp’s effect on reaction rate
Sodium thiosulfate and hydrochloric acid are heated in a water bath before mixing and placing over black mark.
Time how long it takes to disappear. (Turns yellow)
Can be repeated at different temps.
Quicker disappears= faster rate
Gradient
Change in y / change in x
What does the rate of a chemical reaction depend on?
Collision frequency (more successful= faster reaction ) Energy transferred during a collision (need to pass activation energy)
3 factors leading to an increased rate of reaction
TEMPERATURE (+temp = +rate)
CONCENTRATION (+concentration = +rate)
SURFACE AREA (+surface area = +rate)
Why does increasing temp increase rate of reaction?
Particles move faster so in turn, have more collisions.
Higher temp= increased energy of collisions
Why does increasing concentration increase rate?
If a solution is more concentrated, there’s more particles of a reactant in same volume so collisions are more likely.
In a gas, increased pressure crowds particles so collision frequency will increase
Why does increasing surface area contribute to a higher rate of reaction?
Increased surface area to volume ratio means the reactants have more area to work on/ collide w.
Catalyst
A substance which increases rate of reaction while remaining chemically unchanged.
How do catalysts work?
They decrease activation energy by providing an alternative reaction pathway w a lower activation energy.
Enzymes
Biological catalysts which catalyse chemical reactions in living cells.
Eg
Respiration, photosynthesis, protein synthesis
Exothermic reaction
A reaction which gives out energy to the surroundings, usually in form of heat and is shown by a rise in temp of surroundings
Endothermic reaction
One which takes in energy from surroundings, usually in form of heat. Is shown by fall in temp of surroundings
Reaction profile of an exothermic reaction
Products are at a lower energy than reactants
Reaction profile of an endothermic reaction
Products are at higher energy than reactants
Activation energy
The minimum amount of energy needed for bonds to break and for a reaction to start.
What’s the activation energy on a reaction profile?
It’s the energy difference between the reactants and the highest point on the curve.
How do you measure change In temperature of a reaction mixture?
Put polystyrene cup in beaker of cotton wool (insulation)
Add volume of first reagent to cup.
Measure initial temp.
Add volume of second reagent and stir.
Put lid on cup (reducing energy lost via evaporation)
Record maximum/minimum temp mixture reaches during reaction.
Calculate temp change
How do you measure temp changes when dissolving salts in water?
Adding salt to polystyrene cup of water and measuring change in temp when dissolved. Eg Ammonium chloride (ENDO) calcium chloride (EXO)
What type of reaction are neutralisation reactions?
Exothermic
Except ethanoic acid + sodium carbonate
What type of reactions are precipitation reactions?
Exothermic
What happens to bonds during exothermic reactions?
They form and energy is released
What happens to bonds during endothermic reactions?
They break and energy is supplied
Formula for overall energy change
Overall energy change = energy required to break bonds - energy released by forming bonds
Energy change during an endothermic reaction
Positive
Energy change in an exothermic reaction
Negative
Reversible reaction
One where the products can react w each other to produce the original reactants.
Example of reversible reaction
The Haber process
N2 +3H2 (><>) 2NH3
Where’s the nitrogen obtained from in the haber process?
The air (78% nitrogen)
Where’s the hydrogen obtained from in the haber process?
Hydrocarbon extractions (eg natural gas/oil)
How is the haber process carried out?
At 450°C w a pressure of 200 atmospheres and an iron catalyst
What the catalyst in the haber process?
Iron
Dynamic equilibrium
When the forwards and backward reactions occur simultaneously at the same rate, meaning the concentrations of the reactants and products won’t change.
What type of system can a dynamic equilibrium only take place in? Define it?
A closed system
One where none of the products/ reactants can escape
An equilibrium lying to the right
The concentration of products is greater than that that of reactants.
An equilibrium lying to the left
Concentration of reactants are greater than that of products
Le Chatelier’s principle
If there’s a change in concentration, pressure or temperature in a reversible reaction, the equilibrium will move to help counteract that change.
What happens when you decrease the temperature of a reaction in equilibrium?
The equilibrium will shift to the exothermic direction to produce more heat.
What happens when you increase the temperature of a reaction in equilibrium?
The equilibrium shifts to the endothermic direction to take in heat and so decrease temp.
What happens when you increase the pressure of a reaction in equilibrium?
The equilibrium will move towards the side that has fewer moles of gas to reduce pressure.
What happens when you decrease the pressure of a reaction in equilibrium?
The equilibrium moves towards the side w more molars of gas to increase pressure
What happens when you increase the concentration of reactants?
The equilibrium moves to the right to use up reactants and make more products
What happens when you increase concentration concentration of the products?
The equilibrium moves to the left to use up products and make more reactants
3 main elements/nutrients plants need
PNK?
Phosphorus
Nitrogen
Potassium
What do fertilisers do?
Increase growth/life processes and so increase crop yield.
Why are ammonia fertilisers at an advantage to traditional methods of fertilisers?
Their element compositions can be controlled and quantity.
Are soluble so can dissolve into soil easily
Ammonia + oxygen?
Nitric acid
Ammonium sulfate use
Fertiliser
Production of ammonium sulfate
NH3 (+) + H2SO4 (2-)»_space; (NH4)SO4
Lab production of ammonium sulphate characteristics
Raw materials previously prepared
Small scale
Minimal risk
Simple glassware equipment
Industrial manufacture of ammonium sulfate production
Ammonia from haber process and sulfúrico acid from contact process.
Large furnaces/tanks
Large scale
High concentrated acid w high pressure/temps
How much yield does 450°C produce in the haber process?
20-40%
How much yield does 200 atmospheres of pressure produce in the haber process?
20-40%
Pressure in haber process, why?
200 atmospheres
Any higher is dangerous/expensive
Temperature in haber process, why?
450°C
Any lower, reaction would be too slow
Catalyst in haber process?
Iron
Increases rate of reaction
Perfect conditions for haber process
100°C and 800 atmospheres > 90 % yield
