Topic 6-The Rate and Extent of Chemical Changes Flashcards
How can rate of reaction be calculated?
Rate of reaction=amount of reactant used/time
Rate of reaction=amount of product formed/time
What are the factors that affect the rates of chemical reactions?
Concentration Pressure Surface area Temperature Catalysts
What is the collision theory?
Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
What are catalysts?
They are substances that speed up chemical reactions without being changed or used up during the reaction
What do catalysts do?
They decrease the activation energy which increases the proportion of particles with energy to react
Catalysts provide a different pathway with a lower activation energy for a chemical reaction
What are three ways of measuring rate of reaction?
- Loss in mass of reactants
- Volume of gas produced
- Time for a solution to become opaque
Describe and explain the effect of increasing temperature on the rate of reaction
As the temperature increases, kinetic energy of the particles increases (more energetic collisions)
If they’re moving faster, they will collide more frequently
They are not directly proportional to each other
Describe and explain the effect of increasing concentration on the rate of reaction
If the concentration increases, there will also be more reactants meaning more frequent collisions meaning a faster reaction
Describe and explain the effect of increasing pressure of a gas on the rate of reaction
If you increase the pressure of reacting gases, it increases the number of gas molecules in the same volume and so it increases the frequency of collisions and therefore increases the rate of reaction
(Volume and pressure are inversely proportional to each other-increasing the volume slows down the reaction)
Describe and explain the effect of increasing surface area
If solid reactants are in smaller pieces, they have a greater surface area.
Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction
When is dynamic equilibrium reached?
In a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant
Describe Le Chatelier’s Principle
If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract change and restore the equilibrium.
What happens to the equilibrium when you change the concentration?
Increase-shifts to the opposite side
Decrease-shifts to the same side
What happens to the equilibrium when you change the pressure?
Increase-shifts to side with less moles
Decrease-shifts to side with more moles
What happens to the equilibrium when you change the temperature?
Increase-Shifts in endothermic direction
Decrease- Shifts in exothermic direction
Why does adding a catalyst not affect the equilibrium?
It just speeds up both forward and backward reactions equally. Equilibrium is achieved faster
What are the various units for the rate of reaction?
Can include g/s or cm^3 or mol/s
Generally, mass/time, volume/time, moles/time
Describe the method needed for the disappearing cross reaction
Use a measuring cylinder to put 10cm^3 of sodium thiosulfate solution into a conical flask
Place the conical flask onto a printed black cross
Next add 10^3 of hydrochloric acid into the conical flask
Swirl the solution and start a stopwatch
Look down the through the top of the flask-after a certain time, the solution will turn cloudy
Stop the clock when we can no longer see the cross
Carry out the experiment again using lower concentrations of sodium thiosulfate solution
What should you do after you conduct the disappearing cross reaction?
Repeat the whole experiment and calculate mean values for each concentration of sodium thiosulfate solution
Do not include any anomalous results when calculating a mean
What does it mean when a measurement is reproducible?
A measurement is reproducible if it can be repeated by another person or using a different technique or equipment and still get the same result
What is the equation for the disappearing cross reaction?
Sodium thiosulfate+hydrochloric acid—>sulphur (solid)
What does the sulphur do during the disappearing cross reaction?
The sulphur makes the solution go cloudy.
Scientists call this cloudiness turbidity
What is a hypothesis?
A proposal that could explain a fact or an observation. In science, a hypothesis must be testable
What is the problem when it comes to the disappearing cross method and how is it responded to?
The problem with the disappearing cross experiment is that different people have different eyesights
That means that some people can see the cross for longer than others so they may not get the same results
However because all the students use the same size printed cross, this problem may not be too great
