Topic 7 - Rates of Reaction and Energy Changes Flashcards
What is the rate of a reaction?
How quickly a reaction happens
How can you observe the rate of reaction?
How quickly reactants are used up
How quickly products are formed
What is the rate of reaction equation?
Rate of Reaction = (Amount of reactant used or amount of product formed) / Time
What are the experiments to measure the rate of reaction?
Change in mass
Precipitation
Volume of gass given off
What is the precipitation experiment?
Using two see-through solutions to form a precipitate which clouds a mark
What is the problem with the precipitation experiment?
Subjective as different people might not agree on exactly when the mark disappears
How do you measure the rate of reaction that produces a gas?
By using a mass balance
When using change in mass to measure the rate of reaction, how do you tell the reaction is finished?
The mass balance stops changing
What risks are associated with measuring the volume of gas to calculate the rate of reaction?
The plunger may be blown out of the syringe if the reaction is too vigorous
Outline how you could use a mass balance to measure the rate of a reaction where a gas is formed?
Put a conical flask on a mass balance and add your reactants. As gas is produced from the reaction, measure how quickly the reading on the balance drops until the balance stops changing
Give one disadvantage of the precipitation method when used to follow the rate of a reaction?
No quantitive result (except time, no amount of reactant used or product formed)
Subjective meaning different people might not agree on exactly when the mark ‘disappears’
How can you measure how surface area affects the rate of reaction?
Add marble chips to dilute HCL and measure the gas released
Each time crunch the chips up more and more, increasing the surface area, whilst keeping the mass the same
What does finer particles of solid mean?
A higher rate of reaction
How can you increase the volume of gas produced in a reaction?
By increasing the mass of the reactants
How can you measure how concentration affects the rate of reaction?
Add marble chips to dilute HCL and measure the gas released
Each time increasing the concentration of acid
Describe how you could investigate how the surface area of calcium carbonate affects the rate of reaction between calcium carbonate and hydrochloric acid?
Place a measured volume of HCL of a known conc in a conical flask
Add a known mass of calcium carbonate in the form of marble chips
Immediatly add a gas syringe and take readings of the volume of gas produced
Repeat the experiment but with the calcium carbonate crushed
What reactants are used to investigate the effect of temperature on the rate of reaction?
Sodium thiosulfate
Hydrochloric acid
What do sodium thiosulfate and hydrochloric acid react to form?
A yellow precipitate of sulfur
Briefly describe the method of investigating temperature and reaction rate
Measure out fixed volumes of sodium thiosulfate and hydrochloric acid
Heat the reactants to a desired temperature and mix the solutions
Time how long it takes for the precipitation to make the black mark disappear
Repeat experiment at different temperatures
Briefly describe the method of investigating temperature and reaction rate
Measure out fixed volumes of sodium thiosulfate and hydrochloric acid
Heat the reactants to a desired temperature and mix the solutions
Time how long it takes for the precipitation to make the black mark disappear
Repeat experiment at different temperatures
When measuring how surface area affects ROR, what are the control variables?
The same volume and concentration of acid
The same mass of marble chips
When measuring how concentration affects ROR, what are the control variables?
The same mass and surface area of marble chips
The same volume of hydrochloric acid
What does a higher temperature mean?
A higher rate of reaction
Azim carries out an experiment to measure how temperature affects the rate of reaction between sodium thiosulfate and hydrochloric acid
He uses the time taken for a mark underneath the reaction vessel to be obscured as a measure of rate
How would you expect the time taken for the mark to disappear to change as the temperature of the reacting solutions were increases?
As the temperature of the reacting solutions increases, the time taken for the mark to disappear decreases
What must particles collide with to react?
Enough energy
What does the rate of a chemical reaction depend on?
The collision frequency of reacting particles
The energy transferrred during a collision
What is activation energy?
The minimum energy that particles need to react when they collide, so bonds can be broken and a reaction can start
The more collisions, the ____ the rate of reaction
The more collisions, the higher the rate of reaction
What are the three factors that affect the rate of reaction?
Temperature
Concentration
Surface Area
How does temperature affect the ROR?
Increasing the temperature will:
Cause particles to move faster causing a higher frequency of collisions therefore the ROR will increase
Give particles more energy causing a higher frequency of successful collisions therefore the ROR will increase
How does concentration affect the ROR?
Increasing the concentration will:
Mean there are more particles of reactant in the same volume meaning collision are more likely therefore the ROR will increase
How does pressure affect the ROR?
Increasing the pressure will:
Mean particles are more crowded meaning there will be a higher frequency of collisions therefore the ROR will increase
How does surface area affect the ROR?
Increasing the surface area will:
Mean there is more area for collisions to take place meaning there will be a higher frequency of collisions therefore the rate of collisions will increase
What does an increased frequency of collisions lead to?
An increased number of sucessful collisions
Describe the two factors, in terms of collisions, that affect the rate of reaction
The energy transferred during a collisions must exceed the activation energy
The collisions frequency
Explain why breaking a solid reactant into smaller pieces increases the rate of reaction
Breaking a solid into smaller pieces will increases the surface area to volume ratio meaning there will be a more area for reactions to take place meaning the frequency of collisions will speed up therefore the number of sucessful collisions will increase
What is a catalyst?
A substance which increases the rate of a reaction without being chemically changed or used up
Does a catalyst have an affect on the chemical equation?
No it will have no affect on the products or reactants
What is good about a catalyst?
It isnt used up meaning you only need a tiny bit to catalyse large amounts of reactants
How do catalysts work?
By lowering the activation energy by providing an alternative reaction pathway that has a lower activation energy
What are enzymes?
Biological catalysts
What are enzymes used for?
Respiration
Photosynthesis
Protein Synthesis
What do reaction profiles show?
The energy levels of the reactants and the products in a reaction
What do enzymes in fermentation do?
Catalyse the reaction that converts sugars into ethanol and carbon dioxide
The decomposition of hydrogen peroxide can be catalysed by manganese dioxide
Explain why only a small amount of manganese dioxide is needed for the catalysis of this reaction, even when starting with a large quantity of hydrogen peoxide
Catalysts are not used up or chemically changed in a reaction meaning you can use a small amount to catalyse a large quantity of reactant
What is an exothermic reaction?
A reaction which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature of the surroundings
What is an endothermic reaction?
A reaction which takes in energy from the surroundings, usually in the form of heat and usually shown by a fall in temperature of the surroundings
How can you tell if a reaction is exothermic?
There is a rise in the surrounding temperature
How can you tell if a reaction is endothermic?
There is a decrease in the surrounding temperature
What will a exothermic reaction profile look like?
The products will be at a lower energy than the reactants and the difference in height will represent the energy given out in the reaction
What will a endothermic reaction profile look like?
The products will be at a higher energy than the reactants and the difference in height will represent the energy taken in during the reaction
What is the activation energy on a reaction profile?
The difference between the peak and the reactant energy
A student carries out an experiment which results in a change in temperature of the reaction mixture.
They notice the reactants have less energy than the products, explain whether what type of reaction this must be?
Endothermic as the products have gained energy from the surroundings
How do you measure the temperature change because of a reaction?
Put a polystyrene cup into a beaker of cotton wool
Add a known volume of a reagent to the cup and measure the inital temperature of the solution
Add a measured mass of the second reagant and stir
Record the maximum and minimum temperature and then calculate the temperature change
Why is cotton wool used in measuring temperature change?
Because it gives insulation to help limit energy transfer to or from the reaction mixture
Why is a lid put on the apparatus when measuring temperature change?
To reduce energy lost by evaporation
What type of reaction is dissolving salt in water?
Endothermic and exothermic
What type of reaction is neutralisation?
Mostly exothermic
What type of reaction is endothermic?
Exothermic
What type of reaction is precipitation?
Exothermic
When measuring the temperature change of a reaction, why is it important to put the polystyrene cup in a beaker of cotton wool and to keep a lid on the cup?
They help to insulate the reaction mixture, limiting the energy transferred to or from the surroundings and by evapouration
What must be supplied during a reaction?
Energy must be supplied
What actually happens in a chemical reaction?
Old bonds are broken and new bonds are formed
What type of process is bond breaking?
Endothermic
What type of process is bond building?
Exothermic
Describe the energy from bond change for a endothermic reaction
More energy is used to break bonds than released by forming the bonds
Describe the energy from bond change for a exothermic reaction
More energy is released by forming bonds than used to break bonds
How do you remember the overall energy change equation?
Break and Entering
Broken - Erected
What is the energy change equation?
Energy change = Energy required to break bonds - Energy released by forming bonds
What is a positive energy change?
An endothermic reaction
What type of energy change is for an endothermic reaction?
A positive
What is a negative energy change?
An exothermic reaction
What type of energy change is for an exothermic reaction?
A negative
Using the bond energy values below, calculate the energy change for the following reaction, where hydrogen and chorine react to product hydrogen chloride
H-H 436
Cl-CL 242
H-Cl 431
H2 + Cl2 ->2HCl
H-H + Cl-Cl -> 2H-Cl
436 + 242 -> 862
678 - 862 = -184
N2 + 3H2 -> 2NH3 N=N 941 H-H 436 N-H 391 Calculate the overall energy change for the forward reaction
941 + 3(436) = 2249
6(391) = 2346
2249 - 2346 = -97
A student carries out a reaction which produces CO2
He collects the CO2 in a gas syringe
How will he know when the reaction has finished
The gas syringe stops moving (changing volume)
Explain how you could follow the ROR where two colourless solutions react to form a precipitate
Add solutions to conical flask above a mark that you can see from the birds eye view
Time how long it takes for the mark to disappear
When undergoing the precipitation reaction to measure the ROR, why must the solutions be colourless?
So you can see the mark before the reaction has begun
How does the rate of the reaction between sodium thiosulfate and hydrochloric acid change with temperature?
Increases
As particles have more energy meaning more sucessful collisions whilst also moving more meaning more collisions
Describe how you would find the rate of reaction from a straight line graph?
Find the gradient
What effect will raising the temperature have on the ROR?
Increase the ROR
What effect will raising the temperature have on the time taken for a reaction?
Decrease it
What graph is the graph showing temperature and time taken for a reaction?
The inverse proportion graph
How does concentration affect the rate of reaction?
Increases the rate of reaction
As there will be more reactants meaning there will be a higher frequency of collisions
In a gaseous reaction, why would a decrease in pressure result in a slower rate of reaction?
Because less pressure means the particles are less packed together meaning there will be less collisions therefore there will be a slower rate of reaction
What effect does a catalyst have on the activation energy needed for a reaction to take place?
The catalyst lowers the activation energy
Give two examples of reactions catalysed by enzymes
Any two from: Respiration Photosynthesis Protein synthesis Digestion
What change in temperature would you expect to observe in an exothermic reaction?
Increase in surroundings
Describe how you could measure the temperature changes in a neutralisation reaction?
Put polysteryene cup into beaker of cotton wool
Add known volume of alkali and acid and put lid on
Measure temperature change
Is energy required for the breaking of bonds or the forming of bonds?
The breaking of bonds
