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Chemistry > Topic 6 - The Rate and Extent of Chemical Change > Flashcards

Topic 6 - The Rate and Extent of Chemical Change Flashcards

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1
Q

What examples of there are of reactions at a slow rate

A

-Rusting of iron
-Chemical weathering (like acid rain damage to limestone building)

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2
Q

What examples are there of reactions at a moderate rate

A

-Metal magnesium reacting with an acid to produce a gentle stream of bubbles

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3
Q

What examples are there of reactions at a fast rate

A

-Burning
-Explosions which are faster and release a lot of gas (over in fraction of a second)

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4
Q

How do you find the speed of a reaction

A

-Recording the amount of product formed
-Or the amount of reactant used up over time

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5
Q

On rate of reaction graphs what does a steeper line mean

A

-The steeper line the faster the reaction

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6
Q

Why does the rate of a chemical reaction depend on collision theory

A

-The more collisions there are the faster the reaction is
-The energy transferred during a collision as particles have to collide with enough energy for the collision to be successful

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7
Q

What four things does the rate of reaction depend on

A

-Temperature
-the concentration of the reactant or pressure of gas
-The surface area
-Presence of a catalyst

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8
Q

How does temperature increase the rate of reaction

A

-When the temperature is increased, the particles all move faster
-When they move faster they will collide more frequently
-The faster they move the more energy they have, so more collisions will have enough energy to make the reactions happen

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9
Q

How does concentration/pressure increase the rate of reaction

A

-A more concentrated solution means there are more particles in the same volume of water
-When the pressure of a gas is increased it means that there’s a higher number of particles occupying the same space
-Collisions are more frequent

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10
Q

How does increasing the surface area increase the rate of reaction

A

-If one of the reactants is a solid breaking it up into smaller pieces will increase its surface area to volume ratio
-This means for the same volume of the solid the particles around it will have more area to work on so collisions will be more frequent

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11
Q

How do catalysts increase the rate of reaction

A

-They decrease the activation energy needed for the reaction to occur they do this by providing an alternative reaction pathway with a lower activation energy

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12
Q

What is a catalyst

A

-A substance that speeds up a reaction without being used up in the reaction itself

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13
Q

How do you calculate the rate of reaction (equation)

A

Rate of reaction = amount of reaction used or amount of product formed / time

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14
Q

What are the two units used to measure rate of reaction

A

-g/s if a solid
-cm^3/s if a gas

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15
Q

How can precipitation and colour change measure the rate of reaction

A

-You can record the visual change in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution
-If the reactants are coloured and the products are colourless you can time how long it takes for the solution to lose its colour

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16
Q

Outline the rate of reaction precipitation experiment

A

Observe a mark under the bottom of the solution and measure how long it takes for it to disappear - the faster the mark disappears the quicker the reaction

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17
Q

What is wrong with the precipitation and colour change experiment for finding rate of reaction

A

-The results are very subjective and different people might not agree on the exact point when the mark disappears or the solution changes colour
-cant plot a rate of reaction graph from the results

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18
Q

How can rate of reaction be measured by a change in mass

A

-Measuring the speed of a reaction that produces a gas can be carried out using a mass balance
-As gas is released the mass disappearing is measured on the balance
-The quicker the reading on the balance drops the faster the reaction

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19
Q

Can you plot a rate of reaction graph from measuring it through a change in mass

A

-If you take measurements at regular intervals you can plot a rate of reaction graph and find the rate quite easily

20
Q

What is the disadvantage of measuring rate of reaction by measuring the change in mass

A

-It releases gas into the room
-Only one disadvantage as most accurate method due to the accurate mass balance

21
Q

How is rate of reaction measured by the volume of gas given off

A

-Involves use of a gas syringe
-More gas given off during a given time interval, the faster the reaction
m

22
Q

Advantages of using the volume of gas given off as a way to measure rate of reaction

A

-Gas syringes usually give volumes accurate to the nearest cm^3 so they’re quite accurate
-Can take measurements at regular intervals and plot a rate of reaction graph using this method too

23
Q

Disadvantage of using the volume of gas given off to measure the rate of reaction

A

-Have to be careful as if reaction is too vigorous you can easily blow the plunger out of the end of the syringe

24
Q

Sum up the experiment reaction between Magnesium and Hydrochloride to produce H2 gas (6)

A
  1. Add a set volume of dilute HCl to a conical flask and place on a mass balance
  2. Add some magnesium ribbon to the acid and plug the flask with cotton wool
  3. Start the stopwatch and record mass on the balance and take readings at regular intervals
  4. Plot the results in a table and work out the mass lost for each reading, then plot a graph with time on the x-axis and loss of mass on y-axis
  5. Repeat with more concentrated acid solutions (only change this)
  6. The three graphs show that with a higher concentration of acid gives a faster rate of reaction
25
Q

Sum up the experiment between sodium thiosulfate and HCl to produce a cloudy precipitate (6)

A
  1. Add a set volume of sodium thiosulfate to a conical flask
  2. Place flask on a piece of paper with a black cross drawn on it and add some dilute HCl and start the stopwatch
  3. Watch the black cross disappear through the cloudy sulphur, time how long it takes to go
  4. The reaction can repeated with solutions of either reactants at different concentrations (only change one at a time) but the depth must be kept the same each time
  5. These show increasing the concentration of HCl on the rate of reaction when added to an excess of sodium thiosulfate
  6. The higher concentration the quicker the reaction and therefore less time it takes for the mark to disappear
26
Q

What is a disadvantage of the reaction between sodium thiosulfate and hydrochloric acid

A

It doesn’t give a set of graphs only readings of how long it took till the mark disappeared for each concentration

27
Q

What’s on the x-axis of a rate of reaction graph and what unit

A

Time taken in seconds

28
Q

What’s on the y-axis of the rate of reaction graphs

A

Amount of product formed or the amount of reactant used up

29
Q

How do you find the mean rate for the whole reaction by using a rate of reaction graph

A

work out the overall change in the y-value and the. divide this by the total time taken for the reaction

30
Q

What do you use a tangent for on a rate of reaction graph?

A

To find the rate of reaction at a particular point in time

31
Q

How do you use a tangent to find the rate of reaction at a particular point in time

A
  1. Position a rule on the graph at the point where you want to know the rate
  2. Adjust it until the space between the ruler and the curve is equal on both sides of the point
  3. Draw a line along the ruler to make the tangent extend the line right across the graph
  4. Pick two readings from the line and use them to calculate the gradient of the tangent to find the rate
32
Q

What is the equation to find a gradient

A

change in y / change in x

33
Q

How will the backward and forward reactions change as a chemical reaction goes on

A

-As the reactants react their concentrations fall so the forward reaction will slow down
-More and more product will be made and their concentrations rise, the backward reaction will speed up.

34
Q

What is the equilibrium of a reversible reaction

A

-When the forward reaction is going at the exact same rate as the backward one
-Both reactions are still happening but there’s no overall effect meaning the concentrations of reactants and products have reached a balance and won’t change
-Equilibrium is only reached if the reversible reaction takes place in a closed system

35
Q

What is a closed system

A

Meaning that none of the reactants or product can escape and nothing else can get in

36
Q

Why can the amounts of reactants and products not be equal at equilibrium

A

-The concentrations can be different

37
Q

What does is mean if an equilibrium lies to the right

A

The concentration of products is greater than that of the reactants

38
Q

What does it mean if the equilibrium lies to the left

A

-The concentration of reactants is greater than that of the products

39
Q

What different things can change the position of the equilibrium

A

-The temperature
-The pressure (only for equilibria involving gases)
-concentration of the reactants and products

40
Q

How does temperature effect the position of equilibrium of the reaction of ammonium chloride > ammonia + hydrogen chloride

A

Heating the reaction moves the equilibrium to the right (more ammonia and hydrogen chloride) and cooling it moves it to the left (more ammonium chloride)

41
Q

How are reversible reactions exothermic and endothermic and why

A
  • If the reaction is endothermic in one direction it will be exothermic in the other
    -The energy transferred from the surroundings by the endothermic reaction is equal to the energy transferred to the surroundings during the exothermic reaction
42
Q

Give an example of how endothermic and exothermic reactions work during a reversible reaction

A

-Thermal decomposition of hydrated copper sulfate
-hydrated copper sulfate > anhydrous copper sulfate + water
-If you heat blue hydrated copper sulfate crystals it drives the water off and leaves white anhydrous copper sulfate powder. This is endothermic.
-If you then add a couple of drops of water to the white powder you get the blue crystals back again. This is exothermic.

43
Q

What is Le Chatelier’s Principle

A

-The idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change

44
Q

What can be Le Chatelier’s Principle be used to predict?

A

The effect of any changes you make to a reaction system

45
Q

How does changing temperature affect equilibrium position

A

-If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat meaning you’ll get more products for the exothermic reaction and fewer for the endothermic reaction
-If you raise the temperature, the equilibrium will move in the endothermic direction to try and decrease it. You’ll now get more products for the endothermic reaction and fewer products for the exothermic reaction.

46
Q

How does changing the pressure affect equilibrium position

A

-Changing the pressure only affects an equilibrium involving gases
-If you increase the pressure the equilibrium tries to reduce it and it moves in through direction where there are fewer molecules of gas
-If you decrease the pressure, the equilibrium tries to increase it and it moves in the direction where there are more molecules of gas
-You can use the balanced symbol equation for a reaction to see which side has more molecules of gas

47
Q

How does changing concentration affect equilibrium position

A

-If you change the concentration of either reactants or products the system will no longer be at equilibrium
-The system responds to bring itself back to the equilibrium again
-If you increase the concentration of the reactants the system tries to decrease it by making more products
-If you decrease the concentration of products the system tries to increase it again by reducing the amount of reactants

Chemistry (9 decks)

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