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GCSE Chemistry > Topic 7 - Rates Of Reactions And Energy Changes > Flashcards

Topic 7 - Rates Of Reactions And Energy Changes Flashcards

1
Q

What does collision theory explain ?

A

Collision theory explains how reactions happen and why they happen at different rates

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2
Q

What must collide for a chemical reactions to happen ?

A

Particles must collide for chemical reactions to happen.
Importantly, these collisions must happen with enough energy.
This minimum amount of energy is known as the activation energy

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3
Q

What is activation energy ?

A

The activation energy is the minimum energy with which particles must collide in order to cause a chemical reaction

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4
Q

How can you increase a rate of reaction ?

A

We can increase the rate of a reaction by increasing the number of collisions and/or the energy of reactant particles.

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5
Q

What are the 4 main factors that increase the rate of reaction ?

A

Temperature
Concentration
Pressure (for gases)
Surface area

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6
Q

What does collision theory state ?

A

The collision theory says that, in order for chemical reactions to happen particles must collide with sufficient energy

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7
Q

How do you plot reaction rates ?

A

Draw a tangent to the curve (straight line that touches the curve at just 1 point).
Work out the gradient of the tangent:
Rate of reaction = change in y / change in x

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8
Q

What is the equation for rate of reaction ?

A

Rate of reaction = change in y / change in x

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9
Q

How can you interpret graphs ?

A

Graphs showing mass, volume or concentration against time show us how different variables affect rate.
The steeper the gradient of the line, the faster the reaction.
If the reaction plateaus (levels out and stays constant) on the graph, the reaction is complete.
In this graph, we can see the reaction starts off with a steep gradient (fastest part of the reaction) and it slows down until it stops.

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10
Q

As the concentration of products increases, what happens to the concentration of reactants?

A

Decreases

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11
Q

If you increase the concentration of dissolved reactants what can this do to the rate of reaction

A

Increasing the concentration increases the collision frequency. This increases the rate of reaction

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12
Q

How can increasing the Pressure of gas reactants increase the rate of reaction ?

A

Increasing pressure is like increasing the concentration.

It has the same outcome of increasing the collision frequency, which increases the rate of reaction

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13
Q

If you increase the temperature what can this do to the rate of reaction ?

A

Increasing the temperature increases the rate at which collisions happen.
Increasing the temperature increases the energy of reactant particles. This means that a greater proportion of the particles will have more energy than the activation energy that is needed.
Together, these lead to more successful collisions. This increases the rate of reaction.

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14
Q

How can increasing the Surface area of solid reactants increase the rate of reaction ?

A

Increasing the surface area increases the collision frequency. This increases the rate of reaction.
This is often done by breaking up solids into smaller lumps (e.g. powders).
This increase in surface area to volume ratio makes sure that more particles are exposed to attack.

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15
Q

Substances that increase a chemical reaction’s rate without reducing in quantity during the reaction are called what ?

A

Catalyst

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16
Q

Limestone and marble structures crumble when reacted with acid. How will the recent increase in concentration of acid in rainwater affect the rate at which limestone and marble are being worn away?

A

Increased rate

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17
Q

How do catalysts increase the rate of reaction?

A

Catalysts increase the rate of chemical reactions by lowering the activation energy. They do this by changing the reaction pathway

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18
Q

Are catalysts used up in reactions?

A

Catalysts are not used up during chemical reactions. This means that:
They can be reused indefinitely.
They are not found in chemical equations.
But, catalysts will often need cleaning or regenerating, which has knock-on effects for energy expenditure and environmental impact.

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19
Q

What are some examples of catalysts?

A

Different catalysts are appropriate for different reactions:
Iron is used in the process that makes ammonia.
Platinum and palladium are used in catalytic converters in cars.
Enzymes catalyse reactions in biological systems.

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20
Q

What form do catalysts come in?

A

Catalysts often come as powders, pellets or fine gauzes because these types of substance have particularly high surface areas

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21
Q

What does rate of a chemical reaction tell us ?

A

how quickly reactants are converted into products.
In industry, reaction rates are very important.
Faster reaction rates may not always be better

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22
Q

In industry, making the required amount of product as cheaply as possible. Why is this ?

A

maximises all-important profit

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23
Q

Why are faster reactions often better then slow ?

A

Faster reactions are often better as they yield more product in a given amount of time.

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24
Q

What are the drawback of fast rates of reactions ?

A

It may be expensive to generate the conditions for very fast reactions.
There can be safety concerns associated with very fast reactions.
Therefore, reaction rates used in industry are the result of a trade-off between speed, cost and safety.

25
Q

how can we see how changing the surface area affects rate of reaction by measuring how much gas is produced when marble chips (calcium carbonate) of different surface areas react with hydrochloric acid ?

A

Make sure the measuring cylinder is full with water.
Place in a known mass of marble chips. The surface area is the independent variable.
Pour in a known concentration and excess volume of acid.
Start the timer.
Record volume of gas produced every 5 seconds. This is the dependent variable.
Plot a graph of time against volume of gas.
Draw a curve of best fit.
Calculate the gradient to find rate at different times.

26
Q

In the experiment measuring rate of reaction by gas being produced, what is the dependent variable ?

A

Volume of gas produced ever 5 seconds

27
Q

In the experiment measuring rate of reaction by gas being produced, what is the independent variable ?

A

Surface are of the marble chips

28
Q

What do we need an excess amount of when measuring production of gas ?

(When measuring the rate of reaction)

A

Acid

29
Q

How can we see how changing the concentration affects rate of reaction by observing colour changes in the reaction between sodium thiosulfate and hydrochloric acid ?

A

Make sure the x can be seen on a sheet of paper under the reaction vessel.
Place the solutions (except acid) in beakers.
Pour in acid with a known concentration (this is the independent variable) and press start on the stop clock.
Stop the clock when the cross cannot be seen. The time taken is the dependent variable.
Repeat for different concentrations of acid.
Plot a graph of time against concentration of acid.
Draw line of best fit.
Calculate the gradient to find how the rate changes per increase in concentration.

30
Q

When measuring the rate of reaction by a colour change, what is the dependent variable ?

A

Time taken for the cross to disappear

31
Q

When measuring the rate of reaction by a colour change, what is the independent variable ?

A

Concentration of acid

32
Q

To measure the rate of reaction by observing a colour change, what graph do we need to plot?

A

Time vs concentration of acid

33
Q

When do energy changes happen ?

A

Energy changes happen when chemical reactions take place.

34
Q

Changes in heat energy happen when what type of reactions take place ?

A

Displacement reactions
Neutralisation reactions
Precipitation reactions
Salt dissolving in water

35
Q

What happens in a displacement reaction ?

A

a more reactive metal takes the place of a less reactive metal

36
Q

What happens in a neutralisation reaction ?

A

There is a reaction between an acid and a base

37
Q

What happens in a precipitation reaction ?

A

When two solutions react to make an insoluble salt

38
Q

What happens when a salt dissolves in water ?

A

Ionic bonds in a salt break. The ions become more mobile (can move more) in solution.

39
Q

What happens in exothermic reactions ?

A

In exothermic reactions, energy from the reacting chemicals is transferred to the surroundings.

40
Q

What does an exothermic reaction often lead to ?

A

This often leads to an increase in temperature

41
Q

What are examples of exothermic reactions ?

A

Combustion
Hand warmers
Neutralisation
Oxidation

42
Q

How are hand warmers an example of an exothermic reaction ?

A

Disposable hand warmers use the energy released by iron oxidation.
Reusable hand warmers source their energy from the crystallisation of salt solutions.
Boiling the pack re-dissolves the crystals so that it’s ready to be activated once more.

43
Q

What happens in an endothermic reaction ?

A

In endothermic reactions, energy from the surroundings is transferred to the reacting chemicals.

44
Q

What often occurs from an endothermic reaction ?

A

decreases the temperature of the surroundings

45
Q

What are examples of endothermic reactions ?

A

Citric acid + baking soda
Thermal decomposition
Sports injury packs

46
Q

How are Sports injury packs an example of an endothermic reaction ?

A

When squeezed forcefully, ammonium nitrate and water mix in the pack, resulting in instant cooling.
The speed of this reaction makes these packs ideal for scenarios when ice is not immediately available.

47
Q

What are the reactants for the endothermic reaction used in sports injury packs?

A

Ammonium nitrate And water

48
Q

What measures the strength of chemical bonds ?

A

All chemical bonds have a “bond energy” that measures the strength of chemical bonds.

49
Q

How can we see if a reaction is endothermic of exothermic ?

A

By subtracting the total bond energies of the products from the total bond energies of reactants
(energy change in kJ mol-1)

50
Q

In an exothermic reaction, is more energy released by forming bonds or is more energy required to break bonds ?

A

A reaction is exothermic if the total energy released by forming bonds is more than total energy required to break bonds.

I.e. overall energy change of reaction < 0.

51
Q

In an endothermic reaction, is more energy released by forming bonds or is more energy required to break bonds ?

A

A reaction is endothermic if the total energy released by forming bonds is less than total energy required to break bonds.

I.e. overall energy change of reaction > 0.

52
Q

What is the most commonly used unit for measuring bond energies?

A

kJ mol^-1

53
Q

What’s the word equation for energy change ?

A

energy change = total bond energies of reactants − total bond energies of products

54
Q

In a reaction, the total bond energies of the reactants was 29kJ/mol. The bond energies of the products were 1kJ/mol, 4kJ/mol and 9kJ/mol. What was the energy change?

A

energy change = total bond energies of reactants − total bond energies of products

Substitute the values in.
EC = 29 − (1 + 4 + 9)

EC = 15 kJ/mol

55
Q

Is bond breaking exothermic or endothermic?

A

Endothermic

56
Q

Is bond making exothermic or endothermic?

A

Exothermic

57
Q

What do Reaction profiles show ?

A

the progress of reaction on the x-axis
energy level on the y-axis

The amount of energy contained within the reactants and the products.
The activation energy of a reaction.
The overall energy change that happens as a result of a reaction.

58
Q

What does the rate of reaction tell you?

A

How quickly reactants are turned into products

GCSE Chemistry (10 decks)

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