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gcse Chemistry > Reversible Reactions > Flashcards

Reversible Reactions Flashcards

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1
Q

Yield definition

A

How much product is actually made in a chemical reaction

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2
Q

Reasons for not 100% yield (5)

A
  • Impure reactants
  • Unexpected reactants producing unexpected products
  • Reaction is reversible
  • Product is lost when transferring between pieces of apparatus
  • Product is lost during separation processes
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3
Q

How to calculate % yield

A

Write balanced equation
Use moles moles mass
Use % yield equation: product produced/ maximum product produced x100

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4
Q

Benefits of high yield

A

Maximises profit (by making as much product as possible in the quickest amount of time)

More environmentally friendly (less energy is required to make products so more crude oil is reserved)

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5
Q

What is a reversible reaction

A

A chemical reaction in which the products can be converted back into the original reactants

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6
Q

Example of reversible reaction (2)

A

Thermal decomposition of ammonium chloride
White fumes turn back into white solid

Carboxylic acid+alcohol 🔁 ester+water

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7
Q

Equilibrium definition

A

At equilibrium there is no change in the concentrations of the products and the reactants because the rate of the forwards and backwards reaction are equal. The reaction has not stopped.

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8
Q

What is Le Chatelier’s principle

A

The position of equilibrium shifts to try and oppose and cancel out changes imposed on the system

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9
Q

Factors affecting equilibrium (3)

A

Temperature
Concentration
Pressure

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10
Q

How does changing the temperature affect the equilibrium position

A

If the forward reaction is exothermic the temperature needs to decrease to increase the yield of the products.
Equilibrium shifts to the right to heat up as the system opposes the decrease.

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11
Q

How does changing the concentration affect the equilibrium position

A

Increasing concentration of one of the reactants shifts the equilibrium to the right (in the forwards direction) and increase the yield of the products.
System opposes the increase and tries to decrease the concentration by reacting more reactants together

Decreasing concentration of the product also shifts the equilibrium to the right

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12
Q

How does changing pressure affect the equilibrium position

A

If there are more moles of the products that the reactants, pressure needs to decrease to increase the yield of the products.
Equilibrium shifts to the right to increase the pressure and the system opposes the decrease.

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13
Q

Rate of the forwards reaction is…

A

Fastest at the start and slows down as the number of reactants particles decreases

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14
Q

Rate of backwards reaction…

A

Speeds up as more products are formed

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15
Q

How does adding a catalyst affect things

A

Does not affect the yield or the position of equilibrium as it affects the rate of forwards and backwards reactions equally

Speeds up the rate of reaction by providing an alternative pathway with a lower activation energy

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16
Q

Why is ammonia important

A

Source of nitrogen for plants so is put into fertilisers and also used for household cleaners

17
Q

Conditions for production of ammonia

A

450°C
200atm
Iron catalyst

18
Q

Describe the Haber process

A

H2 from methane at high temperatures, N2 from air by fractional distillation

Gas stream contains purified H2 and N2

Converter - H2 and N2 is heated to 450°C and compressed to 200atm

Reaction vessel - gases pass over iron catalyst

Separator - gases are cooled, ammonia is removed as a liquid (condenses)

Unreacted H2 and N2 are recycled back for the reaction vessel

19
Q

Why is this pressure chosen for production of ammonia

A

200atm

More moles of the reactants than products so high pressure

High pressure is expensive and risk of explosion so compromise to intermediate pressure

20
Q

Why is this temperature chosen for production of ammonia

A

450°C

Exothermic so low temperature

Low temperature means slow rate of reaction so compromise with intermediate temperature

Iron catalyst can help speed up the rate of reaction

21
Q

Equation for making ammonia

A

N2(g) + 3H2(g) -> 2NH3(g)

gcse Chemistry (8 decks)

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