Chemistry - THE RATE + EXTENT OF CHEMICAL CHANGE (C6)

Rates of Reaction -

What is the rate of a chemical reaction?

The speed at which reactants - converted into products.

Rates of Reaction -

How can you measure the rate of a reaction?

Measure change is mass, Volume of gas produced, Disappearing cross method (for cloudy reactions).

Rates of Reaction -

What is the formula for rate of reaction?

Rate = amount of reactant used / product formed / time.

Rates of Reaction -

What units are used for rate of reaction?

g/s (grams per second)

cm³/s (cubic centimetres per second)

mol/s (moles per second)

Collision Theory -

What is collision theory?

Reactions occur when particles collide w/ enough energy (activation energy).

Collision Theory -

What is activation energy?

The minimum energy particles need to successfully react.

Factors Affecting Rate of Reaction -

How does increasing temperature affect rate?

Increases particle energy → more frequent + energetic collisions → faster rate.

Factors Affecting Rate of Reaction -

How does increasing concentration ( / pressure) affect rate?

More particles in a given volume → more collisions → faster rate.

Factors Affecting Rate of Reaction -

How does increasing surface area affect rate?

More area for collisions → more frequent collisions → faster rate.

Factors Affecting Rate of Reaction -

What is a catalyst?

A substance that speeds up a reaction w/out being used up.

Factors Affecting Rate of Reaction -

How do catalysts work?

They provide an alternative reaction pathway w/ a lower activation energy.

Interpreting Rate Graphs -

What does a steeper slope on a rate graph mean?

A faster rate of reaction.

Interpreting Rate Graphs -

What does a flat line (horizontal) on a rate graph mean?

The reaction has finished - no more products are being made.

Interpreting Rate Graphs -

How do you calculate the rate from a graph?

Find the gradient: change in y / change in x.

Reversible Reactions -

What is a reversible reaction?

A reaction where the products can react to form the original reactants.

Reversible Reactions -

What symbol is used to show a reversible reaction?

⇌

Reversible Reactions -

Give an example of a reversible reaction.

The dehydration + hydration of copper (II) sulfate.

CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O

Reversible Reactions -

What does it mean if a reversible reaction is exothermic in one direction?

It’s endothermic in the other direction - energy is conserved.

Dynamic Equilibrium -

What is dynamic equilibrium?

The point in a closed system where the forward + reverse reactions occur at the same rate.

Dynamic Equilibrium -

What is a closed system?

A system where no substances can enter / leave, but energy can be transferred.

Le Chatelier’s Principle -

What is Le Chatelier’s Principle?

If conditions change in a reversible reaction at equilibrium, the system shifts to oppose the change.

Le Chatelier’s Principle -

What happens equilibrium if you increase temperature?

The system shifts in the endothermic direction to absorb heat.

Le Chatelier’s Principle -

What happens if you increase pressure in a gas reaction?

The system shifts to the side w/ fewer gas molecules.

Le Chatelier’s Principle -

What happens if you increase concentration of reactants?

The system shifts to make more products.

Disappearing Cross Practical -

What is the visible change in the disappearing cross reaction?

A cloudy sulfur precipitate forms, making the cross underneath the beaker disappear.

Disappearing Cross Practical -

What reaction is used in the disappearing cross method?

Sodium thiosulfate + hydrochloric acid → sulfur + other products

Disappearing Cross Practical -

What is the independent variable in this experiment?

The concentration of sodium thiosulfate.

Disappearing Cross Practical -

What is the dependent variable in this experiment?

The time taken for the cross to disappear.

Disappearing Cross Practical -

What are control variables in the disappearing cross practical?

Volume of reactants

Temperature

Size of beaker

Same cross + lighting conditions

Disappearing Cross Practical -

Why is this experiment considered subjective?

Because different people may judge when the cross disappears at slightly different times.

Measuring Gas Production Practical -

What type of reaction is used to measure gas production?

Magnesium + hydrochloric acid → magnesium choride + hydrogen gas

Measuring Gas Production Practical -

What is measured during this experiment?

The volume of gas produced at regular time intervals.

Measuring Gas Production Practical -

What apparatus can be used to collect the gas?

A gas syringe, / an inverted measuring cylinder in a water trough

Measuring Gas Production Practical -

What is the independent variable in the magnesium + acid reaction?

The concentration of hydrochloric acid.

Measuring Gas Production Practical -

What is the dependent variable?

The volume of hydrogen gas produced.

Measuring Gas Production Practical -

How do you calculate rate from this practical?

Rate - Volume of gas produced / time.