C5 ocr chemistry

5.1 – Concentration Calculations (g/dm^3)

How do you calculate the concentration of a solution in g/dm^3?

Concentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³).

How do you convert 250\ cm^3 to dm^3?

250\ cm^3 \div 1000 = 0.25\ dm^3

If 5\ g of salt is dissolved in 250\ cm^3 of water, what is the concentration in g/dm^3?

5 \div 0.25 = 20\ g/dm^3

5.2 – Titration Practical

What is the purpose of a titration?

To find the exact volume of one solution that reacts with a known volume of another solution.

What is an indicator used for in titrations?

To show the end point of the reaction by changing colour.

Name the steps for a titration.

1. Rinse equipment

• Rinse burette with acid, pipette with alkali, flask with water.

2. Fill the burette

• Fill above 0.00 cm³ using a funnel and remove funnel after filling.

3. Measure the alkali

• Use a pipette to measure exactly 25 cm³ into the flask.

4. Add indicator

• Add 2–3 drops of indicator.

• Place flask on a white tile.

5. Titrate

• Add acid slowly while swirling, drop by drop near endpoint.

6. Record reading

• Read bottom of meniscus at eye level.

7. Repeat & calculate

• Repeat until concordant results; calculate mean.

5.3 – Gas Calculations

How do you calculate the volume of gas at room temperature and pressure (RTP)?

Volume\ (dm^3) = moles \times 24\ dm^3

How many moles of gas are in 48\ dm^3 at RTP?

48 \div 24 = 2\ moles

What is the formula linking moles (n), volume (V), and molar volume?

n = V \div 24 (where V is in dm^3 at RTP)

5.4 – Atom Economy

What is atom economy?

The percentage of reactant atoms that end up in the desired product.

What is the atom economy formula?

(\frac{Mr\ \text{of desired product}}{\text{sum of } Mr\ \text{of all reactants}}) \times 100

Why is high atom economy important?

It reduces waste, saves money, and is better for the environment.

5.5 – Percentage Yield

What is percentage yield?

(\frac{\text{Actual yield}}{\text{Theoretical yield}}) \times 100

Define actual yield.

The amount of product actually made in an experiment.

Why is the actual yield usually less than the theoretical yield?

Because of losses, incomplete reactions, or side reactions.

5.6 – Rates of Reaction

How is the rate of reaction calculated?

Rate = \frac{\text{amount of reactant used}}{\text{time}} OR Rate = \frac{\text{amount of product formed}}{\text{time}}.

What does a fast rate of reaction mean?

Reactants are being used up quickly and products are being made quickly.

Give factors that affect the rate of reaction.

• Temperature

• Concentration

• Surface area

• Catalyst

5.7 – Measuring Rates from a Graph

How do you find the rate from a concentration-time graph?

Rate = gradient\ of\ the\ line\ (\frac{\Delta concentration}{\Delta time})

What does a steeper line on a concentration-time graph mean?

A faster rate of reaction.

How do you find the rate at a specific time on a curve?

Draw a tangent to the curve at that time and calculate its gradient.

5.8 – Factors & Collision Theory

According to collision theory, what must happen for a reaction to occur?

Particles must collide with enough energy and correct orientation.

How does increasing concentration affect rate?

More particles in a given volume leads to more frequent collisions, causing a faster reaction.

How does temperature affect rate of reaction?

Higher temperature makes particles move faster, leading to more frequent and more sucsessfull collisions.

5.9 – Reversible Reactions & Equilibrium

What is a reversible reaction?

A reaction where products can react to form reactants again.

Define dynamic equilibrium.

When the forward and backward reactions occur at the same rate, so concentrations stay constant.

In dynamic equilibrium, are the amounts of reactants and products equal?

Not necessarily — only their rates of formation/depletion are equal.

5.10 – Le Chatelier's Principle

What does Le Chatelier's principle state?

If a system at equilibrium is disturbed, it shifts to oppose the change.

How does increasing concentration of reactants affect equilibrium?

Equilibrium shifts to the right (towards the products).

How does increasing temperature affect an exothermic reaction at equilibrium?

Equilibrium shifts to the left (towards reactants) to oppose the increase in temperature.