chemistry - topic 3: chemical reactions

What is the conservation of mass?

Conservation of mass states that no atoms are created or destroyed during a chemical reaction.

Finish the sentence: If the mass seems to change, there is usually a ___ involved.

Gas.

Why might the mass increase during an experiment?

One of the reactants is a gas that’s found in the air (e.g. oxygen). When the gas reacts to form part of the product, it becomes contained inside the reaction vessel, increasing the mass.

Why might the mass decrease during an experiment?

Some of the reactants are solids, liquids, or aqueous, and at least one of the products is a gas. Before the reaction, any solid/liquid/aqueous is contained in the vessel. Is the vessel isn’t inclosed, the formed gas can escape, meaning the mass in the vessel decreases.

Elements in Group 1 form what ions?

1+ Ions.

Elements in Group 2 form what ions?

2+ ions.

Elements in Group 6 form what ions?

2- Ions.

Elements in Group 7 form what ions?

1- Ions.

How do you work out the chemical formula of an ionic compound?

You need to balance the positive and negative charges. For example, the formula for calcium nitrate (Ca²+ and NO3-) is Ca(NO3)2 — you need two nitrate ions to balance the 2+ charge on Ca.

What do the prefixes ‘di-‘, ‘mono-‘, and ‘tri-‘ tell us?

• Mono = One atom in a molecule

• Di = Two atoms in a molecule

• Tri = Three atoms in a molecule

What are state symbols?

State symbols are symbols next to each substance, telling us what phsyical state the reactants and products are in.

What is an ‘aqueous’ substance?

A substance that dissolves in water.

Finish the sentence: In an ionic equation only the ____ _____ (and the products they form) are included.

Reacting particles.

What do half equations show?

Show how electrons are transferred during oxidation or reduction reactions.

What is Avogadro’s constant?

6.022 × 10²³

What does Avogadro’s constant show?

It defines the number of constituent particles in one mole of any element or compound, and they weigh exactly the same number of grams as the relative atomic mass (or relative formula mass).

How would you work out the amount of atoms in 5 moles of oxygen gas?

1. Multiply Avogadro’s constant by the number of moles (5) to find the number of particles.

2. There are two atoms in one molecule of oxygen, so multiply your answer by 2.

3. Answer: 6.022 × 10^24.

What is the equation for number of moles in a certain mass?

Number of moles = GRAM/RAM

What is the definition of a limiting reactant?

The reactant that’s used up in a reaction.

Finish the sentence: The amount of product formed is ___ ___ to the amount of the limiting reaction.

Directly proportional.

What are the steps to creating a balanced symbol equation?

1. Divide the mass of each substance by it’s relative formula mass to find the number of moles (moles = gram/ram)

2. Divide the number of moles of each substance by the smallest number in the reaction.

3. If needed, make them all whole numbers by multiplying them all by the same number.

4. Write the balanced symbol equation by putting these numbers in front of the formulas.

How can you work out the limiting reactant of a reaction?

1. Divide the mass of each substance by it’s mass to find the moles (moles = gram/ram)

2. Divide these by the smallest number of moles.

3. Compare the ratios between the moles of products with a balanced chemical equation.

4. The lower ratio is the element that is the limiting reactant.

What is an exothermic reaction?

A reaction that gives out energy to the surroundings in forms of heat, causing a rise in temperature of the surroundings.

What is an endothermic reaction?

A reaction which takes in energy in forms of heat, resulting in a fall in temperature of the surroundings.

If the products are at a higher energy than the reactants, is this an endo or exothermic reaction?

Endothermic — energy is absorbed.

What is the definition of activation energy?

The minimum amount of energy needed for the bonds to break and therefore a reaction to start. If the energy input is less than the activation energy, nothing will happen.

Bond breaking is a ___ process.

Endothermic — Energy must be supplied to break existing bonds, and this is greater than the energy needed to form them.

Bond formation is a ___ process.

Exothermic — Energy is released when new bonds are formed, and this is greater than the energy needed to break them.

What is the equation to calculate overall energy change, and what does the result mean?

Overall energy change = Energy to break bonds (reactants) - Energy released forming bonds (products).

A positive energy change - Endothermic and negative energy change - Exothermic.

What is the pH scale?

A measure of how acidic or alkaline a solution is.

What two methods can you use to measure pH?

An indicator (result is a colour), and a pH probe (result is a numerical value).

What is a neutralisation reaction?

The reaction between acid and bases that produces a salt and a water.

What is the ionic equation of a neutralisation reaction?

H+ + OH- —> H2O

How do you know if an acid is strong or weak?

A strong acid ionises almost completely in water, and a large proportion release H+ ions, while a weak acid do not fully ionise in a solution.

What is the definition of ‘concentration’?

How much acid there is in one litre of water, which is basically how watered down your acid is.

What are the products when an acid and metal react?

Salt and hydrogen.

What are the products when a metal carbonate reacts with an acid?

Salt, water, and carbon dioxide.

What are the products when an acid and alkali react?

Salt and water.

What is oxidation and reduction?

Oxidation is the reaction with (or addition of) oxygen, whilst reduction is the removal of oxygen.

What does OIL RIG stand for?

Oxidation Is Loss, Reduction Is Gain.

What is a redox reaction?

A reaction in which both oxidation and reduction occur.

How can you use half equations to see if something has been oxidised or reduced?

If the element loses electrons, it is being oxidised (and is a reducing agent). If it gains electrons, it’s being reduced (and is an oxidising agent).

What is electrolysis?

The breaking down of a substance using electricity.

Describe the process of electrolysis.

An electric current is passed through an electrolyte causing it to decompose. The positive ions (cations) move towards the cathode and are reduced (gain electrons). The negative ions (anions) move towards the anode and are oxidised (lose electrons). The ions formed are then discharged from the solution.