Katherine F. A Level Chemistry tutor, A Level Biology tutor, GCSE Fre...

Katherine F.

£18 - £20 /hr

Currently unavailable: for new students

Studying: Biochemistry (Bachelors) - Oxford, Trinity College University

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About me

Hi!  I'm a first-year biochemist at Oxford University and I am very keen to help with biology and chemistry A levels, or with French and German at GCSE.  I love learning and want to pass on this enthusiasm, as well as helping students to achieve their full potential in exams.

Hi!  I'm a first-year biochemist at Oxford University and I am very keen to help with biology and chemistry A levels, or with French and German at GCSE.  I love learning and want to pass on this enthusiasm, as well as helping students to achieve their full potential in exams.

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10/07/2015

Qualifications

SubjectQualificationGrade
BiologyA-level (A2)A*
ChemistryA-level (A2)A*
FrenchA-level (A2)A*
GermanA-level (A2)A*

General Availability

Before 12pm12pm - 5pmAfter 5pm
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Subjects offered

SubjectQualificationPrices
BiologyA Level£20 /hr
ChemistryA Level£20 /hr
BiologyGCSE£18 /hr
ChemistryGCSE£18 /hr
FrenchGCSE£18 /hr
GermanGCSE£18 /hr

Questions Katherine has answered

What evidence is there to support the delocalised model of benzene over Kekulé's model?

Bond lengths - a single bond is 0.153nm while a double bond is 0.134nm, making Kekulé's model of alternating single and double bonds asymmetric.  Other evidence shows benzene to be symmetrical so it cannot have this combination.  Instead, all bonds are 0.139nm.

Enthalpy change of hydrogenation - if cyclohexene is hydrogenated, the enthalpy change for adding hydrogen across 1 double bond is -120kJ/mol.  If benzene has 3 double bonds, it should have an enthalpy change of -360kJ/mol (3x120).  However, when benzene is hydrogenated, it releases -208kJ/mol, showing it to be more stable than Kekulé's structure.  This supports the theory of a delocalised electron ring as this electronic structure stabilises the molecule.

Resistance to reaction - benzene will not react with halogens or strong acids, or take part in addition reactions so it does not react like an alkene.  This suggests that C=C double bonds are not present.

Bond lengths - a single bond is 0.153nm while a double bond is 0.134nm, making Kekulé's model of alternating single and double bonds asymmetric.  Other evidence shows benzene to be symmetrical so it cannot have this combination.  Instead, all bonds are 0.139nm.

Enthalpy change of hydrogenation - if cyclohexene is hydrogenated, the enthalpy change for adding hydrogen across 1 double bond is -120kJ/mol.  If benzene has 3 double bonds, it should have an enthalpy change of -360kJ/mol (3x120).  However, when benzene is hydrogenated, it releases -208kJ/mol, showing it to be more stable than Kekulé's structure.  This supports the theory of a delocalised electron ring as this electronic structure stabilises the molecule.

Resistance to reaction - benzene will not react with halogens or strong acids, or take part in addition reactions so it does not react like an alkene.  This suggests that C=C double bonds are not present.

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2 years ago

1886 views

How can an aldehyde be distinguished from a ketone?

An aldehyde will react with Tollen's reagent to form a characteristic silver mirror, whereas a ketone will not react.  This is because an aldehyde can be oxidised to a carboxylic acid, reducing Tollen's reagent, whereas a ketone cannot be oxidised so cannot take part in the reaction.

An aldehyde will react with Tollen's reagent to form a characteristic silver mirror, whereas a ketone will not react.  This is because an aldehyde can be oxidised to a carboxylic acid, reducing Tollen's reagent, whereas a ketone cannot be oxidised so cannot take part in the reaction.

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2 years ago

731 views

How do auxins bring about positive phototropism in plants?

1) Phototropins are enzymes found in the plasma membrane of particular cells.  They are phosphorylated when hit by blue light.

2) This phosphorylation triggers the movement of auxin across the plant shoot by active transport as transporter proteins pump the hormone out of cells.

3) As a result, there is more auxin on the shaded side of the plant, producing a concentration gradient.

4) Auxin promotes active transport of hydrogen ions into the cell wall by ATPase action.  Where there is more auxin, more ions are brought in.

5) The hydrogen ions lower the pH of the cell wall, reaching the optimum for the enzymes that hydrolyse bonds withincellulose fibres.  The ions also disrupt hydrogen bonds between cellulose fibres.  This makes the cell wall less rigid.

6) As the cell absorbs water by osmosis, it swells and elongates.  Because of the reduced rigidity of the cell wall, the cell can stretch more than cells with lower auxin concentrations and therefore more restrictive cell walls.

7) Consequently the cells on the shaded side of the plant grow longer than those facing the light due to the difference in auxin concentration.  This causes the plant to bend towards the light.

1) Phototropins are enzymes found in the plasma membrane of particular cells.  They are phosphorylated when hit by blue light.

2) This phosphorylation triggers the movement of auxin across the plant shoot by active transport as transporter proteins pump the hormone out of cells.

3) As a result, there is more auxin on the shaded side of the plant, producing a concentration gradient.

4) Auxin promotes active transport of hydrogen ions into the cell wall by ATPase action.  Where there is more auxin, more ions are brought in.

5) The hydrogen ions lower the pH of the cell wall, reaching the optimum for the enzymes that hydrolyse bonds withincellulose fibres.  The ions also disrupt hydrogen bonds between cellulose fibres.  This makes the cell wall less rigid.

6) As the cell absorbs water by osmosis, it swells and elongates.  Because of the reduced rigidity of the cell wall, the cell can stretch more than cells with lower auxin concentrations and therefore more restrictive cell walls.

7) Consequently the cells on the shaded side of the plant grow longer than those facing the light due to the difference in auxin concentration.  This causes the plant to bend towards the light.

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2 years ago

1039 views

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